Rapidly growing fibro-osseous pseudotumor of the digits mimicking extraskeletal osteosarcoma.

Fibro-osseous pseudotumor of the digits is a rare benign lesion characterized histopathologically by a fibroblastic proliferation with foci of osseous differentiation. We report a case of fibro-osseous pseudotumor of the digits with a rapidly growing lesion in a 30-year-old woman. Because of its rapid growth and aggressive histopathological appearance, this lesion can be mistaken for a malignant neoplasm such as extraskeletal or parosteal osteosarcoma, despite its benign clinical behavior. For a soft tissue mass in the digit with a clinicopathological feature of pseudomalignancy, the clinician should consider fibro-osseous pseudotumor of the digits as a possible diagnosis, thereby avoiding inappropriately aggressive surgery.