[Fibrous dysplasia].

The observations in 222 cases of fibrous dysplasia of the Hamburg Bone Tumor Registry will be presented. This benign lesion is based on the appearance of postzygotic point mutations in a gene encoding the Gsa protein. It occurs as a mono- and a polyostotic variant and may affect every bone of the skeleton. Most often affected are the proximal femur, the skull and the ribs. Polyostotic lesions tend to occur on one side of the body. The monostotic form is 7.6 times more frequent, but both variants show no predilection for gender. Most cases are diagnosed during adulthood. On x-rays the lesion has a ground-glass appearance and is located intramedullary with a sharply defined edge. The cortical bone is arroded and the bone is expanded. Histologically typical signs are slender curved fibrous trabeculae with a C and Y shape embedded in a morphologically bland and moderately cellular fibrous stroma. Diagnostically important are collagen fibres emerging perpendicular from the surface of the trabeculae. Cartilage is present in 8% of the cases. Therapeutically, thorough clinical controls are indicated and operative procedures are rarely needed to prevent progressive deformities and fractures. The formerly applied radiotherapy is now obsolete because of the increased occurrence of malignant transformations.