Exophytic variant of fibrous dysplasia (fibrous dysplasia protuberans).

Two cases of an exophytic variant of fibrous dysplasia (fibrous dysplasia protuberans) are reported in which the lesions protruded far beyond the normal bone contour mimicking surface lesions of bone. The first case was an 18-year-old man who had a pedunculated calcified mass of the sixth rib in association with radiologically diagnosed fibrous dysplasia of the skull. The second case was a 33-year-old man who had an exophytic lesion of the proximal tibia. Both of these patients were shown to have benign fibro-osseous lesions consistent with fibrous dysplasia. The intramedullary portions of the host bone adjacent to the exophytic masses also were involved by the fibro-osseous lesions and this fact suggests that the lesions may arise eccentrically in the medullary spaces and mainly grow outwards. It is important to be aware that fibrous dysplasia occasionally presents as an excrescence on the surface of bone. Careful radiographic and histological correlation is required to make a correct diagnosis of this rare variant of fibrous dysplasia.