Aspiration cytology of heterotopic ossification. A case report.

Heterotopic ossification (myositis ossificans) is usually a posttraumatic reactive process involving new bone formation. This nonneoplastic process can be clinically and radiologically confused with osteosarcoma. A case of heterotopic ossification is reported in which cytologic material was obtained from a partially calcified thigh mass from a 17-year-old, athletic male. Aspiration cytology revealed numerous osteoclastic giant cells containing multiple plump nuclei. These cells were admixed in a benign-appearing stromal background composed of mature fibroblasts and were typified by elongated spindle cells. A small incisional biopsy showed new bone formation, osteoclasts, osteoblasts and fibroblasts consistent with maturing heterotopic ossification. Heterotopic ossification may be distinguished cytologically from osteosarcoma by the presence of numerous uniform benign stromal cells composed of mature fibroblasts and osteoclastic giant cells. The differential diagnosis may be more difficult in the early stages of this reactive and proliferative process. Adequate sampling of suspected heterotopic ossification by aspiration cytology may avoid surgery in a selected group of patients.