Primitive rhabdomyosarcoma presenting with diffuse bone marrow involvement: an immunohistochemical and ultrastructural study.

We recently identified three cases of primitive rhabdomyosarcoma (PRMS) presenting with diffuse bone marrow infiltration but inconspicuous soft tissue primaries, referred to The Johns Hopkins Hospital (JHH) as acute leukemia. In each case, the diagnosis of rhabdomyosarcoma was established using immunohistochemical staining and electron microscopy. Ultrastructural examination of tumor cells showed a feltwork of thin filaments, discontinuous basal lamina, glycogen, and primitive cell junctions without cell processes or neurosecretory granules. This presentation of PRMS may be more common than recognized, since it can be readily misdiagnosed as a hematopoietic tumor. While positive staining for muscle-specific actin, desmin, myoglobin, or other markers of skeletal muscle differentiation may be diagnostic, negative staining is inconclusive, requiring recognition of the "minimal" ultrastructural findings of primitive rhabdomyosarcomas.