Fibro-osseous pseudotumor of the digits.

Twenty-one cases of a benign fibro-osseous pseudotumor occurring in the soft tissues of the digits are described. They affected mostly young adults. Twenty of the 21 cases involved the soft tissues of the fingers, especially the proximal portions of the index or middle finger, and one case involved the toe. Microscopically these tumor-like lesions closely resembled myositis ossificans, but unlike this process they involved chiefly the subcutaneous tissue and adjacent fibrous structures and were marked by an irregular multinodular growth pattern without the typical zoning of myositis ossificans. Because of the focal hypercellularity, cellular atypia and increased mitotic activity six of the cases were initially interpreted as being parosteal or extraskeletal osteosarcoma. Follow-up information indicates that this lesion behaves in a nonaggressive fashion and is curable by complete local excision.