Fibrous Dysplasia of Occipital Bone

Fibrous dysplasia is a non-inherited developmental disorder in which normal bone marrow is replaced by fibro-osseous tissue. Pathologically, there is abnormal differentiation of the osteoblasts which replace the normal cancellous bone by immature bone and fibrous stroma. The exact etiology of the disease is unknown. It is often asymptomatic and is typically seen in adolescents and young adults. Fibrous dysplasia can be either monostotic (involving single bone) or polyostotic (multiple bones) and may occur as a component of McCune-Albright syndrome or the rare Mazabraud’s syndrome. Monostotic lesions are asymptomatic and do not cause significant deformity. The polyostotic variety is more severe with the involvement of multiple sites. It results in significant deformities and, although the lesions become quiescent after puberty, the deformities may progress. McCune-Albright syndrome is an endocrinopathy occurring mainly in girls, consisting of the triad of precocious puberty, polyostotic fibrous dysplasia, and characteristic cutaneous pigmentation. Mazabraud syndrome is the rare combination of fibrous dysplasia and soft-tissue myxomas Classic radiographic features of fibrous dysplasia lesions are intramedullary, expansile, and well defined causing endosteal scalloping. Lesions show varying degrees of hazy density with a ground-glass quality, although some may appear almost completely radiolucent or sclerotic. The amount of the woven bone and extent to which it is mineralized determines the radiographic density of the lesion. CT and MRI are useful for evaluating the soft-tissue components and the entire extent of a lesion. On MRI, fibrous tissue has been described as being hypointense to skeletal muscle on both T1 and T2 weighted images. However, lesions do not follow the same signal characteristics of pure fibrous tissue. The apparent discrepancy between the theoretical and actual appearance of fibrous dysplasia on MRI is explained on the basis of the histology of the lesions. Most of the lesions are largely isointense to skeletal muscle on T1 weighted images. On T2 weighted images, lesions are typically heterogeneously hyperintense with hypointense, isointense or markedly hyperintense areas within. Depending on the appearance on both the T1 and T2 weighted images, the heterogeneous areas may be due to calcification, cystic changes, fatty areas or septations. Complications of fibrous dysplasia include pathological fracture, deformity, recurrence following surgery and associated tumours. Malignant degeneration of fibrous dysplasia complicates less than 1% of all cases, presenting clinically as pain and swelling.