Answer: Alveolar rhabdomyosarcoma, solid variant

Histology: The excision demonstrates an undifferentiated small round blue cell tumor. The tumor grows as solid sheets of primitive cells with little evidence of cytoplasmic differentiation. Focally, however, one can see slight cytoplasmic eosinophilia. A high mitotic rate and single cell necrosis are present.

Rhabdomyosarcoma is the most common pediatric soft tissue sarcoma. Pediatric rhabdomyosarcomas are divided into embryonal rhabdomyosarcomas, which occur at a slightly younger mean age and are typically spindled tumors, and alveolar rhabdomyosarcomas, which occur at a slightly older age and are small round blue cell tumors. Alveolar rhabdomyosarcomas have a worse prognosis than embryonal rhabdomyosarcomas. The majority of alveolar rhabdomyosarcomas are found to have one of two specific chromosome translocations which involve fusion of a PAX gene (PAX3 or PAX7) with the fork head in rhabdomyosarcoma gene (FKHR). These specific chromosome translocations are t(2;13) (q35;q14) or t(1:13) (p36;q14). Diagnostic chimeric RNA transcripts resulting from these gene fusions can be detected by reverse-transcriptase polymerase chain reaction (RT-PCR) performed from fresh frozen tissue, and often from paraffin embedded tissue if necessary.

Alveolar rhabdomyosarcoma typically has an architecture that resembles lung parenchyma in that the tumor cells are discohesive when they are away from the fibrovascular septa. Tumor cells that cling to the fibrovascular septa hence resemble pneumocytes lining alveolar septa of the lung, with the discohesive tumor cells resembling alveolar macrophages. Multinucleated tumor giant cells with wreath-like nuclei are characteristic of this entity. The solid variant of alveolar rhabdomyosarcoma, as this case illustrates, can lack both of these helpful features.

Discussion: Embryonal rhabdomyosarcoma shows a similar staining pattern as alveolar rhabdomyosarcoma; however, this is typically a spindled cell tumor. Ewing’s sarcoma/primitive neuroectodermal tumors are also small round blue cell tumors; however, the diffuse immunoreactivity for muscle markers and lack of staining for MIC2 essentially excludes this diagnosis. While blastemal Wilms tumors are occasionally reported to stain for desmin, they do not typically stain for actin or for other muscle markers. In addition, Wilms tumor typically metastasizes to the liver, lymph nodes, and lung. Soft tissue metastases of Wilms tumors are exceedingly uncommon.