Track: Case Reports

Abstract:

Alveolar rhabdomyosarcoma (ARMS), a rare aggressive malignant tumor of skeletal muscle origin, is predominantly encountered in adolescents and young adults. It represents approximately 20 to 25% of rhabdomyosarcoma (RMS) cases. ARMS typically arises in deep soft tissues of the extremities and is commonly associated with chromosomal translocations that result in fusion oncoproteins: PAX3::FOXO1 and PAX7::FOXO1. These fusions are thought to drive tumorigenesis and are linked to the increased metastatic risk and overall poor prognosis. An 18-year-old female presented with an enlarging mass (5-6 cm) on her distal lateral arm. Biopsy showed a small round blue cell tumor in a desmoplastic stroma with focal plasmacytoid features, suggesting possible hematolymphoid, neuroendocrine, melanocytic or skeletal muscle differentiation. Initial ancillary immunohistochemistry showed membranous  positivity for CD56 and CD99 with a high proliferative index on Ki-67 (at least 20%). Keratin, S100 protein, synaptophysin and SMA were negative. Desmin was only positive in the plasmacytoid appearing cells, suggesting possible muscle differentiation. Confirmation was obtained with positive MyoD1 and myogenin by immunohistochemistry. Thus, hematolymphoid, melanocytic and neuroendocrine malignancies were excluded. Further, a sarcoma molecular panel showed the fusion oncoprotein PAX3::FOXO1.  Despite the absence of nodal involvement or distant metastasis, the tumor’s unfavorable location and PAX3::FOXO1 positivity portends a high-risk disease course with poor prognosis. This case highlights the importance of correlating clinical findings and imaging studies with histopathology, immunohistochemistry and molecular tumor profiling to timely secure correct diagnoses. Such integration furthers communication toward the best patient treatment options and informs probable prognosis.