Astrocytomas

The most common supratentorial brain tumor is the astrocytoma, representing between 50% and 60% of all primary pediatric brain tumors. Astrocytomas are seen in all age groups, and presenting symptoms are typically of intracranial pressure, with headache, nausea, and vomiting commonly seen. Astrocytomas can be reliably diagnosed by CT and MRI ( Fig. 66-9 ). They range from benign to malignant in their histologic grade, with grade 1 lesions classified as benign, grade 2 as low-grade glioma, grade 3 as anaplastic glioma, and grade 4 as glioblastoma. The lesions are usually quite large with both solid and cystic components. Calcifications can occur but are unusual. Parietal, frontal, and temporal lobes are common sites of astrocytoma. On fluid-sensitive sequences, the cystic component of the lesion may show features of complex fluid, with decreased signal intensity relative to CSF on T2-weighted sequences. After contrast infusion, homogeneous enhancement of the solid component of the lesion is characteristic. Higher-grade, more malignant tumors often have less distinct margins, more heterogeneous enhancement, and increased peritumoral edema and mass effect. Newer techniques including perfusion-weighted MRI and MR spectroscopy have been evaluated as noninvasive techniques to determine tumor angiogenesis and capillary permeability. For example, in higher-grade gliomas increased blood flow to the tumor lesion has been demonstrated using this technique. Also, as the tumor grade increases to a more malignant variant, there is an increase in the Cho-to-NAA ratio, reflecting the increased metabolic activity present in higher grade lesions. Complete surgical resection is often not possible because of involvement of adjacent major neural structures. The 5-year survival rate for low-grade astrocytomas is between 40% and 50%, with less than 5% survival for the higher-grade malignancies. Because of a propensity to spread to the spine via the CSF, the staging evaluation must also include imaging of the entire spine.

A 4-year-old patient with hemispheric astrocytoma.

T2-weighted ( A ) and post-Gd T1-weighted images ( C and D ) show cystic and solid mass with peripheral enhancement located in the frontoparietal region. MRS from the solid component is notable for decreased N-acetylaspartate and elevated choline (Cho) and lipid peaks, typical of a highly cellular tumor ( B ).

The infratentorial/cerebellar form of astrocytoma, also known as juvenile pilocytic astrocytoma, is distinct from the diffuse supratentorial astrocytoma discussed here and is described in further detail below.

Oligodendrogliomas

Oligodendrogliomas, although common in adults, are relatively rare in children and adolescents. The most common location is in the frontotemporal region, with less common involvement of the posterior fossa and spinal cord. Imaging evaluation shows a hypodense lesion on CT. Because the majority of these contain varying degrees of calcification, they are distinct as being the most common supratentorial tumor in childhood to calcify. The noncalcified solid component is typically isodense to the adjacent brain on CT. On MRI, the lesions are characterized by low signal intensity on T1-weighted images, hyperintensity on T2-weighted images, and minimal enhancement after infusion of gadolinium contrast ( Fig. 66-10 ). Although these tend to be slow growing lesions, depending on their size there may be edema evident in the surrounding brain.

Oligodendroglioma.

CT showing calcified low-attenuation lesion in the occipital cortex, with imaging features characteristic of oligodendroglioma.

Gangliogliomas

Gangliogliomas and ganglioneuromas comprise about 5% of the pediatric brain tumors. As with their counterparts in the peripheral nervous system, calcification is seen in up to 40% of the cases. The temporal lobes, frontoparietal lobes, and hypothalamic regions are the most common sites of involvement. The brainstem, posterior fossa, and spinal cord are less commonly involved. MRI demonstrates a low signal-intensity lesion on T1-weighted images with hyperintensity on T2-weighted images and hyperintense homogeneous enhancement after gadolinium infusion ( Fig. 66-11 ).

Ganglioglioma.

A 4-year-old patient with left temporal-lobe ganglioma showing increased signal on T2-weighted images ( A ), and bright enhancement after gadolinium administration ( C and D ). Depending on their location, these lesions may be more conspicuous on fluid-attenuated FLAIR images (B) .

Tumors of the midline deep gray matter, including the basal ganglia, thalamus, hypothalamus, and chiasmatic/optic pathway region tend to be astrocytomas. As with the cerebral hemispheric astrocytomas, tumor grades range from benign to highly anaplastic. The imaging features are similar to the lesions located in the cerebral hemispheres with decreased signal intensity on T1-weighted images, hyperintensity on T2-weighted images, and variable contrast enhancement. Because of their location, behavioral changes, emotional and memory changes, movement disorders, visual abnormalities, and hydrocephalus are all common symptoms associated with these deep gray-matter tumors. Surgical resectability is often not possible, and the imaging evaluation is directed at identifying involvement of adjacent critical structures and monitoring posttherapy changes.

Intraventricular tumors include choroid plexus papillomas, neurocytomas, and dermoid/epidermoid tumors. Choroid plexus papillomas/carcinomas are relatively common, making up between 10% and 20% of the intraventricular tumors identified in the first year of life and comprising approximately 5% of all CNS neoplasms. The imaging features are characteristic and typically reveal large lobulated intraventricular lesions ( Fig. 66-12 ). These tumors are highly vascular and are often associated with calcification and occasionally hemorrhage. Depending on the size and location of the lesion, there may be obstructive hydrocephalus. MRI shows a low signal-intensity lesion on T1-weighted images with variable hyperintensity on T2-weighted images. Magnetic susceptibility is seen if hemorrhage is present, and after contrast infusion these lesions typically show marked contrast enhancement. The combination of a lobulated, intensely enhancing intraventricular lesion is nearly diagnostic of choroid plexus papilloma. Five percent to 10% of these lesions degenerate into choroid plexus carcinomas, and this latter diagnosis is suggested by accompanying necrosis within the lesion and metastatic spread into the adjacent brain, either by direct extension into the adjacent brain or by CSF dissemination. Because of their intraventricular location, metastatic spread throughout the CNS via CSF dissemination is unfortunately common for choroid plexus carcinomas, resulting in a uniformly poor prognosis.

Choroid plexus papilloma.

8-month-old and 10-month-old infants, both with large lobulated lesions causing obstructive hydrocephalus. A and B , In one patient the tumor is centrally located, producing severe global hydrocephalus. C and D , In the other patient the peripheral location is accompanied by extensive vasogenic edema (arrow) and left occipital-horn hydrocephalus (asterisk).

Central Neurocytomas

Neurocytomas are rare in children and have imaging features that mimic oligodendroglioma, showing an intraventricular calcified mass with heterogeneous contrast enhancement. These tumors typically involve the lateral ventricle in the midline; however, surgical resection is necessary to distinguish this lesion from other intraventricular tumors such subependymal astrocytomas and ependymomas.

Craniopharyngiomas

Craniopharyngioma is the most common suprasellar tumor. It has fairly characteristic imaging features with lesions containing solid, cystic, and calcified elements. Symptoms usually result from increased intracranial pressure and local effects upon the hypothalamic pituitary region and optic chiasm. Because these are relatively slowly growing lesions, they may be quite large at diagnosis. The presence of a calcified suprasellar mass at CT scan is usually suggestive of the diagnosis; however, MRI is superior in evaluating the extent of involvement ( Fig. 66-13 ). Depending on the type of material present in the cystic component of the mass, variable signal intensity on T1- and T2-weighted images may be seen. It is common, for example, for the complex proteinaceous, cholesterol-laden fluid to be variably hyperintense on both T1- and T2-weighted images (see Fig. 66-13 ). After contrast infusion, the cyst wall commonly enhances. However, the solid components of the cyst often demonstrate inhomogeneous enhancement as well. Of note, craniopharyngiomas commonly cause T2 prolongation along the optic tracts. This is a nonspecific sign and has been reported in other tumors centered in this region. It is thought to signify optic tract edema caused by compression by the tumor.

Craniopharyngioma.

CT showing calcified suprasellar mass ( A ), with heterogeneous increased signal on both T2-weighted images ( B ) and T1-weighted images ( C ), with peripheral enhancement after gadolinium ( D ).

Pineal-Region Tumors

Pineal-region masses are characterized by their location in the quadrigeminal plate cistern. These tumors typically arise from primordial germ cells and include pineoblastomas, pineocytomas, and immature germ-cell tumors. It is important to distinguish these neoplastic lesions from benign pineal cysts, which are commonly seen within the pineal gland. Pineal cysts have a characteristic appearance on both CT and MRI and do not distort or compress the adjacent ventricles. Simple pineal region cysts typically do not achieve sizes greater than 1 cm. Also of note is normal calcification within the pineal gland. Calcification is unusual before the age of 10 but is commonly seen in adolescents and older children. Therefore the presence of calcification in the pineal region in an infant or young child should raise suspicion for a pineal region mass/germ-cell tumor and prompt further investigation.

Pineal-gland tumors, including pineocytomas and pineoblastomas, are relatively rare. Pineocytoma is benign and usually nonaggressive, whereas the pineoblastoma, representing an immature undifferentiated lesion, often metastasizes. Pinealoblastoma is also a component of the “trilateral retinoblastoma” and is seen with increased incidence in patients diagnosed with bilateral retinoblastoma. Pineocytomas are more often more calcified than pinealoblastomas. Pinealoblastomas are typically larger with low signal intensity on T1-weighted images, variable hyperintensity on T2-weighted images, and fairly homogeneous contrast enhancement ( Fig. 66-14 ). Very large pinealoblastomas frequently produce mass effect on the adjacent quadrigeminal cistern, and because of their large size often show areas of necrosis.

Pinealoblastoma.

Two patients with pinealoblastoma. The larger cystic/solid mass seen in one patient causes obstruction and hydrocephalus at the level of the quadrigeminal plate cistern ( A ). In the accompanying case ( B ), even the smaller calcified mass causes hydrocephalus as a result of its central location abutting the ventricles. As noted in the text the lesions are often hyperintense on fluid-attenuated sequences ( C , FLAIR images ) and isointense to cortex on T2-weighted images ( D ).

Germ-Cell Tumors

The pineal region is the most common location for intracranial germ-cell tumors. Histopathologically these are most often germinomas and are usually associated with calcification. This is well demonstrated by CT. MRI is indicated to better delineate the extent of the noncalcified solid portion of the mass as well as heterogeneous cystic portions that may also be present. On T2-weighted images, the solid portion is typically hyperintense with bright enhancement after contrast infusion. Other tumors of germ-cell origin are also seen, with choriocarcinomas more commonly showing hemorrhagic components and displaying more aggressive features. Benign teratomas, in contrast, display imaging features more in keeping with their well-differentiated dermoid/epidermoid components, including low attenuation fluid on CT, hyperdense fluid on T1-weighted sequences, and heterogeneous contrast enhancement ( Fig. 66-15 ).

Pineal germ-cell tumor.

CT ( A ) and T1-weighted axial MRI ( B ) showing low attenuation fluid on CT and hyperintense fluid on T1-weighted sequences, typical of a well-differentiated pineal germ-cell tumor (teratoma).

Hypothalamic Hamartoma

This is a rare congenital lesion, usually identified in infants under the age of two. Seizure disorders are common, and these patients often have the presenting symptoms of intractable gelastic seizures. The presence of precocious puberty or diabetes insipidus may provide clinical clues to suggest this diagnosis.

On imaging, CT scanning shows an isodense mass in the region of the hypothalamus. There is little to no contrast enhancement either on CT or MRI. On MRI, the lesion is usually isodense to the brain on T1-weighted images, minimally hyperintense on T2-weighted images, and interestingly may be hyperintense on FLAIR images ( Fig. 66-16 ). Because of the location of these tumors, surgical resection may be challenging, and MR- or CT-guided ablation techniques are now increasingly being used ( Fig. 66-17 ).

Hypothalamic hamartoma.

A , CT shows a low density suprasellar/hypothalamic lesion. MRI shows the lesion to be isointense to brain on T2-weighted sequences ( B ), mildly hyperintense on FLAIR images ( C ), with no significant enhancement after gadolinium infusion ( D ).

MRI-guided laser ablation of hypothalamic hamartoma in a 4-year-old female with medically intractable gelastic seizures.

Coronal reformat of a magnetization-prepared rapid acquisition with gradient echo shows the needle tip within the lesion in the hypothalamus with enhancement around the tip indicating the areas that have been ablated by the procedure.

Medulloblastomas

Medulloblastomas are slightly more common in incidence than cerebellar astrocytomas. They typically occur in the first decade of life. Presenting symptoms usually relate to increased intracranial pressure secondary to obstructive hydrocephalus, most often related to mass effect upon the fourth ventricle. Medulloblastomas are highly malignant and have a propensity to disseminate and seed the CNS via the CSF route. Hematogenous spread to other sites may also occur in medulloblastoma.

CT scanning is usually the first imaging study performed to evaluate new-onset nausea, vomiting, headache, or seizures. CT scanning usually demonstrates a hyperintense solid lesion within the cerebellum, often near the midline ( Fig. 66-18 ). MRI imaging is indicated to better characterize the lesions and to establish sites of local and metastatic spread. As with other highly cellular CNS tumors, on T1-weighted images medulloblastomas are usually hypointense relative to the adjacent cerebellum, with increased signal intensity on T2-weighted images (see Fig. 66-18 ) and homogeneous enhancement after gadolinium infusion. Calcification and cystic areas are unusual in medulloblastoma. Because of the propensity to disseminate to other sites via the CNS, the staging evaluation must include a contrast-enhanced evaluation of the entire spine.

Medulloblastoma.

A , Unenhanced CT shows hyperdense mass in the posterior fossa, compressing the fourth ventricle with resultant obstructive hydrocephalus. B , Gadolinium-enhanced T1-weighted MRI shows enhancement of the majority of the mass; these features are all consistent with a highly cellular medulloblastoma.

Cerebellar Astrocytomas

Astrocytomas are the next most common posterior fossa tumor. As with their intracerebral counterpart, cerebellar astrocytomas range in histologic grade from benign to anaplastic. The characteristic juvenile pilocytic form makes up the majority of the cerebellar astrocytomas. Both cystic and solid components are normally present in this type of astrocytoma ( Fig. 66-19 ). Calcification is unusual and occurs in 10% of tumors. After resection, survival is excellent with a 10-year overall survival rate greater than 90%. As with medulloblastoma, CT scanning is usually the first imaging modality used to evaluate initial symptoms. CT shows a heterogeneous, predominantly cystic lesion in the posterior fossa, typically with a small mural nodule eccentrically along the margin of the tumor. These lesions are often midline, near the vermis, but may also be eccentric in location. MRI imaging is indicated to better characterize the extent of cerebellar involvement. Although the presence of a predominantly cystic component suggests a lower-grade neoplasm, foci of local/metastatic spread may be present and will be manifest as a focus of increased enhancement after contrast administration. Therefore the imaging evaluation should include conventional T1- and T2-weighted imaging as well as postcontrast enhanced scanning in all three imaging planes. Because higher-grade neoplasms may also disseminate via the CSF, as with medulloblastoma, the initial staging evaluation should also include evaluation of the entire spine.

Cerebellar pilocytic astrocytoma.

A , Unenhanced CT shows a large, predominantly cystic mass with a solid mural component in the posterior fossa with moderate hydrocephalus. B , T2-weighted MRI shows high signal-intensity cystic fluid and intermediate signal mural nodule. C and D , Only the mural nodule enhances after gadolinium infusion.

Brain-Stem Gliomas

Brain-stem gliomas make up the majority of the neoplasms involving the brain stem. These tumors are of astrocytoma origin and because of their location result in nearly uniformly poor survival. These predominantly cellular solid lesions are best evaluated by MRI, and they typically appear as hypodense lesions on T1-weighted images, with increased signal intensity on T2-weighted images, and show heterogeneous enhancement. Lower-grade lesions, or lesions with larger cystic components may only show very minimal contrast enhancement ( Fig. 66-20 ). As with the other tumors in these locations, spread is most often via the CSF to other portions of the brain.

Low-grade pontine glioma.

Axial T2 ( A ) and post-Gd T1-weighted ( B ) images show T2-bright lesion with faint enhancement of a small nodular component of the tumor along the lateral margin (arrow).

Ependymomas

After medulloblastomas and juvenile pilocytic astrocytomas, the next most common posterior fossa tumor is the ependymoma, constituting approximately 8% to 15% of intracranial neoplasms. The majority of these tumors are benign and arise from the ependyma of the fourth ventricle. They typically present as a calcified intraventricular mass. Symptoms result from obstruction of CSF flow at the level of the fourth ventricle leading to obstructive hydrocephalus. There are two histologic types, with the benign undifferentiated type more common, typically less invasive locally, and with a lower likelihood of CSF dissemination. Malignant anaplastic ependymomas, in contrast, tend to spread via the CSF early in the disease course to other sites within the CNS. Because of their propensity to insinuate through neural foramina, resulting in a lobulated appearance, ependymomas have a fairly characteristic appearance on imaging ( Fig. 66-21 ). Depending on the degree of calcification, these lesions may be predominantly hypointense on T1- and T2-weighted images. In the absence of calcification, these are usually hyperintense lesions on T2-weighted images with heterogeneous intense enhancement after gadolinium infusion. Because of their propensity to seed throughout the CSF and ventricular system, the imaging evaluation must include examination of both the brain and spinal canal.

Ependymoma.

A 7-year-old patient with the symptom of lethargy. T2-weighted ( A ) and gadolinium-enhanced ( B ) MRI show a large heterogeneously enhancing posterior fossa mass compressing the brain stem and beginning to extend out through the foramen magnum.

Hemangioblastomas

Hemangioblastomas are benign vascular lesions that are more common in adults than children. However, the cerebellum is still the most common location in children, and multiple hemangioblastomas may be encountered in familial predisposition syndromes such as the von Hippel-Lindau disease. The imaging features demonstrate a predominantly cystic lesion with an enhancing mural nodule that may be difficult to distinguish from JPAs. As with JPAs, these lesions show cystic characteristics on MRI with low signal intensity on T1-weighted images and T2 hyperintensity, with intense enhancement of the solid mural nodular component ( Fig. 66-22 ). Depending on the size of the mural nodule, the degree of enhancement, and its location, these lesions may also be difficult to discriminate from a benign posterior fossa arachnoid cyst.

Hemangioblastoma.

A 28-year-old patient with von Hippel-Lindau disease and cerebellar hemangioblastoma showing typical features on T2-weighted ( A ), fluid-attenuated FLAIR ( B ), and post-gadolinium enhanced images ( C and D ).

Metastatic Disease

Metastatic disease to the CNS may result from leptomeningeal seeding, hematogenous dissemination, or direct extension. Leptomeningeal seeding results from a number of primary brain tumors, most notably medulloblastomas. Leptomeningeal seeding can also result from non-CNS primary tumors and systemic malignancies. The imaging appearance in this setting is similar to that of leptomeningeal dissemination occurring secondary to a primary CNS tumor.

Hematogenous metastases to calvarium or dura are most commonly seen in patients with neuroblastoma, leukemia, and lymphoma. These lesions present as lytic calvarial lesions or as nonspecific, enhancing dural masses. CNS involvement by leukemia is also common, although this is more often diagnosed by lumbar puncture and pathologic evaluation of the CSF rather than imaging.

Hematogenously disseminated metastases to the brain parenchyma are rare in childhood. When they do occur, primary tumors are sarcomas, particularly rhabdomyosarcoma, Ewing sarcoma, and osteosarcoma. A tumor with a particular propensity to metastasize to the brain is the rhabdoid tumor of the kidney, and imaging of the CNS is essential in children with this diagnosis. Parenchymal metastases are usually multiple and located at the interface between the gray and white matter. They are associated with marked vasogenic edema and demonstrate avid enhancement secondary to loss of the blood-brain barrier.

Although the imaging features may be variable depending on the primary malignancy, the presence of any focal area of signal abnormality or enhancement with a known primary tumor should raise suspicion for metastatic spread of disease to the CNS.

Spinal Cord Tumors

In comparison to pediatric brain tumors, spinal cord tumors are relatively rare in children. The majority of the intramedullary spinal tumors in children are low-grade astrocytomas, with the remainder being ependymomas. The imaging features of these lesions mimic their intracranial counterparts. The imaging assessment is usually prompted by neurologic symptoms and should include multiplanar T1, T2, and post–gadolinium-enhanced imaging. Because it may be difficult to discriminate a primary intraspinal tumor from metastatic disease, imaging of the brain must also be performed.

As in the brain, the imaging examination should be directed at identifying the site of origin of the tumor. It should be possible to make a distinction between primary intramedullary tumors, extramedullary intradural tumors, and extradural/epidural lesions. This is important, because the majority of the spinal tumors in children are extramedullary. Intradural extramedullary lesions are, by definition, located within the subarachnoid space and are typically the result of drop metastases, which as discussed earlier, spread via the CSF from primary brain tumors ( Fig. 66-23 ). Their appearance on imaging mimics their counterparts in the brain. Extradural extramedullary tumors may arise from local extension of other primary neoplasms such as neuroblastoma, lymphoma, or Ewing sarcoma ( Fig. 66-24 ). In evaluating these other primary pediatric neoplasms, when their location is adjacent to the vertebral column, it is essential that a thorough imaging evaluation be undertaken to evaluate the presence and extent of intraspinal involvement. In particular, with neuroblastoma, this will influence staging and surgical resectability. The presence of intraspinal extension may be suggested by CT, but will be typically better delineated by MRI ( Fig. 66-25 ).

Spinal cord astrocytoma.

Sagittal T2-weighted ( A ), sagittal post-Gd T1-weighted ( B ), and axial T2-weighted ( C ) images demonstrate a T2-hyperintense, ovoid lesion with enhancing central components consistent with a intramedullary thoracic spinal cord tumor. Pathology of resected specimen showed a low-grade astrocytoma.

A and B , Coronal and axial MRI images show displacement of nerve roots and intension through the neural foramen in a patient with extramedullary intraspinal Ewing sarcoma. C and D , Patient with calcified left upper thoracic neuroblastoma initially detected on chest x-ray. MRI is superior in detecting the degree of intraspinal extension ( D ).

Upper thoracic paraspinal neuroblastoma.

A 5-year-old patient with 2 weeks of upper-respiratory–infection symptoms. Chest x-ray revealed a right upper lobe opacity (not shown). This was found on CT to be a paraspinal mass. Intraspinal extension is evident on CT ( A , arrow ) but much better delineated by MRI ( B , arrow ).

Anterior Mediastinal Masses

The anterior mediastinum is defined as the prevascular space situated between the sternum and the heart, pericardium, and great vessels. The anterior mediastinum extends superiorly from the thoracic inlet to the level of the diaphragm. Organs located in the anterior mediastinum include thymus, thyroid, and parathyroid glands and prevascular-space lymphoid tissue. With respect to mediastinal mass formation, the thymus and anterior mediastinal lymph nodes are the two most important structures to be considered.

In infants, it is important to distinguish normal thymus from a mediastinal mass. The normal thymus typically has an undulating contour and on chest radiographs can be accurately identified based on subtle deformity by the adjacent anterior ribs’ costal cartilages ( Fig. 66-29 ). The normal thymus can be quite large and may extend posteriorly between the superior vena cava and aorta in the middle mediastinum or superiorly into the lower neck, making the distinction from neoplasm difficult. In the appropriate clinical setting thymic hyperplasia may also be seen and is characterized by a homogeneously enlarged thymus. The thymus also may rapidly involute in response to physiologic stress, and this can also provide an indirect means of confirming its identity as normal thymus.

Anteroposterior chest radiograph of an infant shows prominent thymic shadow and undulating contour conforming to the adjacent ribs, findings all consistent with normal thymic tissue.

Lymphoma.

Lymphoma is the most common tumor arising in the mediastinum and most commonly arises in either the anterior mediastinum or middle mediastinum. Both Hodgkin and non-Hodgkin lymphomas can present as mediastinal masses. The imaging features of Hodgkin lymphoma characteristically reveal a bulky anterior mediastinal mass with a nodular appearance. In addition, Hodgkin lymphoma is characterized by contiguous lymph-node spread, which may aid in the distinction from other types of lymphoma. Approximately two thirds of pediatric patients with Hodgkin lymphoma show lymphadenopathy involving the mediastinum. Non-Hodgkin lymphomas include lymphoblastic lymphoma and other subtypes. Approximately one third of pediatric non-Hodgkin lymphomas have a mediastinal mass, and more than 50% of the lymphoblastic lymphomas have mediastinal involvement. Although the distinction between some subtypes of non-Hodgkin and Hodgkin lymphoma may be difficult, acute T-lymphoblastic lymphomas often demonstrate a homogeneous infiltration and enlargement of the thymus gland with encasement/compression of the vessels ( Fig. 66-30 ). All forms of lymphoma can result in significant tracheal compression/narrowing, vascular compression, and symptoms related to the superior mediastinal syndrome. Pleural and pericardial effusions are seen in both non-Hodgkin and Hodgkin lymphomas of the mediastinum and are not a distinguishing feature. Bone-marrow involvement is seen in patients with either Hodgkin or non-Hodgkin lymphoma. Recent studies have shown that FDG-PET is a sensitive method for detecting bone-marrow involvement in children with Hodgkin lymphoma and can safely replace routine bone marrow biopsy for establishing the presence of marrow involvement. Direct bone invasion by Hodgkin lymphoma is unusual; non-Hodgkin lymphomas often show more aggressive local involvement.

A , Chest x-ray showing marked tracheal deviation, pneumomediastinum, and subcutaneous emphysema along the right neck in a child with lymphoblastic lymphoma, emphasizing the severity of airway obstruction and potential for respiratory compromise (arrows) . B – E , CT shows large anterior mediastinal mass with mediastinal air around the heart, pulmonary vessels, aorta, and esophagus ( C ). The trachea is markedly narrowed (arrow), likely the cause of the pneumomediastinum. Subcutaneous air is also evident in the anterior chest wall. Five days later the findings had progressed considerably ( C and D ).

The imaging evaluation should always include a chest radiograph. In many instances this is the most important imaging study to guide the subsequent care of the patient. The presence of significant tracheal narrowing or tracheal displacement in the context of a large anterior mediastinal mass, particularly if accompanied by respiratory symptoms (see Figs. 66-2 and 66-30 ), should raise immediate concern for impending respiratory/cardiopulmonary compromise and prompt intensive care unit (ICU)–level monitoring. Pneumomediastinum and pneumothorax may also be identified on these initial imaging studies (see Fig. 66-30 ). The placement of the patient in a recumbent position for CT scanning may further exacerbate the patient’s already tenuous respiratory status, impair central venous return, and result in an acute cardiorespiratory event. In this setting it may be necessary to delay CT imaging, and the staging evaluation will have to be postponed until stabilization of the patient’s clinical status.

In a stable patient, staging of anterior mediastinal lymphomas must include imaging of the neck to evaluate the Waldeyer ring of lymphoid tissue and should also include the abdomen and pelvis. Intravenous contrast should be used to provide an accurate assessment of the mediastinal vascular structures. Oral contrast should be provided to opacify the bowel and aid in the distinction from mesenteric lymphadenopathy.

MRI, particularly with faster scanning techniques, may be effective in evaluating mediastinal masses and staging lymphoma but currently does not have an established role in the more acute stages of the evaluation. Gallium scintigraphy previously and currently FDG-PET imaging are recognized as standard of care in identifying sites of metabolically active lymphoma and in identifying sites of disease otherwise undetected by conventional imaging techniques ( Fig. 66-31 ). FDG-PET scanning, in particular, has also been shown to play an important role in the posttreatment evaluation, helping to stratify patients into early responder and nonresponder groups to allow appropriate modulation of therapy based on treatment response ( Fig. 66-32 ).

FDG-PET and image fusion delineate equivocal areas of abnormality on CT (arrows), identifying foci of disease recurrence in a patient with Hodgkin lymphoma.

Hodgkin Lymphoma

CT and FDG-PET obtained at diagnosis ( A and B ) and after 2 cycles of chemotherapy ( C and D ) show excellent functional and anatomic response to therapy. Residual mediastinal mass on CT with no residual FDG uptake emphasizes the potential importance of functional imaging in response assessment.

After treatment, particularly of Hodgkin disease, it is common for the mediastinal mass to slowly regress and for residual inflammatory tissue to persist even months after therapy. In contrast, lymphoblastotic lymphomas often respond quite rapidly to therapy with very little residual soft-tissue abnormality remaining in the anterior mediastinum, even after a relatively short duration of treatment ( Fig. 66-33 ). The use of FDG-PET for early response assessment in Hodgkin lymphoma is now considered standard of care, with images being simultaneously acquired and coregistered as fused PET-CT images to distinguish persistent metabolically active disease from residual posttreatment inflammatory tissue. There is a large body of literature from both pediatric and adult patients showing that the presence of residual metabolically active disease after two cycles of chemotherapy is prognostically significant, presumably reflecting chemosensitive versus chemoresistant disease. Patients with residual CT abnormalities that are FDG-avid have a worse overall outcome as compared with patients whose scans are FDG-negative, which has led to a paradigm shift in Hodgkin lymphoma therapy such that patients in whom early response assessment shows no metabolically active disease would be eligible for reduced chemotherapy and radiation in an effort to reduce treatment-related complications. Whether this approach will maintain the good outcomes (>90% overall survival) typically seen in Hodgkin lymphoma remains to be seen. There is currently no convincing data showing the prognostic value of FDG-PET imaging in non-Hodgkin lymphoma.

A 4-year-old child with T-cell ALL and a large mediastinal mass with airway and vascular compression seen by CXR (A) and CT (B) , showing near complete resolution of the mass after just 2 weeks of therapy (C) .

Germ-Cell Tumor.

Mediastinal germ-cell tumors are primarily located in the anterior mediastinum near the thymus gland and make up about 10% to 20% of all childhood mediastinal tumors. Germ-cell tumors are second only to lymphoma as the cause of a thymic/anterior mediastinal mass. When they present as predominately solid masses, they may be difficult to distinguish from lymphoma. Histologically, mature teratomas are the most common germ-cell tumor arising in the mediastinum. At diagnosis, calcification of lymphoma is unusual, and the calcifications that are often present in teratoma may suggest the diagnosis. Germ-cell tumors are often asymptomatic and may be identified incidentally as mediastinal masses on chest radiographs obtained for other indications. When symptoms are present, they are usually the result of compression of the tracheobronchial tree.

Either CT or MRI scanning are effective at further characterizing these masses. The presence of fluid attenuation, mixed attenuation fluids and gelatinous material, fat, and calcification all should suggest the diagnosis of mediastinal germ-cell tumor ( Fig. 66-34 ). Ten percent to 20% of all mediastinal germ-cell tumors are malignant and present with signs of local invasion into the pleura or pericardium. Malignant tumors include seminomatous and nonseminomatous histologic subtypes. Seminomas tend to be noncalcified, bulky solid masses that either remain localized or spread to local lymph nodes, whereas other malignant germ-cell tumors display greater heterogeneity with cystic and solid elements and more aggressive, locally invasive features. Germ-cell tumors may rupture into the pleural or pericardial spaces, the lung parenchyma, or the tracheobronchial tree. Imaging evaluation should be directed at determining the extent of rupture and sites of tissue involvement. Chemical pneumonitis can result from parenchymal rupture, and expectoration of blood, hair, and sebaceous material can result from rupture into the airways.

A 15-year-old boy with mediastinal mass seen incidentally on a chest x-ray obtained for pain after a football injury (A) . Subsequent CT ( B and C ) shows fat (F) and adjacent solid elements in addition to calcification, in this case taking the form of tooth (T), characteristic of germ cell tumor, in this case mature teratoma.

Thymic Masses

Thymoma.

Thymoma is rare in children, accounting for less than 4% of pediatric mediastinal tumors. As with adults, thymoma may present with symptoms related to associated autoimmune disorders, including myasthenia gravis, diabetes, and Hashimoto thyroiditis. Thymomas are typically solid, lobulated masses arising in or around the thymus ( Fig. 66-35 ). The imaging evaluation should be directed at assessing for aggressive features such as invasion into adjacent pericardial or pleural structures. MRI and contrast-enhanced CT reveal a heterogeneously enhanced low-density mass that is isointense on T1-weighted images and mildly T2 hyperintense relative to muscle. Both CT and MRI are excellent in delineating the relationship of these masses to the adjacent vascular structures (see Fig. 66-35 ). Nodular pleural or pericardial thickening should raise concern for invasive thymoma. Because complete surgical resection is the most effective treatment for ensuring long-term survival for patients with thymoma, postsurgical radiation therapy may be considered when invasive features are present.

A 15-year-old girl with a 2-week history of chest pain.

Chest x-ray and CT show a large, lobular, heterogeneous anterior mediastinal mass abutting the great vessels, with a poorly defined plane between the mass and normal mediastinal structures. Biopsy revealed thymoma, and the patient underwent complete resection.

Thymolipomas.

Thymolipoma, in contrast to thymoma, is relatively more common, making up about 3% to 9% of pediatric thymic tumors. These benign tumors are most often seen in older children, but they may present in infancy. These heterogeneous masses may contain calcification and cystic areas, making distinction from germ-cell tumors challenging. A predominantly fatty mass should, however, suggest the diagnosis of thymolipoma ( Fig. 66-36 ). The fat characterization can be sensitively and specifically evaluated by MRI, showing a high signal-intensity mass on T1-weighted imaging with loss of signal on fat-suppressed images. A low-attenuation mass with negative Hounsfield-unit density on either unenhanced or contrast-enhanced CT can also suggests the presence of a fat-containing mass (see Fig. 66-36 ).

Thymolipoma in a child with an incidentally detected mediastinal mass on chest x-ray.

Note the fat-density mass (arrow) located within the thymus.

Middle Mediastinum

Tracheobronchial Tree Masses.

The middle mediastinum is defined as the vascular space including the pericardium/heart, the great vessels, and trachea/proximal bronchi and associated lymph nodes. Although the most common middle mediastinal mass is lymphoma, lymphoma is rarely isolated to the middle mediastinum and usually occurs in conjunction with a large anterior mediastinal mass. The most common masses isolated to the middle mediastinum are benign bronchopulmonary foregut malformations, including bronchopulmonary sequestration, congenital cystic adenomatoid malformation (C-CAM), and foregut/bronchogenic cysts. These benign masses are not discussed further in this section except to emphasize that the distinction between cystic adenomatoid malformation and pleuropulmonary blastoma cannot be made on the basis of imaging alone ( Fig. 66-37 ). Surgical resection and histopathologic evaluation is required to distinguish the benign C-CAM from its neoplastic counterpart.

Pleuropulmonary blastoma ( A and B ). Note the difficulty in distinguishing this malignancy from C-CAM ( C and D ).

Masses of the endobronchial tree are also exceedingly rare in children. The most common endobronchial neoplasm in children is the bronchial carcinoid tumor, making up about 50% of all bronchial neoplasms. Although these tumors may secrete neuroendocrine peptides, the classic carcinoid syndrome is relatively unusual in children with bronchial carcinoids, and these patients are more likely to have presenting respiratory symptoms such as hemoptysis or lobar atelectasis/partial collapse caused by the obstructing endobronchial lesion ( Fig. 66-38 ). These lesions are commonly hilar in location and may be very difficult to identify on chest radiographs. The presence of a persistent area of segmental or lobar collapse that does not resolve after appropriate therapy should prompt further investigation by CT scanning to directly assess the airways. Multidetector-row CT scans can provide exquisite 3D reformations of the airways and allow accurate detection of endobronchial abnormalities. Chest CT of patients with suspected endobronchial carcinoid tumor may be performed without intravenous contrast for the purpose of identifying the endobronchial lesion; however, contrast infusion typically shows prominent enhancement of these highly vascular lesions. There may be intraluminal, mural, and extraluminal components of these lesions, and the extraluminal extent of the lesion may exceed its intraluminal component. Although the presence of calcification in an endobronchial mass should suggest the diagnosis of a carcinoid tumor, calcification is still relatively rare in childhood bronchial carcinoid tumors. Surgical resection is usually curative, and although the extent of local invasion around the primary lesion is variable, metastatic disease is relative rare (5% to 20%). Uptake of the neuroendocrine-specific radiotracer ¹¹¹ I-octreotide provides highly specific scintigraphic imaging confirmation of the diagnosis (see Fig. 66-38, C and D ) and may aid in identifying occult sites of metastatic disease. FDG-PET imaging of metabolically active carcinoid tumors has also been performed and, although not specific for neuroendocrine tumors, may prove to be a sensitive means of identifying occult metastases.

A and B , Bronchial carcinoid tumor with CT showing a hypervascular enhancing endobronchial mass ( A , arrow ), causing post­obstructive collapse of the right lower lobe ( B , arrow ). ¹¹¹ In-octreotide scintigraphy ( C ) and FDG-PET ( D ) both show accumulation in the mass and were helpful in staging this patient’s tumor.

Mucoepidermoid carcinoma is the second most common primary bronchial neoplasm. These patients also have primarily respiratory presenting symptoms related to airway obstruction. Because primary airway neoplasia in children is rare and commonly characterized by nonspecific clinical symptoms, the diagnosis is often delayed and difficult to distinguish from infectious processes such as pneumonia. The most common finding on plain chest radiographs in patients with mucoepidermoid carcinoma is a central mass or nodule. These tumors range from low to high grade, with high-grade neoplasms tending to invade the adjacent pulmonary parenchyma. As with carcinoid tumors, multidetector-row CT with multiplanar and 3D reformats can be used to accurately identify an airway abnormality. However, in contrast to carcinoid tumors, mucoepidermoid carcinoma is relatively hypovascular and shows minimal enhancement. Recent studies have also suggested that FDG-PET imaging may be used to advantage in staging patients with mucoepidermoid carcinoma, with intense FDG uptake visualized ( Fig. 66-39 ).

Mucoepidermoid carcinoma.

Chest x-ray for symptoms of nonspecific upper respiratory infection showed right upper lobe mass ( A ). CT shows posterior segment upper lobe mass compressing the posterior segment bronchus with mild associated air-trapping ( D ). FDG-PET ( B and C ) showed intense uptake and was helpful for staging.

Tumors of the heart and pericardium are very rare in children. The majority of these are rhabdomyomas arising from the neoplastic transformation of cardiac myocytes. There is a well-known association between cardiac rhabdomyoma and tuberous sclerosis ( Fig. 66-40 ), with approximately half of the patients with this diagnosis having rhabdomyomas. Indeed the presence of a rhabdomyoma should prompt further evaluation, because rhabdomyomas may be the initial presenting sign of tuberous sclerosis.

Cardiac rhabdomyoma.

A 5-day-old infant with a large chest mass and tuberous sclerosis. A , Cardiac MRI showed multiple rhabdomyomas (asterisks), the largest obliterating the right ventricle. B , Multiple smaller tumors are in the left ventricle, both adherent to the septum and along the free wall. C , Brain MRI shows multiple subependymal and cortical tubers (arrows).

When symptoms are present, they are usually the result of tumor protruding into the cardiac lumen, resulting in outflow obstruction and congestive failure. Echocardiography is usually sufficient to suggest the diagnosis; however CT scanning and cardiac MRI are superior at defining the extent and attachments of these cardiac tumors and may be indicated for surgical planning. Cardiac fibromas and myxomas are much less common. Indeed it is more common to see secondary extension into the heart by other primary pediatric neoplasms such as neuroblastoma and Wilms tumor, both of which have a propensity to extend into the heart via the inferior vena cava (IVC); hepatoblastoma, which may show direct intracardiac extension ( Fig. 66-41 ); and mediastinal lymphoma, which may result in chest wall/pericardial involvement. Contrast-enhanced CT scanning usually allows an accurate delineation of sites of local extension in the thorax. Either abdominal ultrasound, multiphase contrast-enhanced CT, or MRI can be used to demonstrate intravascular/intracardiac extension from primary abdominal malignancies.

Coronal contrast-enhanced CT showing direct extension into the IVC in a 15-month-old patient with hepatoblastoma.

Posterior Mediastinal Masses

The posterior mediastinum is defined dorsally by the chest wall/vertebral column, ventrally by the pericardium and posterior wall of the great vessels, superiorly by the thoracic inlet, and inferiorly by the diaphragm. The primary components of the posterior mediastinum are the paravertebral sympathetic ganglia, azygos and hemiazygos veins, descending thoracic aorta, esophagus, and lymph nodes. The majority of the posterior mediastinal masses in children are neurogenic in origin and include ganglion-cell tumors, nerve-sheath tumors, and other nervous-tissue neoplasms such as paragangliomas. Ganglion-cell tumors arise from sympathetic chain ganglia and range from well-differentiated and benign ganglioneuromas to malignant neuroblastomas. Ganglioneuroblastomas contain elements of both benign and malignant tissue types. The distinction between these different histologic subtypes cannot be made reliably based on imaging features, although age at presentation is important in narrowing the differential diagnosis. Neuroblastoma, the most common non-CNS solid tumor in children, typically occurs in young children with a median age at presentation of younger than 2 years and more than 95% of cases occurring by the age of 10 years. In contrast, the median age at presentation of ganglioneuroblastoma is approximately 5.5 years, whereas mature ganglioneuromas occur in later childhood/early adolescence, typically after the age of 10. Whereas the majority of neuroblastomas occur in or around the adrenal gland, approximately 15% arise in the posterior mediastinum.

It is important to emphasize that these lesions may be present in a child who is otherwise asymptomatic, and a thorough inspection of the chest radiograph using multiple windows and levels is necessary to identify small foci of disease. This is of particular importance, as shown in Figure 66-42 , because a localized low-stage favorable histology neuroblastoma identified in a child under the age of 1 year may make the difference between being treated on low-risk protocols with higher likelihood of overall and disease-free survival versus more advanced stages of disease, when the patient’s risk of relapse and treatment failure increases. Primary intraabdominal neurogenic tumors may also have a significant posterior mediastinal extent and often have an associated posterior mediastinal/paravertebral component near the thoracic inlet (Virchow node). For this reason, it may be prudent to include imaging of the chest, in addition to the abdomen and pelvis, in the staging evaluation of patients with suspected neuroblastoma.

Posterior mediastinal neuroblastoma in a 5-month-old infant who showed symptoms of an upper respiratory infection.

A posterior mediastinal mass was identified by chest x-ray ( A ) and MRI ( B ). Surgery revealed neuroblastoma. Subsequent staging confirmed low-stage disease.

Radiographically, ganglion-cell tumors appear as paraspinal soft-tissue masses. Calcifications may be present. The presence of chest-wall invasion or destructive bony changes are more commonly seen with neuroblastoma, although benign ganglioneuromas can produce a considerable neural foraminal widening. As with primary adrenal neuroblastomas, staging should include ¹²³ I-MIBG scintigraphy to assess for other sites of disease, including marrow involvement. ^99m Tc-MDP bone scintigraphy is also commonly performed to identify sites of cortical bone involvement, although recent data suggests that bone scintigraphy rarely affects that clinical stage of the patient. The presence of a paraspinal mass can usually be adequately assessed by CT scanning, particularly with sagittal and coronal reconstructions. However, the extent of intraspinal extension is better depicted by MRI ( Fig. 66-43 ). MRI also provides a more accurate assessment of chest-wall involvement, nerve-root compression, and impingement upon the spinal cord. MRI of ganglion-cell tumors typically reveals a low signal-intensity lesion on T1-weighted images and intermediate to high signal intensity on T2-weighted and fast spin echo inversion recovery (FSEIR) images, with variable patterns of enhancement. Bone-marrow metastases are well demonstrated by MRI, with replacement of normal bone-marrow signal by areas of low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. Indeed, several studies have suggested that MRI may be sufficient to identify sites of bone-marrow involvement in neuroblastoma, although is has not yet become common in clinical practice to rely on the MRI findings of bone-marrow involvement for staging of the patient. The use of FDG-PET in neuroblastoma has received attention as an alternative to conventional imaging techniques. However, a number of studies have shown variable patterns of FDG avidity in neuroblastoma, and FDG-PET has not yet been shown to offer a clear advantage over MIBG scintigraphy in either the staging or response assessment of patients with thoracic neuroblastoma.

A 7-year-old child with a cough.

Chest x-ray showed left upper lobe opacity. A , CT shows paraspinal mass. B – D , MRI shows the mass to much better advantage. Cor FSEIR and post–Gd-enhanced images ( C and D ) show the mass abutting the neural foramina and left subclavian vessels. MIBG scanning showed barely detectable uptake in the mature ganglioneuroma (not shown) .

Nerve-sheath tumors, including schwannomas and neurofibromas may be similar in appearance to mature ganglion cell tumors, presenting as relatively sharply marginated, lobulated paraspinal masses. The presence of rib erosions and neural foraminal widening is commonly seen with nerve-sheath tumors, although intraspinal extension is less common. Paraspinal paragangliomas are catecholamine-secreting tumors arising from extra-adrenal chromaffin cells present in sympathetic chain ganglia and occur much less often than either primary ganglion-cell or nerve-sheath neoplasms. Distinguishing between these tissue types is difficult on the basis of imaging, and surgical resection is the mainstay of therapy. Local recurrence can occur, however, and close imaging follow-up is necessary in these patients.

Mesenchymal Hamartomas

Mesenchymal hamartomas of the chest wall are rare, benign neoplasms occurring during infancy. The presence of a large chest-wall mass, often containing calcifications with associated rib destruction and chest-wall deformity should prompt this diagnosis. Because these lesions may contain a significant necrotic/cystic component, the distinction between Langerhans cell histiocytosis, lymphoma, and metastatic neuroblastoma may be difficult to make. These masses often become quite large, and the radiographic finding may be sufficient to suggest the diagnosis, showing a large calcified extrapleural mass with expansion/destruction of multiple ribs. CT or MRI allows more accurate measure of size, characterization of the soft-tissue component of the lesion, and mass effect on the lung and adjacent structures ( Fig. 66-45 ). In particular, the presence of hemorrhagic cystic components has been described as characteristic of these benign neoplasms. There is some data to suggest that spontaneous resolution of these masses may also occur, although the extent of local invasion and tissue destruction may prompt surgical resection, which if complete, is usually curative. The importance of the imaging evaluation is to suggest this diagnosis in order to guide appropriate management of these benign lesions.

A 20-day-old infant with a prenatally diagnosed, calcified, left lower lung mass.

Chest x-ray shows calcified mass ( A, arrow ) which on CT is seen to arise from posterolateral left ribs ( B ). T1-weighted MRI ( C ) shows presence of high signal intensity fat (arrow) , all consistent with the diagnosis of mesenchymal hamartoma.

Pulmonary Metastases

Metastatic disease to the lung represents the most common pulmonary neoplasm encountered in childhood. The lung is a common site for metastatic disease spread but is an uncommon site in childhood for primary malignancy.

Extrathoracic pediatric tumors that are commonly associated with pulmonary metastases include Wilms tumor, rhabdomyosarcoma, hepatoblastoma, Ewing sarcoma, and osteosarcoma. Neuroblastoma does not typically result in hematogenously spread pulmonary metastases, although in its disseminated form, pulmonary metastases may be seen.

The imaging evaluation in a patient with a known primary malignancy with a propensity to metastasize to the lung should include chest CT. CT is more sensitive than conventional radiographs in detecting pulmonary metastatic disease, although the increased sensitivity of nodule detection has led to difficulty in distinguishing benign lesions from malignant pulmonary nodules ( Fig. 66-46 ). Once the staging evaluation has been completed and the absence of pulmonary metastatic disease and other sites of metastatic disease have been established, it may be reasonable to proceed with routine chest radiographs for subsequent follow-up care, reserving CT for patients with suspected relapse or at high risk for relapse. In particular, in osteosarcoma, which has a high frequency of developing pulmonary metastases and for whom resection of these metastases may be curative, routine follow up chest CT scanning is justifiable.

A -year-old child with metastatic Wilms tumor.

A , The nodule at the left lung base (arrow) was resected at diagnosis and found to be metastatic disease. B , The nodule at the right lung base discovered 1 year later was resected, and found to be a pulmonary hamartoma.

It is often difficult, particularly with small pulmonary nodules, to distinguish benign causes from malignancy. Calcification within a pulmonary nodule is usually associated with benign causes (except in the case of osteosarcoma); however, there are no specific imaging findings that can be used to reliably make this distinction. Benign pulmonary nodules may result from postinflammatory change or represent small benign intrapulmonary lymph nodes. Short interval follow-up care and documentation of stability in areas of low suspicion is usually adequate to confirm the benign nature of these findings. For more suspicious or enlarging lesions, biopsy or surgical resection is necessary to arrive at a definitive diagnosis. The use of FDG-PET scanning has been advocated in an attempt to distinguish benign from malignant causes of pulmonary metastatic disease. However, while CT can accurately identify lesions of only 1 to 2 mm in size, even the newest generation PET scanners have inherently lower limits of resolution, on the order of 5 mm, because of both the intrinsic spatial resolution of PET detectors and the distance traveled by positrons in the tissue before undergoing annihilation and emission of 511 keV coincident photons.

Controversy still exists as to the significance of pulmonary nodules detected by chest CT in patients with Wilms tumor. There are conflicting data as to the importance of pulmonary nodules not visualized by chest radiography but detectable by chest CT in predicting relapse-free and overall survival in patients with Wilms tumor, with the most recent data from the National Wilms Tumor Study Group (NWTSG) 4 and 5 showing that patients with lung lesions visible only on CT have improved event-free, but not overall, survival and do not appear to benefit from pulmonary radiation. Nonetheless it has been shown that the sensitivity of malignant nodule detection is enhanced by CT, and current treatment protocols routinely advocate the use of chest CT in staging these patients. In an effort to develop a better understanding of CT features that might distinguish benign from malignant pulmonary nodules, current Children’s Oncology Group (COG) Wilms tumor protocols call for surgical resection or biopsy of persistent, suspicious lung nodules detected by CT in patients with intermediate and high-risk disease.

CT scanning of the chest for the purposes of identifying pulmonary metastatic disease need not include intravenous contrast, and the presence of hyperdense contrast material in small pulmonary vessels may confound the interpretation of a subtle parenchymal nodules. In addition, multidetector-row CT scanners allow the acquisition imaging data to be reviewed in 1-mm thick increments or less, as well as in sagittal and coronal planes. This allows the radiologist to make an accurate judgment as to the presence or absence of a suspicious pulmonary nodule and distinguish it from branching vessels. There is no data to suggest that additional high-resolution CT (HRCT) imaging, which is performed using a different image acquisition technique, provides any additional benefit or higher resolution over the 1-mm thick slices obtained during the multidetector helical acquisition. MRI for the detection of pulmonary nodules has been shown to be feasible, but there are no studies showing that MRI can replace CT for the routine identification of pulmonary metastatic disease. Similarly low-dose attenuation-correction CT images of the lungs obtained as part of combined PET/CT examinations are not sufficient to exclude small pulmonary nodules, and a diagnostic chest CT obtained during suspended inspiration is still recommended for evaluation of patients at risk and in whom pulmonary metastases will impact clinical management.

Although the imaging features of small pulmonary nodules may be nonspecific and the difficulty in distinguishing benign from nonneoplastic etiologies is challenging, the presence and persistence of lung nodules in a child with a solid tumor that has a propensity to metastasize to the lung is usually indicative of metastatic disease. The use of CT in detecting these sites of disease spread remains an essential component of the staging of most pediatric solid tumors.

Hepatocellular Carcinomas

Hepatocellular carcinoma, although rare in childhood, is the second most common primary hepatic malignancy. It typically occurs in older children, with a mean age at diagnosis of approximately 12 years. There is a slight male predominance and an association between development of hepatocellular carcinoma and preexisting liver disease, such as hepatitis B, biliary atresia, and Fanconi syndrome, as well as certain metabolic diseases such as hereditary tyrosinemia and glycogen storage disease. The imaging features of hepatocellular carcinoma are not unique, and as for hepatoblastoma, ultrasound examination is often the first imaging study performed. Multiphase CT scanning with multiplanar reconstruction affords the greatest likelihood of detecting subtle lesions and in assessing relationships to adjacent structures and the vasculature, although the use of diffusion-weighted MR imaging and hepatobiliary MR contrast agents has added to the appeal of MRI for evaluating these patients. The CT appearance of hepatocellular carcinoma depends on tumor size and phase of contrast administration. Lesions are typically isointense or mildly hypointense relative to liver and show hyperattenuation on early arterial phase scanning, becoming isointense during later venous phases of imaging ( Fig. 66-51 ). Early arterial phase, portal venous phase, and delayed equilibration venous phase images have all been shown to contribute to enhancing lesion conspicuity, providing the greatest sensitivity and specificity in lesion detection. MR imaging, with opposed-phase imaging to detect microscopic fat and hepatobiliary agents to identify lesions with hepatocyte function can help in distinguishing hepatocellular carcinoma from benign liver lesions such as hepatic adenoma and focal nodular hyperplasia (FNH) ; however, biopsy may still be necessary to confirm the diagnosis ( Fig. 66-52 ). Gadolinium-enhanced MRI, like CT, shows hyperenhancing lesions during early arterial phase imaging, with larger tumors showing variable signal intensity and patterns of enhancement and absent enhancement on delayed hepatocyte phase imaging when hepatobiliary agents are used. Superparamagnetic iron-oxide particles, which are taken up by Kupffer cells present in normal liver and in FNH, have been used to help differentiate hepatocellular carcinoma, which contains few if any Kupffer cells, from benign liver masses and to enhance lesion conspicuity relative to the surrounding liver tissue.

An 18-year-old female with fibrolamellar hepatocellular carcinoma.

Biphasic contrast-enhanced CT demonstrates a large hyperenhancing lesion in the right lobe, which on delayed images becomes hypodense and less distinct from the background liver tissue.

A 19-month-old child with hepatocellular carcinoma.

T2-weighted ( A ) and DWI ( B ) images show a large heterogeneous mass in the left liver lobe with restricted diffusion. C , After Gd-contrast administration with gadoxetate (Eovist), early dynamic images shows heterogeneous enhancement. D , Delayed imaging shows absence of gadoxetate (Eovist) enhancement on the hepatocyte phase, in contrast to the enhancement of the adjacent normal liver.

The fibrolamellar form of hepatocellular carcinoma occurs in adolescents and has imaging features that are distinctive, showing a central scar and often associated with calcification ( Fig. 66-53 ). Fibrolamellar hepatocellular carcinoma is not associated with elevated AFP, and overall survival is higher than in other forms of hepatocellular carcinoma, in part because of its presentation as a solitary localized mass amenable to complete surgical resection. Ultrasound reveals a hypoechoic mass, often with a focal central hyperechoic scar. After the initial characterization by ultrasound, CT or MRI is indicated for further staging. On CT, fibrolamellar hepatocellular carcinoma is usually relatively low attenuation, well-circumscribed mass with heterogeneous enhancement, initially becoming hyperintense relative to the surrounding liver parenchyma and ultimately becoming isointense to the liver during the equilibration phase of scanning. A nonenhancing central scar is seen in the majority of the cases (see Fig. 66-53 ). On MRI, lesions are isointense to hypointense relative to the liver on T1-weighted images and isointense to hyperintense on T2-weighted sequences. The pattern on gadolinium enhancement is similar to that observed on CT, with lesions becoming hyperintense early after contrast infusion and ultimately isointense to liver on delayed scanning. The distinction between fibrolamellar hepatocellular carcinoma and FNH, both of which have similar imaging characteristics and patterns of early contrast enhancement, may be difficult to make (compare Figs. 66-53 and 66-54 ). However, as shown in Figure 66-54 , the uptake and retention of a hepatobiliary contrast agent by the lesion on delayed hepatocyte phase imaging is nearly diagnostic of FNH.

A 16-year-old patient with fibrolamellar hepatocellular carcinoma.

Unenhanced ( A ) and contrast enhanced ( B ) CT show the calcifications and poorly enhancing central scar (arrows) typical of this form of hepatocellular carcinoma.

A 15-year-old patient with abdominal pain and a liver mass noted on outside CT ( A ).

B , MRI performed with gadoxetate (Eovist) shows a mildly T2-hyperintense lesion that becomes isointense to liver shortly after contrast infusion ( C ), but that retains the contrast on delayed hepatocyte phase imaging ( D ). The features, together with the central nonenhancing scar, are characteristic of FNH.

Embryonal Sarcomas (Hepatic Mesenchymomas)

Embryonal sarcomas of the liver are the fourth most common hepatic neoplasm in children after hepatoblastoma, hepatocellular carcinoma, and infantile hepatic hemangioma. These tumors are highly malignant, usually present in later childhood (between the age of 6 and 10), and are typically accompanied by symptoms of abdominal pain and an abdominal mass. Embryonal sarcomas may be difficult to distinguish from mesenchymal hamartomas, although the latter tumors are more commonly diagnosed before the age of 2 years. At presentation embryonal sarcomas are often large and contain both cystic and solid areas with areas of necrosis. Ultrasound reveals a heterogeneous echogenic mass with many features overlapping with mesenchymal hamartoma and infantile hemangioma. Calcifications are rare. CT reveals a large hypodense mass, often with septations and areas of internal high attenuation; MRI shows a high signal-intensity lesion on T2-weighted images ( Fig. 66-55 ), with predominantly low signal on T1-weighted images. After contrast enhancement, imaging by either CT or MRI demonstrates peripheral enhancement and central areas of heterogeneous signal intensity that likely reflect solid tumor elements, cystic spaces, necrosis, and foci of hemorrhage. Despite being highly malignant, complete surgical resection and adjuvant chemotherapy can be curative in the majority of these patients.

Mesenchymal hamartoma.

MRI showing the complex nature of the multiple cysts in a 13-month-old infant with mesenchymal hamartoma. Note the importance of obtaining T1-weighted images before contrast administration ( A and B ), in addition to fluid-sensitive T2-weighted sequences ( C ), because areas of complex fluid may be T1 bright and should not be misinterpreted as enhancing solid components of the mass. Note the similarity to the large embryonal sarcoma shown in ( D ).

Mesenchymal Hamartomas

Mesenchymal hamartoma, in contrast to hepatoblastoma, is a predominantly cystic mass most often occurring in infants under the age of 2 years. It commonly presents as a painless, palpable abdominal mass comprised of mesenchymal tissue, bile ducts, hepatocytes, and hematopoietic cells. In contrast to hepatoblastoma, which is the primary differential consideration in an infant, the AFP levels are normal. On imaging, radiographs reveal a large right upper quadrant mass. Calcifications, which are seen in hepatoblastoma, are unusual in mesenchymal hamartoma. Ultrasound examination typically shows a complex, predominantly cystic mass. MRI and CT may be of value in further characterizing the lesion and typically show large multilocular cystic masses with septations. Only minimal enhancement is seen. Some of the masses may have a predominantly solid component yielding a so-called Swiss cheese appearance to the mass, and contributing to some difficulty in the differential diagnosis. MRI may also demonstrate the complex nature of the cystic fluid, with gelatinous, more complex fluids showing high signal on T1-weighted images instead of the more common T2 bright/ T1 dark serous fluid contained in the majority of the cysts (see Fig. 66-55 ). Treatment is surgical resection, and recurrence is uncommon.

Infantile Hepatic Hemangiomas

Infantile hepatic hemangioma are proliferative endothelial cell neoplasms that may be solitary or may diffusely involve the liver. Infantile hepatic hemangiomas have characteristic phases of cellular proliferation followed by spontaneous involution. They are often referred to as hepatic hemangioendothelioma, creating some confusion in nomenclature and must be distinguished from epithelioid hemangioendothelioma. The latter is a proliferative tumor with malignant potential that does not involute in contrast to the benign hepatic hemangioma. Infantile hepatic hemangiomas share the same growth characteristics as the more common cutaneous infantile hemangiomas. The majority present by 6 months of age, and about half are associated with cutaneous lesions. The majority of the infantile hepatic hemangiomas involute spontaneously, and adverse outcomes are only observed when there is massive hepatic involvement and arteriovenous (AV) shunting with accompanying high-output cardiac failure. The presence of extensive hepatic involvement and near complete replacement of the liver parenchyma occurs in the diffuse infantile hemangioma variant and is associated with profound hypothyroidism secondary to overproduction of type III iodothyronine deiodinase.

Ultrasound is usually the first step in the evaluation and typically shows either focal or multiple hepatic masses with heterogeneous echotexture. It may be possible to identify a large draining varix or varices. Both arterial and venous waveforms are often detected, and calcifications may be present. More characteristic findings are seen on CT and MRI. MRI typically shows homogeneously hypointense lesions on T1-weighted imaging with heterogeneous hyperintense masses on T2-weighted images. After contrast enhancement, imaging by either CT or MRI reveals a typical pattern of peripheral contrast enhancement with gradual filling in centrally (centripetal enhancement) ( Fig. 66-56, A and B ). Diffuse infantile hepatic hemangiomas may nearly completely occupy the entire liver parenchyma (see Fig. 66-56, C and D ). The associated hypothyroidism is the result of the elaboration of deiodinase enzymes affecting thyroid hormone synthesis, which can in turn result in cardiac failure and neurologic impairment. These endocrinologic findings are important in securing the diagnosis, because the multiple hepatic masses may share overlapping imaging features with malignant hepatic lesions such as hepatoblastoma or metastatic neuroblastoma.

Involuting hepatic hemangioma and diffuse infantile hemangioma.

Fat-suppressed T2-weighted ( A and C ) and post–gadolinium-enhanced ( B and D ) MRI showing large solitary peripherally enhancing lesion typical of involuting hepatic hemangioma ( A and B ) and the multifocal hepatic involvement in a patient with hypothyroidism and diffuse infantile hemangioma ( C and D ).

High-output cardiac failure associated with large varices and significant high-volume AV shunting can be effectively treated using interventional radiologic techniques, including coil and particle embolization. This may be essential in stabilizing the patient in order to initiate treatment to accelerate the involution of these benign proliferative lesions.

Hepatic Adenoma

Other benign hepatic lesions include solitary hepatic adenomas. Hepatic adenomas are unusual in children in the absence of a predisposing condition, such as glycogen storage disease, galactosemia, or tyrosinemia. The imaging features of hepatic adenomas are not specific, and imaging is usually directed at monitoring these lesions for increases in size and number. Rapid increase in size is generally considered to be an indication of degeneration of benign adenoma to hepatocellular carcinoma, particularly in patients with glycogen storage disease. Regular surveillance, typically by ultrasound, is indicated in these patients. MRI should be considered to further aid in characterizing new or multiple lesions.

Focal Nodular Hyperplasia

FNH is rare in children, most commonly occurring in teenage and adolescent years. It is usually found incidentally during imaging evaluation for other reasons. This benign lesion typically shows a central fibrous scar with surrounding hyperplastic hepatocytes and small bile ducts. As discussed, these imaging features are also seen in the fibrolamellar variant of hepatocellular carcinoma, and the distinction between FNH and fibrolamellar hepatocellular carcinoma may be difficult. CT and MRI scans usually show rapid homogeneous contrast enhancement during early arterial-phase imaging (see Figs. 66-53 and 66-54 ). Lesions then characteristically become isodense to the liver during later phases of enhancement, with the central scar remaining hypodense even during the delayed imaging period. With newer MRI imaging techniques performed either with iron-oxide particles that accumulate in reticuloendothelial cells (Kupffer cells) present in the hyperplastic FNH lesions or with hepatocyte selective gadolinium based agents such as gadoxetate (Eovist), the diagnosis of FNH can generally be made with confidence. For equivocal cases, scintigraphy with ^99m Tc-sulfur colloid can also be performed, although in practice this is rarely necessary. These masses may be monitored by ultrasound to ensure stability and do not typically require surgical resection unless they become symptomatic or undergo progressive enlargement. FNH has also been observed in patients who have completed therapy for nonhepatic primary tumors. Whereas larger masses may be by detected by ultrasound or CT, it has been found that dynamic contrast-enhanced MRI is a highly sensitive technique for identifying these lesions. Occasionally these lesions can be characterized by ultrasound using high-resolution, high-frequency transducers; however, they are unequivocally shown by contrast-enhanced MRI ( Figs. 66-57 and 66-58 ). These FNH lesions are also isointense to the liver on conventional imaging sequences and display rapid early arterial enhancement, after which they become nearly isointense to the liver and indistinguishable from the surrounding parenchyma. Hepatobiliary contrast agents are now used routinely to further characterize these lesions as benign FNH (see Fig. 66-53 ) and distinguish them from hepatic metastatic deposits. It is important recognize these characteristic patterns of enhancement so as to not mistake this benign entity for recurrent disease or a new site of disease relapse.

A 19-month-old boy with a history of treatment for stage 4 neuroblastoma.

Follow-up MRI imaging using dynamic contrast enhancement shows multiple brightly enhancing lesions throughout the liver ( A and B ). These rapidly become isointense to liver during delayed scanning. They are not seen on any other sequences. The enhancement characteristics are typical of FNH. Intraoperative ultrasound localized lesions for biopsy ( C ) and confirmed them as FNH. Similar lesions in a 2-year-old patient treated in infancy for stage 4 neuroblastoma are shown in D .

A 6-year-old patient treated for stage 4 neuroblastoma as an infant who developed multiple enhancing liver lesions proven to be FNH on biopsy.

Subsequent MRI with gadoxetate (Eovist) shows representative lesions (arrows) to be isointense to liver on T2-weighted imaging (A), hyperenhancing on an early–arterial-phase image ( B ), and with persistent enhancement on the 20-minute delayed hepatocyte phase sequence ( C ). These features are characteristic of FNH.

Other lesions occurring in the liver include sarcomatous tumors such as malignant angiosarcomas and rhabdomyosarcomas of the biliary tree, malignant vascular tumors, and metastatic disease. The most common metastatic lesions occurring in the liver include neuroblastomas, locally invasive and hematogenously-disseminated Wilms tumors, rhabdomyosarcomas and Ewing sarcomas. The imaging characteristics of these metastatic hepatic lesions are nonspecific with lesions characteristically hypoechoic on ultrasound relative to normal liver parenchyma, decreased attenuation on CT scan, low T1 and increased T2 signal intensity on T1- and T2-weighted MR images, respectively, and variable patterns of contrast enhancement with both CT and MRI.

Pancreatoblastomas

Pancreatoblastomas can occur at any age but are most common in children between the ages of 1 and 8 years. Pancreatoblastomas are reported as arising in or around the head of the pancreas in roughly half of the cases. Because they are nonfunctional, they are typically quite large at the time of diagnosis. Presenting symptoms include abdominal pain, nausea, and weight loss. Obstructive symptoms such as jaundice or pancreatitis are less common despite the large size of these masses, which has been attributed to the soft, gelatinous, and compliant consistency of these tumors. These tumors are typically well encapsulated, have heterogeneous patterns of echogenicity on ultrasound, and show variable patterns of contrast enhancement on CT ( Fig. 66-61 ). Calcifications are commonly seen and are probably the sequelae of hemorrhage and necrosis known to occur within these tumors. MRI shows a similar pattern of heterogeneous signal intensity and enhancement, with masses usually hypointense on T1-weighted images and variably hyperintense on T2-weighted and post–gadolinium-enhanced images. As with hepatoblastoma, pulmonary metastatic disease is often present, and the staging evaluation must include a CT of the chest. Because of their large size, the tissue of origin may be difficult to clearly delineate, and differential considerations includes exophytic hepatoblastoma, renal and suprarenal masses, and retroperitoneal neoplasms.

Pancreatoblastoma in a 4-year-old boy with a palpable abdominal mass.

Contrast-enhanced CT shows a large, calcified retroperitoneal mass centered in the pancreas. Both adrenal glands were normal. Lung metastases were also present (not shown). The biopsy showed pancreatoblastoma.

Solid Pseudopapillary Neoplasms

Solid pseudopapillary neoplasms of the pancreas (also known as solid and papillary epithelial neoplasms ) are a rare primary pediatric pancreatic tumor. In contrast to other pancreatic tumors occurring in children, however, the prognosis after complete surgical resection is favorable. Studies have reported up to a 90% survival rate after complete resection. In one small study, median age at presentation was 13 years, with abdominal pain, a palpable mass, dyspepsia, and pancreatitis the most common presenting symptoms. On imaging, usually by ultrasound and/or CT, solid pseudopapillary tumors appear as solid, well-demarcated masses. On ultrasound there is heterogeneous echotexture, occasionally with fluid-filled cystic spaces ( Fig. 66-62 ). CT shows a heterogeneous mass with peripheral contrast enhancement. Although these tumors are slow-growing tumors with low-grade malignant behavior, FDG uptake on PET imaging has been shown in solid papillary tumors of the pancreas. FDG-PET scanning may therefore be of value in follow-up imaging to assess for recurrence. Despite high rates of overall 5-year survival, if metastases are present and not resectable, survival is poor with no clear, beneficial role reported for adjuvant chemotherapy or radiation therapy.

Pseudopapillary pancreatic tumor.

This 9-year-old patient had hypertension and increased abdominal girth. A , CT shows a low attenuation, heterogeneously enhancing lobulated mass arising from the pancreatic body and tail (arrow) , which also abutted the greater curvature of the stomach (not shown). B , Ultrasound shows a lobulated solid mass that could not be distinguished from the pancreas.

Pancreatic Sarcomas

Mesenchymal tumors of the pancreas include rare synovial-cell sarcomas, myofibroblastic tumors, and metastatic rhabdomyosarcomas. Extremity rhabdomyosarcomas seem to have an unusual predilection for metastases to unusual cites such as the breast, ovary, testicle, and pancreas. Two patients shown in Figure 66-63 had metastatic rhabdomyosarcoma to the pancreas. Local presentation may include pancreatitis and abdominal pain or may be incidental (see Fig. 66-48, B ). A recent report from three major pediatric centers also noted an association of alveolar rhabdomyosarcoma with pancreatic metastasis, emphasizing the importance of follow-up imaging in these patients and including an increasing role for routine FDG-PET/CT imaging to identify unusual sites of metastatic spread in patients with rhabdomyosarcoma.

CT shows metastatic rhabdomyosarcoma to the pancreas in two patients with extremity rhabdomyosarcoma.

A , The patient had symptoms of pain. B , The patient had no abdominal symptoms, and the lesion was detected by FDG-PET during routine surveillance.

Pancreatic Lymphomas

Burkitt lymphoma is well known to involve organs of the gastrointestinal tract, although isolated pancreatic involvement is unusual. Typically pancreatic involvement is accompanied by other sites of visceral and mesenteric disease. Diffuse pancreatic infiltration in the setting of Burkitt lymphoma is shown in Figure 66-64 in a child who had symptoms of pancreatitis. On ultrasound the pancreas was diffusely enlarged and heterogeneous in echogenicity. CT scan showed diffuse pancreatic enlargement, poor enhancement, and a homogeneous low attenuation. As is common for the rapidly dividing Burkitt lymphoma, a prompt response to chemotherapy was observed, with near complete resolution of pancreatic abnormalities 10 days after onset of chemotherapy.

Burkitt lymphoma of the pancreas.

An 8-year-old patient presented with pancreatitis. A , Ultrasound showed diffuse pancreatic infiltration, confirmed by CT ( B ), with disease limited to the pancreas. Ten days after initiating chemotherapy the pancreas appeared normal ( C ), typical of the rapid responses observed with Burkitt lymphoma.

Pancreatic Islet-Cell Tumors

Functioning islet-cell tumors of the pancreas are rare but important clinically because of symptomatic endocrine abnormalities that result from constitutive secretion of hormones normally tightly regulated by the endocrine pancreas. As shown in Figure 66-65 , in which the patient had symptomatic hyperinsulinemic hypoglycemia, these tumors are often quite small and may be difficult to diagnose and characterize. As with other neuroendocrine neoplasms, optimal scanning techniques for detection of pancreatic islet-cell tumors include early arterial-phase scanning as part of a multiphase imaging technique. Thin sections (≈1 mm) are now routinely acquired by most multidetector-row CT scanners and the evaluation should include a careful review of sagittal and coronal reconstructed images. Although there are no controlled studies comparing CT and MRI in evaluating these tumors, a well-administered MRI examination with respiratory triggering/breath-holding, fast gradient-echo imaging, diffusion imaging, and dynamic-contrast administration is likely to yield comparable, if not superior results to CT. These small lesions are often difficult to see by conventional ultrasound; however, intraoperative ultrasound is invaluable for localizing small, nonpalpable lesions (see Fig. 66-65, B ).

Pancreatic islet-cell tumor.

This patient had profound hypoglycemia. A , CT shows a small enhancing lesion in the pancreatic head, adjacent to the superior mesenteric vein, detected only during the early arterial phase of scanning and consistent with insulinoma. B , Intraoperative ultrasound was essential to localize this small lesion before resection.

Wilms Tumors

Wilms tumors are the most common renal malignancy in children, representing 6% to 10% of all pediatric malignancies and making up greater than 80% of pediatric renal masses. The peak age of incidence is between 3 and 4 years, and bilateral disease is seen in between 5% and 10% of patients. Wilms tumor is often associated with syndromes and nephroblastomatosis and typically presents as a painless, palpable abdominal mass. Occasionally, Wilms tumor is an incidental finding during a radiologic assessment for other indications such as trauma. Hypertension and hematuria only occur in about 25% of the cases.

Tumor staging based on the NWTSG classification depends on surgical, pathological, and radiologic findings, and the imaging evaluation should be directed at determining spread to adjacent organs such as the pancreas, spleen, and liver and for intravascular extension into the renal vein, IVC, and right atrium ( Table 66-2 ). In addition, the imaging examination should assess for the presence of distant metastatic disease (lung and liver) and for the presence of contralateral kidney involvement. Wilms tumor is a rapidly growing malignancy, and intraabdominal rupture at diagnosis, during chemotherapy, or at the time of surgical resection is a well-appreciated complication and a major risk factor for abdominal recurrence, increasing the recurrence rate to as much as 20%. Accurately identifying tumor rupture can be a challenge. In a recent review of 70 patients treated through the COG, preoperative CT imaging was found to have moderate specificity (88%) but relatively low sensitivity (62%) in the detection of Wilms tumor rupture, although ascites seen outside of the cul-de-sac, irrespective of fluid density, was most predictive of rupture. When tumor rupture is identified, these patients are upstaged and treated as having stage 3 disease.

The initial examination is often a conventional abdominal x-ray obtained for symptoms of abdominal pain or a palpable mass. The typical findings include displacement of bowel and a masslike opacity in the abdomen. An ultrasound should serve as the initial evaluation and usually confirms the renal origin of the suspected mass. Sonographically Wilms tumors are often quite large with relatively homogeneous echogenicity. There may be focal areas of hypoechogenicity or cystic change. Calcifications are uncommon. Ultrasound is also accurate and sensitive at detecting intravascular invasion, and both color and Doppler imaging are effective at establishing vascular patency ( Fig. 66-68 ). Intravascular extension occurs in 4% to 10% of patients with Wilms tumor but is rare in neuroblastoma, helping to differentiate between these two tumors. In a recent study evaluating the diagnostic performance of CT versus Doppler ultrasound in identifying intravascular extension of Wilms tumor, investigators from the COG found CT to be superior to ultrasound for detection of intravascular and cavoatrial thrombus. Nonetheless because ultrasound is often the initial examination performed in these patients, early identification of intravascular extension into the right atrium is essential and may increase the sedation/anesthesia risk for a child for whom subsequent CT or MRI is planned.

Wilms tumor.

A 6-year-old child with a left renal Wilms tumor and left renal vein/IVC extension shown by ultrasound ( A and B ) and CT ( C and D , arrows ). Ultrasound confirms the complete absence of flow in the left renal vein and documents the level of intravascular invasion into the IVC (asterisks). s, Spine .

Once the diagnosis has been suggested by ultrasound, further imaging of suspected Wilms tumor requires either CT or MRI to accurately stage the disease. Multidetector-row CT scanning allows for rapid scanning and in some instances may allow sedation to be avoided. High-resolution imaging techniques with multiplanar reconstructions are essential in determining the extent of tumor spillage, local invasion, invasion into vascular structures, and presence of pulmonary metastatic disease. Early studies suggested that pulmonary metastases detected only by CT should not result in upstaging of the patient, relying on the chest x-ray to identify pulmonary metastases. Subsequent work by European investigators in the International Society of Paediatric Oncology (SIOP) and in North America with the NWTSG identified a small cohort of patients with pulmonary lesions visualized only by CT scanning in whom disease stage would be advanced based on the CT results. One study showed an increased risk of relapse in patients with positive CT findings. Others, however, reported either no difference in prognosis between patients with positive or negative chest CT scans or improvement in event-free survival rates, but with no impact on the overall survival rate and with no benefit from whole-lung radiation for patients with CT-only pulmonary nodules. These studies are limited by their retrospective nature, and it remains controversial whether patients would have benefited from increased therapy. Furthermore other investigators have argued that CXR and CT data are concordant in the majority of cases, and it as been shown that not all pulmonary lesions detected by CT represent metastases, even in the hands of experienced radiologists, suggesting that surgical confirmation may be necessary to establish the malignant nature of suspicious lesions detected by CT in patients with Wilms tumor. These considerations aside, it is current practice to obtain a chest CT for staging of all patients with Wilms tumor, with ongoing studies investigating the positive predictive value of this practice and its impact on overall clinical outcome.

Because of the need for chest CT in the staging evaluation, MRI is usually obtained for follow-up evaluation, either after resection or chemotherapy. In addition, MRI is the imaging modality of choice in monitoring nephrogenic rests that are seen in approximately 40% of patients with unilateral Wilms tumor and nearly all patients with bilateral Wilms tumors ( Fig. 66-69 ). Nephrogenic rests are remnants of primitive blastemal tissue that persist after birth and have the potential to transform into Wilms tumor. Nephroblastomatosis is the presence of multifocal or diffuse nephrogenic rests and is prevalent in patients with hemihypertrophy. The distinction between nephrogenic rests and Wilms tumor may be difficult; however, there are certain distinguishing MRI features that may help discriminate between the two. On T1-weighted MRI images, Wilms tumors appear as well-defined heterogeneous masses with slightly lower signal intensity as compared with the adjacent renal cortex. On T2-weighted images Wilms tumors appear isointense to slightly hyperintense relative to the normal renal cortex. After intravenous gadolinium-contrast administration, Wilms tumors typically enhance heterogeneously but less than the surrounding renal parenchyma. Nephrogenic rests also exhibit decreased signal intensity on T1-weighted images, but in contrast to Wilms tumors maintain low signal on T2-weighted images. Furthermore, nephrogenic rests show minimal enhancement after contrast infusion relative to the normal renal parenchyma, whereas foci of Wilms tumors show heterogeneous enhancement. A role for FDG-PET imaging in the routine management of patients with Wilms tumor has not been established. However, studies documenting increased FDG accumulation in metabolically active Wilms tumors suggest there may be role for FDG-PET imaging in Wilms tumor staging, posttherapy response evaluation, and possibly in helping discriminate benign nephrogenic rests from foci of transformed malignant tumor.

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