Unusual Case of Histiocyte-Rich Rhabdomyoblastic Tumor in a Pediatric Patient

Histiocyte-rich skeletal rhabdomyoblastic tumor (HR-RMT) is a rare recently described skeletal muscle neoplasm of uncertain malignant potential. We report an unusual tumor in the right arm of a 5-year-old boy, which is the first case of a pediatric HR-RMT. Immunohistochemically, most cells in the tumor were positive for CD163 and CD68 staining. The neoplastic cells themselves showed a skeletal muscle phenotype with diffuse expression of desmin and focal expression of myoD1. Mitotic activity was low (1/10 HPF) and no necrosis was observed.