Ewing sarcoma superimposed on a previous osteochondroma in multiple osteochondromatosis

It has been reported that patients with hereditary multiple exostoses (called multiple osteochondromatosis by the World Health Organization) are at increased risk for malignant transformation of osteochondromas to secondary chondrosarcomas. A review of the literature found 14 cases showing transformation of osteochondromas into osteosarcomas; however, Ewing sarcoma has never been reported superimposed on an osteochondroma. This article presents the case of a boy who underwent biopsy of a previously existent osteochondroma for which the pathology report showed cytologic and immunohistochemical properties consistent with Ewing sarcoma. A 13-year-old boy with hereditary multiple exostoses (multiple osteochondromatosis) presented to an orthopedic clinic because of waxing and waning pain superficial to a previous osteochondroma on the lateral aspect of the right leg, below the knee, of 1 month's duration. On examination, inflammation was noted over a bony mass associated with tenderness to palpation of the affected area. There was no evidence of penetrating injury or trauma, and the patient reported no constitutional symptoms, including fever. Radiographs showed marked osteolysis and signs of periosteal reaction. Magnetic resonance imaging showed evidence of cortical bone erosion and extension of the mass into soft tissue. Malignant transformation was suspected, and the patient underwent biopsy. The pathology findings were consistent with Ewing sarcoma. The highly uncommon presentation of this malignancy must serve as a red flag to other physicians who treat patients with hereditary multiple exostoses. Ewing sarcoma tends to be of higher grade and have a worse prognosis than other malignancies that are more commonly seen in these patients.