Adipocytic Tumors: Liposarcomas

The major changes in the recent 2013 World Health Organization (WHO) classification has been the removal of the terms ‘round cell LPS’ and ‘mixed-type LPS’ and introduction of new subtype “LPS, not otherwise specified.” Round cell LPS are currently thought to represent high-grade myxoid LPS, and mixed-type LPS are considered variant of dedifferentiated LPS based on molecular testing.

Atypical lipomatous tumor/well-differentiated liposarcoma

ALT and well-differentiated LPS are intermediate (locally aggressive) neoplasms that do not metastasize. ALT and well-differentiated LPS have a predilection for the extremities, retroperitoneum, and paratesticular and the inguinal regions ( Fig. 1 ). Although imaging findings of lipoma and ALT/well-differentiated LPS overlap, owing to the predominant fat component, features that favor ALT/well-differentiated LPS over lipoma include age greater than 60 years, lesion size greater than 10 cm, lower extremity location, presence of thickened septa (>2 mm), globular/nodular enhancing foci, and greater portion of nonfatty (solid or amorphous) areas ( Fig. 2 ). The sclerosing variant of well-differentiated LPS seen most commonly in the retroperitoneum, has a predominant nonfatty component, simulating dedifferentiated LPS or other aggressive neoplasms.

A 76-year-old man with retroperitoneal well-differentiated liposarcoma. Coronal contrast enhanced computed tomography of the abdomen reveals a large multicompartmental intraabdominal mass ( arrows ) with predominantly fat attenuation compressing and displacing the ureter medially ( arrowhead ). The thick internal septations and mass effect differentiate the mass form the normal intraabdominal fat. Surgical resection included right nephrectomy.

A 57-year-old male with atypical lipomatous tumor. Axial T1 turbo spin echo ( A ) and fat-suppressed postgadolinium ( B ) T1-weighted MRI demonstrates a heterogeneous mass ( arrows in A and B) in the medial thigh. The mass has predominantly T1 hyperintense signal areas, which are suppressed on fat-suppressed sequence consistent with macroscopic fat. There are, however, linear T1 hypointense enhancing areas in the mass, which is atypical for simple lipoma and raising the possibility of liposarcoma. Histopathology confirmed atypical lipomatous tumor with spindle cell histology.

Dedifferentiated liposarcoma

Dedifferentiated LPS are aggressive sarcomas seen in up to 15% of well-differentiated LPS of the retroperitoneum, and rarely in the extremities and mediastinum. Dedifferentiation can occur in primary tumors or within the recurrences and metastases. Both well-differentiated and dedifferentiated LPS are characterized on pathology by 12q13 to 15 amplification and MDM2 and CDK4 positivity. Dedifferentiated areas most frequently resemble undifferentiated high-grade pleomorphic sarcoma in 90% of cases, but may exhibit heterologous differentiation (eg, myogenic, osteo/chondrosarcomatous). On imaging, dedifferentiated component is identified by focal nodular nonlipomatous region greater than 1 cm, often showing enhancement ( Fig. 3 ). Calcifications/ossification may be present. The proportion of lipomatous component decreases with increasing grade of tumor, and measurement of this component maybe more significant than the entire tumor size when evaluating rate of growth and response to treatment. Distant metastases from dedifferentiated LPS occur in 15% to 20% of cases, most commonly to the liver and lungs.

A 69-year-old woman with retroperitoneal dedifferentiated liposarcoma. Coronal contrast-enhanced computed tomography of the abdomen reveals a large heterogeneous predominantly soft tissue containing retroperitoneal mass ( arrows ) extending into the right inguinal region with variable areas of fat attenuation ( arrowhead ). The inferior vena cava and the right iliac vessels are displaced by the mass.

Myxoid liposarcoma

Myxoid LPS is another subtype of LPS, often occurring in the extremities of young to middle-aged adults, usually in the deep soft tissues of the thigh. On imaging, myxoid LPS are usually seen as well-circumscribed, multilobulated (often septated), deep subcutaneous or intramuscular masses. On MRI, the myxoid content leads to characteristically low signal intensity (SI) on T1-weighted imaging and very high SI on T2-weighted imaging, with marked heterogeneous enhancement ( Fig. 4 ). Macroscopic fat is usually less than 10% of the tumor volume, although quite variable. Myxoid LPS is graded using a 3-tier system as low, intermediate, or high grade, based on the degree of cellularity. On imaging, large tumor size (>10 cm), deep location, lack of lobulations, and greater than 5% of nonfatty, nonmyxoid contrast enhancing component was associated with higher grader tumors. Myxoid LPS has an unusual pattern of metastases to extrapulmonary sites, often to other fat-containing areas, such as the paraspinal regions, intermuscular fat pad, bone, retroperitoneum, and the opposite extremity. Osseous metastases, although frequent and readily apparent on MRI, are exceedingly hard to detect on computed tomography (CT) and PET-CT, because lesions cause minimal disruption of the bony trabeculae and may not be avid owing to the myxoid matrix. Several investigators, including ourselves, have found whole body MRI to be a promising tool for screening for osseous and soft tissue metastases in patients with myxoid LPS, and should be strongly considered, especially in high-grade tumors ( Fig. 5 ).

A 66- year-old woman with myxoid liposarcoma. Axial short-tau inversion recovery ( A ) and fat-suppressed postgadolinium ( B ) T1-weighted MRIs demonstrates a well-circumscribed markedly T2 hyperintense lobulated intramuscular mass in the anterior compartment of the leg, which shows marked heterogeneous enhancement after contrast administration ( arrow ). There is no significant surrounding edema.

A 55-year-old man with myxoid liposarcoma and presumed solitary thoracic vertebral body metastasis. Whole body MRI was subsequently performed. Sagittal whole-body short-tau inversion recovery MRI demonstrates multifocal T2 hyperintense osseous disease ( arrowheads ) throughout the spine and the pelvic bones (images not shown).

Dermatofibrosarcoma Protuberans

Dermatofibrosarcoma protuberans was included in STS for the first time in the 2013 WHO classification (previously was categorized under tumors of the skin). Dermatofibrosarcoma protuberans is a rare soft tissue tumor, comprising 6% of STS. Tumors are characterized by t(17; 22) translocation, resulting in the activation of platelet- derived growth factor receptor. Although the majority of the tumors are intermediate in grade and rarely metastasizing, fibrosarcomatous transformation to higher grade with metastatic potential may occur in up to 20% of cases. Dermatofibrosarcoma protuberans affects men more often than women, and most commonly occurs in the third to fifth decades of life. The trunk is most commonly involved (50%), followed by proximal extremities (35%–40%) and the head and neck (14%) ( Fig. 6 ). Surgery with wide local excision is the treatment of choice. However, owing to infiltrative margins, local recurrence is common, ad is seen in up to 20% of cases. Although imaging is not performed routinely owing to its superficial location, it is helpful for preoperative planning especially in deep and infiltrative tumors. MRI demonstrates a lobular or nodular enhancing intermediate SI lesion involving the subcutaneous fat and the skin, often causing a focal protuberance ( Fig. 7 ). Myxoid lesions may be extremely T2 hyperintense. Satellite nodules and plaque like lesions with ill-defined infiltrative margins are also reported. Hemorrhage and necrosis is uncommon, except in fibrosarcomatous transformation.

A 54-year-old male with dermatofibrosarcoma protuberans. Axial noncontrast computed tomography image of the chest shows an ovoid, lobulated, homogeneous, low-density, subcutaneous mass in the posterior chest wall without invasion of the underlying muscles ( arrow ).

An 80-year-old woman with high-grade fibrosarcomatous variant of dermatofibrosarcoma protuberans. Axial T2 ( A ) and postcontrast ( B ) T1-weighted MRIs of the chest reveal a large lobulated heterogeneous high signal intensity mass with heterogeneous enhancement protruding from the skin of the right anterior chest wall ( arrows ), with infiltrative margins.

Myxofibrosarcoma

Previously known as the myxoid variant of malignant fibrous histiocytoma, MFS is recognized currently as a distinct entity. MFS is among the most sarcoma in elderly patients (median age, 65 years), occurring mainly in the extremities (75%), followed by trunk (12%) and retroperitoneum/mediastinum (8%). Masses may be subcutaneous (50%–70%) or intramuscular in location. Histologically, MFS is characterized by multinodular growth pattern with variable degrees of fibrous tissue, cellularity, and pleomorphism on a background of myxoid stroma (usually >5% of tumor). Treatment includes preoperative chemoradiation followed by wide en bloc resection. Local recurrence is seen in 50% to 60% of patients, and is independent of the histologic grade. Low-grade tumors can demonstrate histologic progression and evolve into high-grade lesions after local recurrence. Metastases can occur anywhere, but most commonly to the lungs.

The MRI findings are nonspecific, with heterogeneity on all pulse sequences reflecting variable amounts of myxoid stroma (low T1 SI, very high T2 SI, usually markedly enhancing), fibrous tissue, often seen as septa (low SI on T2 with minimal enhancement), cellular (intermediate on T1-weighted imaging, intermediate to high SI on T2-weighted imaging, and moderately enhancing), hemorrhage (high SI on T1 fat-suppressed sequences, fluid levels, nonenhancing), and necrosis (low SI on T1-weighted imaging, high SI on T2-weighted imaging, and nonenhancing) ( Fig. 8 ). T2 hypointense pseudocapsule may be present in some cases. The main differential diagnosis includes other myxoid neoplasms (eg, myxoid LPS) and other pleomorphic soft tissue tumors (eg, unclassified pleomorphic sarcoma). A characteristic feature of MFS is the presence of unencapsulated margin and a centrifugal infiltrative growth pattern along the fascial planes and intermuscular septa, resulting in positive surgical margins despite wide resection, and a high rate of local recurrence (50%–60%). This growth pattern is often manifest on MRI as the “tail sign,” a T2-hyperintense enhancing curvilinear projection (tail) extending from the primary tumor mass into the adjacent soft tissues. The “tail sign” was found to be moderately specific and sensitive for the diagnosis of MFS compared with other myxoid neoplasms and was also found to be associated with worse local recurrence-free survival. Recognition of the infiltrative pattern of tumor spread on preoperative MRI is critical for surgical planning and facilitating complete tumor resection. Thick fascial enhancement without a discrete mass may also be a sign of local recurrence after resection.

A 77-year old male with myxofibrosarcoma. Axial short-tau inversion recovery ( A ) and postgadolinium fat-suppressed ( B ) T1-weighted MRIs of the thigh reveal a heterogeneous extremely T2 high signal intensity intramuscular mass ( arrows ) with heterogeneous enhancement. There is a T2 hypointense pseudocapsule and T2 hyperintense enhancing peritumoral edema.

Smooth Muscle Tumors

LMS is a common mesenchymal tumor of smooth muscle differentiation, accounting for 9% of all STS. The most common sites of involvement are the uterus, retroperitoneum, and large blood vessels, and less commonly the soft tissues and skin. LMS is the second most common retroperitoneal tumor after LPS, and is the most common sarcoma of the large blood vessels ( Fig. 9 ). There was no difference between the imaging features and metastatic pattern of non–inferior vena cava and inferior vena cava LMS, with tumors of both subtypes appearing as large, heterogeneously enhancing masses, and frequently metastasizing to the lungs, peritoneum, and liver. Extremity LMS, most commonly affecting the thigh, may be deep (intramuscular and intermuscular) or superficial (subcutaneous) in location, and very rarely arise from the bones.

A 78-year-old man with inferior vena cava (IVC) leiomyosarcoma. Axial contrast-enhanced computed tomography image reveals a large mass ( arrows ) in the retroperitoneum centered in the IVC ( arrowhead ), with intraluminal tumor extension.

On MRI, imaging features of extremity LMS are nonspecific. Lesions are typically isointense to muscle on T1-weighted images and variably hyperintense relative to muscle on T2-weighted images, with moderate heterogeneous contrast enhancement ( Fig. 10 ). Histologically, LMS are usually highly cellular, and may account for T2 hypointensity seen in some of the tumors. Large lesions and those in the deep soft tissue locations are usually more heterogeneous secondary to hemorrhage, necrosis, and cystic change, and may exhibit thick peripheral irregular rim enhancement. Calcification/ossification may be present, and bone involvement is seen in up to 10% of cases.

A 58-year-old woman with right gluteal leiomyosarcoma. Axial short-tau inversion recovery ( A ) and fat-suppressed postgadolinium ( B ) T1-weighted MRIs demonstrate a heterogeneous T2 intermediate signal mass ( arrow ) in the right gluteal region with large central areas of T2 hyperintensity and hypointensity consistent with hemorrhage ( asterisk ). Postcontrast shows moderate homogenous enhancement of the mass.

Rhabdomyosarcomas

RMS arising from striated muscle or its precursor undifferentiated mesenchymal cells is the most common STS in children but also affects adults. Although it can occur anywhere in the body, the most common primary site in adults is the extremities (26% of cases) followed by the genitourinary system (25%), whereas in children the head and neck parameningeal region (25% of cases) is the most common site. Three histologic subtypes of RMS are recognized. The most common embryonal subtype (50%) occurs commonly in children under the age of 10 and affects the head and neck region and genitourinary system. The alveolar subtype (30%) is often seen in the trunk and extremities of adolescents and young adults, 10 to 25 years old, and in the perimaxillary region in adults. The pleomorphic subtype, seen in adults older than 45 years, usually involves the muscles of the thigh and has a poor prognosis.

Embryonal RMS usually presents as heterogeneous, poorly circumscribed mass, with moderately high SI on T2-weighted imaging, with heterogeneous contrast enhancement Hemorrhage, necrosis, and calcification are rare. Multiple ring enhancing regions resembling bunch of grapes referred to as “botyroid sign” was noted in some cases. Destruction of the adjacent bone is frequent, seen in approximately 25%, almost exclusively with tumors of the head and neck region. Alveolar RMS present as infiltrative heterogenous soft tissue masses with moderately high SI on T2-weighted, and prominent areas of necrosis. Serpentine flow voids and lobulated architecture may also be present. The pleomorphic subtype tends to be markedly hyperintense on T2-weighted imaging with marked heterogeneous enhancement. Lymph node involvement is seen in 33% to 45% of patients with adult RMS, and is most frequent with the alveolar subtype. Lung is the most common site of distant metastases followed by the bones, although any organ may be involved.

Vascular Tumors

Angiosarcoma of soft tissue

Soft tissue angiosarcomas are highly aggressive tumors, with high rates of local recurrence and distant metastasis. Angiosarcomas contain both epithelioid and spindle cells, forming rudimentary vascular channels, with frequent infiltration. The majority of soft tissue angiosarcomas are cutaneous tumors, with less than 25% presenting in the deep soft tissues. Risk factors include chronic lymphedema, previous radiation therapy, implanted foreign body, and familial syndromes including type 1 neurofibromatosis and Klippel-Trénaunay-Weber syndrome. MR signal characteristics of angiosarcoma include intermediate SI on T1-weighted images, and high SI on T2-weighted images, with marked, often heterogeneous enhancement ( Fig. 11 ). Areas of high signal on T1-weighted imaging suggestive of hemorrhage may be present. Although flow voids maybe seen in larger deeper tumors, it is characteristically absent with cutaneous lesions. A recent study of MRI findings in 17 women with secondary radiation associated angiosarcoma of the breast demonstrated multifocal T2 hyper or hypointense and rapidly enhancing dermal and intraparenchymal nodules, against a background of T2 hyperintense diffuse skin thickening.

An 84-year-old woman with radiation-associated angiosarcoma, presenting with violaceous skin lesion and bruising over the left breast 4 years after lumpectomy and radiation for invasive ductal carcinoma. Axial T2-weighted ( A ) and postgadolinium fat-suppressed T1-weighted ( B ) MRIs of the breast reveals a heterogenous T2 high signal intensity left medial breast skin thickening, which enhances rapidly after contrast administration ( arrows ). The diffuse T2 high signal thickening in the skin of the rest of the breast ( arrowhead ) and within the breast parenchyma do not enhance and represent postradiation edema. The patient underwent neoadjuvant chemotherapy before radical mastectomy and chest wall resection.

Synovial Sarcoma

Synovial sarcoma (SS) is a relatively common sarcoma, mainly affecting young adults, and accounts for 5% to 10% of STS. Pathologically, it is classified as monophasic (composed of only spindle cells), biphasic (composed of both spindle cell and epithelial components) and poorly differentiated. The characteristic t(X;18) translocation is seen in about 95% of tumors. Despite the name, less than 5% of SS originate in a joint or bursa. The most common location are the extremities (80% of cases), especially around the knee and adjacent to tendon sheaths ( Fig. 13 ).

A 66-year-old woman with synovial sarcoma. ( A ) Plain radiograph of the hand shows a soft tissue mass centered on the thenar eminence of the left hand with destruction of the first metacarpal bone and areas of calcification within. ( B ) Sagittal T2-weighted MRI of the hand reveals a large heterogeneously hyperintense mass with abnormal signal of the first metacarpal bone ( arrowhead ) and invasion of the extensor tendons of the first digit ( arrow ).

On imaging, SS are multilobulated or septated masses with heterogeneous SI on both T1 and T2 W images. Heterogeneity is secondary to areas of cystic change, necrosis, calcification, and hemorrhage within the tumor, and maybe manifest as “triple sign” on T2-weighted imaging (consisting of hypo-, iso-, and hyperintense signal) ( Fig. 14 ). “Fluid–fluid levels” secondary to layering hemorrhage and “bowl of grapes” appearance from T2 hyperintense lobulated areas and T2 hypointense septa, may be present in some cases. Calcification is seen in up to one third of cases, ranging from fine stippling to marked peripheral calcifications, and is best seen on CT. Up to 20% show erosion or invasion of adjacent bone (see Fig. 13 ). Smaller tumors may be well-circumscribed and homogeneous and can be mistaken for a benign cystic lesions. SS are commonly high grade tumors, with high incidence of pleuropulmonary metastases, as well as lymph nodal metastases (10%–30%). Although imaging findings are nonspecific, a calcified soft tissue mass in the lower extremity of young adults should prompt the diagnosis of synovial sarcoma.

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