Proliferative myositis. Report of thirty‐three cases

Proliferative myositis is a pseudosarcomatous process of muscle characterized by an ill‐defined proliferation of basophilic giant cells and fibroblasts chiefly involving the perimysium, the epimysium and the neighboring fascia. Because of its rapid growth, its cellularity and the presence of giant cells, the condition had been initially confused with a sarcoma in 14 of the 33 cases reviewed. Clinically, the process usually presented as a painless firm solitary mass that grew rapidly and chiefly affected, as a poorly circumscribed scarlike process, the muscles of the shoulder region, the thorax and the thigh. In 19 of the 33 cases the lesion had been noted for two weeks or less prior to surgery. Proliferative myositis differed from other pseudosarcomatous lesions such as nodular fasciitis or myositis ossificans principally by its infiltrative diffuse growth, the presence of bizarre giant cells and the high incidence in patients older than 45 years. Lesions closely resembling proliferative myositis, but affecting only the fascia or the subcutaneous fat, were observed in 12 additional patients. Follow‐up data on 30 patients with proliferative myositis revealed a benign and probably self‐limiting process having no tendency to recur. Treatment by simple local excision is recommended. Radical or mutilating surgery should be discouraged.