Pathologic Classification

Table 20-2 lists the histologic subtypes with reference to their frequency, morphology, site of origin, and age distribution.

Specific assays may be necessary to aid in the differential diagnosis of rhabdomyosarcoma from the other small round cell tumors of childhood (i.e., lymphoma, Ewing's sarcoma, primitive neuroectodermal tumor, neuroblastoma). These assays include:

• Electron microscopy

• Immunocytochemistry

• Cytogenetics.

Table 20-2. Histologic Subtypes of Rhabdomyosarcoma and Undifferentiated Sarcoma

Pathologic subtypea

Morphology

Usual ages Usual primary sites (years)

Rhabdomyosarcoma Embryonal (57%)

Botryoid variant

Spindle cell variant (3%)

Alveolar (24%)

Undifferentiated (3.5%)

Resembles skeletal muscle in 7- to 10-week fetus.

Moderately cellular with loose myxoid stroma. Actin and desmin positive.

Cambial layer of tumor present, several cells thick. Loose myxoid stroma. Typically grapelike configuration is present grossly.

Spindle-shaped cells with elongated nuclei and prominent nucleoli. Low cellularity. Collagen rich and poor variants.

Resembles skeletal muscle in 10- to 21-week fetus.

Basic cell is round with scanty eosinophilic cytoplasm; alveolar pattern may be lost if densely packed; cross striations more common than embryonal variety.

Cell of origin appears mesenchymal with no evidence of myogenesis or differentiation. Cells are round, larger than mature lymphocytes and have scanty or moderate cytoplasm; may be a heterogeneous group.

Heterogeneous.

Head and neck, 3-12 orbit, genitourinary tract

Bladder, vagina, nasopharynx, bile duct

Paratesticular

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