Circumscribed spontaneous heterotopic ossification in the soft tissues simulating sarcoma

Eight cases are described of circumscribed heterotopic ossification in the soft tissues characterised by spontaneous onset, no history of trauma, and rapid course which have frequently led to a mistaken preoperative diagnosis of highly malignant tumour. The terminology differentiates it from myositis ossificans, which is a different entity. The authors emphasize the need for accurate evaluation of all the clinical and radiographic data in order to avoid an erroneous diagnosis of extraosseous osteogenic sarcoma or parosteal osteogenic sarcoma. In this regard, both radiography and histological examination demonstrate the so-called zoning pattern, namely a radiolucent central zone corresponding to the more immature area and consisting of fibrous tissue with active histiofibroblastic proliferations and a radiopaque peripheral area, where ossification is more mature the closer it gets to the periphery. Histological specimens obtained exclusively from the central area may lead even the most expert pathologists to a diagnosis of fibrosarcoma or osteogenic sarcoma. Finally, the pathogenetic aspects of the anomaly are discussed, as well as those concerned with the differential diagnosis from other types of heterotopic ossification.