Rhabdomyosarcoma (RMS) is an aggressive malignant soft tissue tumor that arises from/recapitulates primitive striated muscle cells (rhabdomyoblasts). It is the most common soft tissue malignancy in children and adolescents and most commonly arises in the head and neck region, but can arise from almost anywhere in the body. (1) RMS is usually divided into two broad histologic groups: alveolar RMS (ARMS), representing 30% of cases and associated with a poorer prognosis, and embryonal RMS (ERMS), representing approximately 70% of cases and associated with a more favorable prognosis. Among patients with ARMS, a translocation between the PAX3 or PAX7 gene and the FOXO1 gene is present in approximately 80% of cases and can be identified by fluorescence in situ hybridization (FISH).(2)
The diagnosis of RMS can also be confirmed with immunohistochemistry (IHC) which is found to be strongly positive for desmin and myogenin markers (myogenin/myg4, myoD1). IHC is often mandated because these tumors frequently lack evidence of lineage differentiation on the basis of cytology alone. In the present case, immunohistochemical stains performed on the mediastinal lymph node did not show epithelial differentiation (negative for keratin AE1, 3/Cam5.2, keratin 7 and 20, MOC-31) or germ cell differentiation (negative for PLAP, HCG, and AFP), and was found to have mesenchymal differentiation. One of the biggest diagnostic challenges with RMS is differentiating it from other small round cell tumors, including Ewing’s sarcoma, synovial sarcoma, neuroblastoma, peripheral neuroectodermal tumor, non-Hodgkin’s lymphoma, retinoblastoma, and hepatoblastoma. To diagnose RMS and exclude these other differential diagnoses, the marker myogenin is used (in conjunction with desmin) which expresses reactivity in RMS and is virtually diagnostic of RMS. The other small round cell tumors are negative for the myogenin marker. (3)