Answer: Rhabdomyosarcoma

Histology: The biopsy specimen shows variation in cellularity with highly cellular zones alternating with sparsely cellular zones of myxoid stroma. The cellular zones is composed of highly compact small primitive cells, some of which are rounded and others which are spindled. The mitotic rate is exceedingly high. By light microscopy, there is no evidence of any specific differentiation such as keratinization, rosette/pseudo-rosette formations, neuropil or melanin production. Immunohistochemical studies were performed. The tumor cells were found to be immunoreactive for myogenin and desmin; and they were not immunoreactive for LCA, S100, synaptophysin, cytokeratin or CD99.

Discussion: Of the rhabdomyosarcomas arising in the head and neck, the embryonal variant of rhabdomyosarcoma (RMS) is the most common, especially in the pediatric patient population. In contrast to the alveolar variant shown above, the cellular component of embryonal RMS includes both spindled cells with elongated cytoplasm and small round cells resembling lymphoid cells, and the cellular aggregates are often separated by a loose and myxoid stroma. As with the alveolar RMS, the diagnostic challenge is to distinguish RMSs from other neoplasms comprised of small round blue cells, including lymphoma, olfactory neuroblastoma, Ewings sarcoma, peripheral neuroectodermal tumor and malignant melanoma. This distinction generally requires the use of immunohistochemical stains to confirm myogenic differentiation in the primitive tumor cells.