Benign calcifying fibrous-myofibroblastic tumor mimicking myositis ossificans in a 22-month-old girl.

Myositis ossificans circumscripta (MOC), with nonneoplastic heterotopic bone formation in soft tissue and skeletal muscle, is rare in children. Extraskeletal osteosarcoma is a very rare malignant mesenchymal neoplasm of soft tissues in children. At onset, it may be difficult to distinguish MOC from a musculoskeletal infection or neoplasm, particularly in the absence of trauma, and a biopsy is frequently required. However, differentiating MOC from malignant neoplasm is imperative. We describe the case of a 22-month-old girl who presented with a benign fibrous-myofibroblastic tumor mimicking MOC. However, extraskeletal osteosarcoma was also considered in the differential diagnosis due to the absence of attachment of the lesion to the skeleton. Pathologic findings after a previous needle biopsy and posterior marginal resection exclude both differential diagnoses. Close follow-up during 3 years postoperatively showed no signs of recurrence.