Massive bone marrow involvement by clear cell variant of rhabdomyosarcoma.

A 16-year-old boy with generalized myalgia and petechial hemorrhage was found to have a diffuse infiltrative disease in his bone marrow (BM). The BM aspirate contained a dense population of vacuolated blast-like cells. The BM biopsy displayed compact sheets of small round cells with clear cytoplasm, reminiscent of Ewing sarcoma. Immunostains were not diagnostically conclusive while transmission electron microscopy on the BM cells demonstrated a clear skeletal muscle differentiation. The morphologic findings led to a tentative designation of metastatic embryonal rhabdomyosarcoma (RMS). It was not until cytogenetic analysis revealed the specific translocation t(2;13)(q35;q14) did the alveolar RMS finally get confirmed. Despite an exhaustive search by imaging studies, a primary tumor was not detected. This case illustrates that the massive BM involvement by atypical alveolar RMS of unknown origin may pose serious diagnostic challenges. Multidisciplinary studies are required to reach a definitive diagnosis.