Molecular analysis has defined many growth-related molecules through their alteration in tumors [27, 29–35]. The definition of molecular changes that fulfill Knudson’s two-hit or two-stage hypothesis has also been reviewed [36]. Although Knudson’s explanation involved one abnormal RB1 allele from the germline (predisposition or first hit), followed by somatic RB1 gene mutations in susceptible tissue (second hit in retina), epigenetic changes can also be placed on this pathway to neoplasia. This is reflected in the fact that most germline RB1 mutations originate on the paternal chromosome, implying a role for genomic imprinting/DNA methylation. Characterization of the RB1 gene as a cell cycle regulatory element places it within cell proliferation/cell death pathways.

Wilms’ tumor (Table 15.3) is a malignant embryonal neoplasm derived from nephrogenic blastemal cells. Several lines of differentiation are commonly seen and often replicate the histology of developing kidneys. Approximately 10 % of patients with Wilms’ tumor also have congenital abnormalities and malformation syndromes. The most common malformations are hemihypertrophy and genitourinary anomalies. The common syndromes associated with Wilms’ tumor include BWS, WAGR syndrome (Wilms’ tumor, aniridia, genitourinary abnormalities and mental retardation), and Denys-Drash syndrome (mesangial sclerosis, pseudohermaphroditism and nephroblastoma) [37–41]. Abnormalities involving the Wilms’ tumor locus, 11p13, are consistently found in the tumors of patients with WAGR and Denys-Drash syndrome. The 11p13 Wilms’ tumor locus encodes two coordinately regulated zinc-finger transcripts, WT-1 and WIT-1. These genes are highly expressed in the developing urogenital system [42–44]. The WT-1 protein binds to several sites on promoters of an IGF2 gene as well as to a promoter of the platelet-derived growth factor (PDGF) A-chain gene [45, 46]. The gene controls mesenchymal-epithelial transition during renal development. Furthermore, WT-1 expression induces transcription of one of the seven proapoptotic genes, Bak, and blocks cellular proliferation and DNA synthesis [47]. Interference with these normal regulatory influences of WT-1 may be an important factor in the genesis of nephroblastoma, which typically express high levels of IGF2 [48] and may also overproduce PDGF [49]. The expression patterns of WT-1 and Pax-2 in the metanephros overlap to a considerable extent; however, expression of WT-1 peaks as that of Pax-2 is decreasing. Therefore, it is possible that WT-1 represses Pax-2, and Pax-2 expression fails to down-regulate in the epithelial component of Wilms’ tumor and in nephroblastomatosis, the putative precursor of nephroblastoma [50]. Despite the strong association of WT1 mutations with Wilms’ tumor predisposition, WT-1 is mutated in only a minority of sporadic Wilms’ tumors [51]. This low prevalence of WT-1 abnormalities in sporadic Wilms’ tumor led to the recognition of other genes involved in pathogenesis. Evidence supporting this is provided by the linkage of familial BWS to a locus at chromosome 11p15, designated WT-2 [52, 53]. Approximately 1 % of the patients with Wilms’ tumor have a positive family history for the same neoplasm. Most of the pedigrees suggest autosomal dominant transmission with variable penetrance and expressivity. This suggests that genetic loci other than WT-1 and WT-2 are responsible for the pathogenesis of many familial as well as sporadic Wilms’ tumors [54, 55].

In gross examination, the most important factors to document are capsular invasion and hilar and sinus involvement along with adequate sampling of the tumor. Histology blocks should include at least one block per centimeter of the largest tumor diameter, tumor-kidney interface, capsule, hilar area, macroscopically normal kidney (at least two), and other areas of pathological significance. Some cases may be markedly cystic; in these, the tumor must be carefully examined for the presence of nodular solid areas. Particular attention should be paid to assessment of the capsule, hilar vessels, including the cut end, and renal sinus.

A triphasic Wilms’ tumor includes epithelial, stromal, and blastemal elements microscopically. Relative proportions of the epithelial components may vary widely such that one is markedly dominant (Fig. 15.1). The stromal component may vary widely from areas of stellate cells within a myxoid background to spindle cells recapitulating primitive mesenchyme. Smooth and striated muscle are the most common differentiated stromal cell types. Occasionally, a wide range of other stromal components may be present, including cartilage, osteoid, and even glial tissues.

Post-chemotherapy features include necrosis, fibrosis, and foamy and hemosiderin-laden macrophages. Rapidly dividing cells, such as those of the blastemal or primitive epithelial components, tend to respond to chemotherapy, whereas the stromal elements will not. Thus, tumors with significant stromal differentiation may not reduce much in size during chemotherapy. Also, stromal components may appear to be more prominent in a post-chemotherapy specimen. The tumor that remains predominantly viable following chemotherapy and in which the main component is blastema is associated with an adverse prognosis.

Nuclear anaplasia in the context of Wilms’ tumor has very specific criteria required for its definition, such as nuclear hyperchromasia, nuclear enlargement in which affected nuclei are at least three times the size of adjacent non-anaplastic nuclei, and abnormal multipolar mitoses. Using such criteria, anaplasia is present in approximately 5–10 % of Wilms’ tumors, occurring more commonly in those diagnosed in later childhood and in black patients. Anaplasia may either be focal or diffuse. Focally, one or a few, well-circumscribed, localized regions of anaplasia are found within the primary tumor that is completely excised, with the majority of tumor containing no significant nuclear atypia. Diffuse anaplasia occurs in multiple fields or outside the primary tumor and is associated with a poor response to chemotherapy. Anaplasia is of major clinical significance when present in tumors, which are potentially incompletely excised. Focal anaplasia or anaplasia in stage I cases may be of importance. Anaplasia is identified as an adverse prognostic factor. Transcription profiling indicates that a cluster of genes separates anaplastic from non-anaplastic Wilms’ tumor.

Immunohistochemical staining is often extremely useful in the diagnosis of Wilms’ tumor from a needle core biopsy. A core biopsy may simply demonstrate small round blue cells, which could represent the blastema of Wilms’ tumor or several other embryonal tumors of infancy and childhood. The most useful immunohistochemical marker is WT-1. Areas of stromal differentiation, and well-differentiated epithelial structures, may not express nuclear WT-1. Cytokeratin staining may help with the identification or confirmation of epithelial differentiation. CD56 is also useful, since primitive renal blastema is diffusely and strongly CD56 positive. Nuclear p53 is expressed in 75 % of anaplastic nephroblastomas. At present, there are no diagnostic or prognostic markers useful for the evaluation of Wilms’ tumor.

Nephrogenic rests are classically divided into perilobar and intralobar types. Perilobar rests are more common and are found in subcapsular peripheral locations with well-demarcated margins on low power. Intralobar rests present anywhere within the kidney and often have a more irregular and intermixed margin. The clinical relevance is their potential increased risk for subsequent development of Wilms’ tumor. Wilms’ tumors in association with intralobar rests develop at a younger age on average. The risk appears highest for intralobar rests in patients under 1 year of age. When a Wilms’ tumor develops within a nephrogenic rest, there is a superimposed nodular growth of the proliferating component and formation of a fibrous pseudocapsule around the tumor with the compressed rest at the periphery.

Cystic nephroma (CN) is predominantly seen in children under 5 years of age. Its characteristic imaging appearance is of a well-circumscribed mass distinct from the surrounding kidney and predominantly cystic with thin septa. In CN, the septa contain no solid nodules and only mature elements. In cystic partially differentiated nephroblastoma, the septa also contain embryonal elements that do not form solid nodules, thus conforming to the outlines of the septa. If classical components of Wilms’ tumor are present, forming nodular structures that distort the septal contours, the lesion should be classified as cystic Wilms’ tumor. Both CN and cystic partially differentiated nephroblastoma are adequately treated by resection alone with an excellent prognosis and no chemotherapy required. Predominantly, cystic Wilms’ tumor, although almost always stage I and therefore with a correspondingly good prognosis, should be treated as an appropriately staged Wilms’ tumor. There is an association between CN and pleuropulmonary blastoma [57].

The classical pattern of CN demonstrates nests of ovoid cells separated by fibrovascular septa. Cord cells may be ovoid, epithelioid, or spindled with bland nuclei without prominent nucleoli. Septa demonstrate a marked regularly branching “chicken wire” pattern of capillaries. A wide variety of other patterns have been described. These include myxoid, sclerosing, cellular epithelioid, palisaded, spindled, storiform, and anaplastic variants. Most cases are composed of a mixture of such patterns.

Clear cell carcinoma of the kidney comprises 3 % of malignant pediatric renal tumors. The average age at diagnosis is 3 years. There are no specific diagnostically or prognostically useful genetic features. Metastatic disease is present in 5 % of cases and up to 60 % of those have bone metastases. The tumor is solid, cystic, and cream to tan in color. Immunohistochemical stains vimentin and bcl2 are usually positive. Cytokeratin, WT1, and CD99 staining is negative [57]. The vimentin staining is unlike the dot-like pattern characteristic of renal rhabdoid tumors.

Rhabdoid tumors are rare, comprising 2 % of pediatric renal malignancies. These tumors are predominantly shown in young children, with average age at diagnosis of 1 year. This tumor is exceptionally rare in children over 5 years of age.

In patients with renal rhabdoid tumors, genetic abnormalities of the hsNF5/INI1 tumor suppressor gene on chromosome 22 are characteristic. The gene is important for chromatin remodeling, and inactivation is tumorigenic. Mutations, deletions, and chromosome losses inactivating INI1 vary widely; therefore, a single readily available RT-PCR-based assay is not available.

Patients with renal rhabdoid tumors have an increased risk of malignant embryonal tumors of the posterior fossa; approximately 10–20 % develop a posterior fossa tumor. Rhabdoid tumors are aggressive, and the majority of patients (up to 80 %) present with metastatic disease. Grossly, rhabdoid tumors have large, solid and cystic areas with extensive areas of hemorrhage and necrosis. They are poorly defined and locally infiltrative. Microscopically, rhabdoid tumors contain eccentric nuclei and eosinophilic intracytoplasmic inclusions (Fig. 15.2). Vimentin immunostaining demonstrates characteristic strong dot-like perinuclear cytoplasmic staining in a significant proportion of cells. INI1 immunostaining is characteristically absent in the nucleus of rhabdoid tumor cells. INI1 nuclear staining is positive in the nucleus of almost all other pediatric tumors, although, renal medullary carcinoma and some cells of synovial sarcoma may also be INI1 negative.

Congenital mesoblastic nephroma is a low-grade spindle cell neoplasm affecting the renal medulla. Representing 2–5 % of pediatric tumors, congenital mesoblastic nephroma is also the most common congenital renal neoplasm. A vast majority of cases present in the first year of life (90 %). Some patients may develop hypertension. Congenital mesoblastic nephroma demonstrates a t(12;15)(p13;q25) ETV6-NTRK3 translocation, also found in congenital/infantile fibrosarcoma and secretory carcinoma of the breast. RT-PCR may be useful diagnostically [58]. Overall, 95 % of patients have an event-free survival; however, prognosis is worse with stage III with cellular congenital mesoblastic nephroma in patients 3 months or older. Grossly, the tumor has a cystic and solid whorled fibrous appearance resembling a fibroid. The cellular subtype may appear softer and more hemorrhagic.

Microscopic subtypes are classical, cellular, and mixtures and may be identified on the basis of molecular characterization and morphological features. The cellular subtype represents 40–60 % of cases and is morphologically indistinguishable from an infantile fibrosarcoma. There is increased cellularity with mitoses and focal necrosis. Mixed subtypes comprise less than 10 % of cases. The presence of incomplete excision is the main cause of local recurrence. The majority of cases with recurrence have been the cellular subtype with an abnormal gene fusion product present.

Renal cell carcinoma is uncommon in childhood and comprises 2–5 % of pediatric renal tumors. It usually occurs in children 10–15 years of age. Genetic changes are similar to adult cases. A range of familial and syndromic conditions may be associated with renal cell carcinoma development such as Von Hippel-Lindau (VHL) syndrome, hereditary papillary renal carcinoma syndrome, hereditary leiomyomatosis and renal cell cancer, Birt-Hogg-Dubé syndrome, and tuberous sclerosis. Twenty percent have metastases at diagnosis. The tumor may have a predominant clear cell, cystic papillary, or chromophobe pattern. Renal cell carcinoma is cytokeratin positive in all tumors and CD10 positive in greater than 90 %. EMA, vimentin, and CEA staining may also be positive.

The incidence of neuroblastoma is 1 per 100,000 [59]. Seventy percent of patients have metastatic disease at presentation [60]. Age, clinical stage, molecular findings, and an undifferentiated subtype all influence overall survival [61]. The International Neuroblastoma Pathology Classification system [62] is as follows:

Molecular studies are mandatory for risk assessment and management in neuroblastic tumors, and it is important for optimal specimen handling to allow FISH and other molecular studies. Current biological markers associated with adverse prognosis are N-MYC amplification, DNA diploidy, 1p deletion and 17q gain. Mitosis karyorrhexis index is calculated per 5,000 neuroblasts, usually within 6–8 high-power fields in cellular tumors in a 3 μm-thick evenly stained section. The mitosis karyorrhexis index is not carried out on small needle biopsy specimens or post-treatment specimens.

Specimens are assigned to 1 of 3 prognostic categories (see Table 15.4). On immunohistochemical staining CD56 is positive and also highlights neurofibrillary stroma. NB84 is positive. CD44 is positive and associated with a better prognosis. Schwannian stroma is S100 positive.

This is a malignant mesenchymal tumor with skeletal muscle differentiation. In childhood, there are two main types: embryonal (ERMS) and alveolar (ARMS). The pleomorphic type affects adults.

Hepatoblastoma is the most common hepatic malignancy of childhood and comprises 1 % of pediatric malignancies. Ninety percent occur in the first 5 years of life and less than 5 % are congenital. Fifty-fold have increased risk of birth weight of less than 1,000 g. There are rare predisposing syndromes in 5 % of cases such as BWS, trisomies, and familial adenomatous polyposis (FAP). Ten percent of sporadic hepatoblastomas have APC gene mutations.

In 80 % of cases, there is a single mass, whereas 20 % have multiple lesions. Anemia is present in 70 % and thrombocytosis in 50 %. Greater than 90 % of cases show elevated serum AFP concentration. Forty to 60 % are unresectable at diagnosis, but 85 % are resectable post-chemotherapy. Ten to 20 % of cases have lung metastases at diagnosis. The overall survival rate is 70 %.

Histopathologically, the tumor has a mixture of features that may be highly variable in both fetal and embryonal epithelial cells, stroma of connective tissue, osteoid, skeletal muscle or rare other epithelial types as well as teratoid features. Extramedullary hematopoiesis may be present. Immunohistochemical staining is especially useful other than to exclude other tumors.

Desmoplastic small round cell tumor affects children and young adults, with more males affected than females. Characteristic and diagnostic is the t(11;22)(p13;q12), Ewing sarcoma (EWS)-WT1 fusion. The patient usually presents with disseminated abdominal disease. Presentation depends on the site; if aggressive, the patient has a poor prognosis. Histopathological features include infiltrative growth with desmoplastic spindle cell stroma that is variably vascular. Tumor cells are uniform, ovoid, and hyperchromatic with scant cytoplasm. Intracytoplasmic inclusions may be present. The cells are arranged in nests of variable shapes and sizes. Mitoses and necrosis may be present.

The tumor has polyphenotypic expression with CK, EMA, desmin, vimentin, and NSE staining. Anticarboxyterminus WTI is nuclear positive, and myogenin and MyoD1 are negative. Molecular techniques are required for confirmation of diagnosis.

This tumor occurs in infants and children. Congenital tumors are reported. There is an association with family history of a range of malignancies and posterior fossa tumors. Like renal rhabdoid tumors, there are abnormalities of the hSNF5/INI1 gene on chromosome 22 [57]. Clinical features consist of mass lesions at almost any site. The tumor is aggressive and has a poor prognosis; however, this has improved with current chemotherapy regimes. Histopathological features include sheets of malignant cells with vesicular nuclei, prominent nucleoli, eosinophilic cytoplasm, mitoses, and necrosis. There is a variable extent of intracytoplasmic inclusions, which are often easier to identify at the periphery or in myxoid areas. The small cell variant has cells with scant cytoplasm. Some tumors may have a markedly myxoid stroma. On immunohistochemical staining, there is co-expression of CK and vimentin. Perinuclear “dot-like” expression highlights inclusions that are composed of intermediate filaments by electron microscopy. There is variable non-diagnostic expression of other antibodies. Negative INI1 nuclear expression is essentially diagnostic.

EWS and primitive neuroectodermal tumor (PNET) are primary malignant small round cell tumors of bone and soft tissue. Because they have a similar phenotype and share an identical chromosome translocation, they are viewed as the same tumor, differing from each other only in their degree of neural differentiation. Tumors that demonstrate neural differentiation by light microscopy, immunohistochemistry, or electron microscopy have been traditionally labeled PNET, and those that are undifferentiated by these analyses are diagnosed as EWS. These tumors account for approximately 6–10 % of primary malignant bone tumors. Most patients are 10–15 years old, and approximately 80 % are younger than 20 years. Boys are affected more frequently than girls, although the disease occurs much more often in girls than boys during the first 3 years of life. There is a striking predilection for whites; blacks are rarely afflicted.

In EWS/PNET, approximately 85 % have a t(11;22)(q24;q12) translocation, 5–10 % of cases have a t(21;22)(q22;q12) translocation, and in less than 1 % of tumors, a t(7;22)(p22;q12) translocation is present. The fusion gene (EWS-FLI1) generated from the translocation plays a vital role in the molecular genesis of this neoplasm [57]. The aberrant gene acts as a dominant oncogene, and the resultant chimeric proteins function as aberrant transcription factors that stimulate cell proliferation.

EWS/PNET can arise in any portion of the skeleton but usually develops in the diaphysis or metadiaphysis of long tubular bones and the flat bones of the pelvis. In young children, the pelvis, ribs, and proximal long bones are preferentially affected. The tumor typically presents as a painful enlarging mass, and the affected site is frequently tender, warm, and swollen. Systemic findings that mimic infection include fever, elevated sedimentation rate, anemia, and leukocytosis.

Arising in the medullary cavity, EWS/PNET usually transgresses the cortex and periosteum, producing a soft tissue mass. The tumor is tan-white or fish flesh-like in appearance and frequently contains areas of hemorrhage and necrosis. The cells grow in sheets or irregular islands with an organoid growth pattern within which Homer-Wright rosettes may be present. Occasionally, tumor cells are larger, have irregular nuclear contours, and prominent nucleoli. Mitoses are easily identified, and necrosis may be extensive. Special histochemical stains demonstrate glycogen in up to 75 % of tumors. In a minority of cases dense core neurosecretory granules are seen by electron microscopy.

EWS/PNET cells express vimentin, CD99 (MIC-2), FLI-1, C-kit and one or more neural markers including NSE, Leu-7, S100, neurofilament, and keratin in approximately 20 % of cases [57]. EWS/PNET is usually treated with a combination of chemotherapy and surgery. The chemotherapy is given preoperatively, and chemotherapy-induced necrosis of 90 % or greater is considered a good response. Radiotherapy is reserved for tumors that are surgically inaccessible, inadequately excised tumors, and patient palliation. Effective chemotherapy has dramatically improved the prognosis to a 75 % 5-year survival of which at least 50 % are long-term cures.

Most germ cell tumors are seminomas in young adults; however, in younger children, they are usually teratomas and yolk sac tumors.

Seminomas. Seminomas are composed of sheets of uniform round to ovoid cells with minimal pleomorphism; in most cases, they are associated with lymphoid infiltrates. PLAP and CD117 are positive in almost all cases [57].

Embryonal Carcinoma . Embryonal carcinoma is composed of sheets, papillae, and cysts with poorly differentiated cells containing overlapping polygonal, vesicular nuclei and prominent nucleoli. CD30, CK and focal PLAP stains are positive.

Yolk Sac Tumor . This is the most common malignant germ cell tumor in infancy. Ninety percent are associated with elevated serum AFP levels and contain ovoid vacuolated cells with scattered hyaline globules. Yolk sac tumors are mitotically active. There are numerous growth patterns, and most cases contain many of the patterns: microcystic/reticular, macrocytic, solid, alveolar, endodermal sinus (Schiller-Duval bodies), papillary, myxomatous, polyvesicular/vitelline, hepatoid, and enteric (Fig. 15.7). They are cytokeratin and AFP focally positive.

Choriocarcinoma . This tumor is very rare in childhood even as a component of mixed malignant germ cell tumor. Choriocarcinoma is positive for cytokeratin, inhibin, and hCG.

Teratoma . Teratoma occurs in infancy and is composed of tissues recapitulating all three germ layers. It is well-demarcated. Malignant areas may be present including focal PNET. It has a benign behavior in infancy.

Mixed Germ Cell Tumor . Normal immature fetal germ cells express PLAP and CD117. Infantile/congenital testicular tumors are most likely juvenile granulosa cell tumors rather than germ cell tumors.

Leydig Cell Tumors . This comprises less than 5 % of pediatric testicular tumors. Leydig cell tumors occur in children between ages 3 and 9 years, presenting as a painless unilateral testicular enlargement. Endocrine features may be present, including gynecomastia and precocious puberty. The tumor is a well-circumscribed, yellow-brown nodule. Histopathologically, it is composed of diffuse sheets of large polygonal cells with eosinophilic cytoplasm; Reinke crystals are present in less than 40 %. The cells have regular nuclei with prominent nucleoli. Malignant behavior occurs in 10 % of tumors, particularly tumors greater than 5 cm in diameter and those containing cytological atypia, mitoses, necrosis and vascular invasion. The cells are vimentin and inhibin positive [57].