A case of malignant transformation of myositis ossificans.

A 49-year-old white man presented for evaluation of an enlarging left distal thigh mass with increasing pain over the last several months. The mass was first noticed 11 years prior to this presentation. During his initial examination, the patient was diagnosed with myositis ossificans and had a partial resection of the mass to improve knee function. His current examination revealed a low grade parosteal osteosarcoma arising from pre-existing mature heterotopic ossification. To our knowledge, no reported cases of secondary parosteal osteosarcoma arising from myositis ossificans have been reported in the literature; only a few reports of malignant transformation of myositis ossificans were found.