Systemic rhabdomyosarcoma with diffuse bone marrow involvement. Case report of an unusual presentation.

Rhabdomyosarcoma presenting as a systemic disease is rare. This report concerns a 12-year-old girl who came to medical attention for what was thought to be a hematologic malignancy. Diffuse lytic bone lesions and large primitive cells in the bone marrow exhibiting erythrophagocytosis supported this diagnosis. During the course of the disease, a soft tissue mass in the region of the left ankle was removed and was a typical alveolar rhabdomyosarcoma. Retrospective review of the marrow, including electron microscopy, demonstrated that the primitive marrow cells were probably rhabdomyoblasts as well. The clinical course was a rapid downhill one in which lytic bone lesions and hypercalcemia were prominent. Although rhabdomyosarcoma eventually may disseminate, initial widespread disease without a clinically apparent primary can be a diagnostic dilemma. This clinical presentation, in combination with the recognized aggressiveness of the alveolar histologic subtype, identifies a rare subgroup of patients with rapidly fatal disease.