Proximal epithelioid sarcomatous dedifferentiation in secondary chondrosarcoma in a known case of multiple osteochondromatosis

Osteochondroma is the most common benign bone tumor. Approximately 15% of osteochondromas occur as multiple lesions. Multiple osteochondromatosis has a higher risk of developing chondrosarcomas, which are of low grade with good prognosis. About 10% of all chondrosarcomas may undergo dedifferentiated change, which has a poorer prognosis. Dedifferentiated peripheral chondrosarcoma developing within an osteochondroma is extremely rare. Dedifferentiation usually occurs in the form of osteosarcoma, malignant fibrous histiocytoma, fibrosarcoma, or rhabdomyosarcoma. We report a case of proximal epithelioid sarcomatous dedifferentiation in secondary chondrosarcoma in a 39-year-old male with multiple osteochondromatosis in bilateral arm. To the best of our knowledge, epithelioid sarcomatous dedifferentiation has not been described in the literature.