Radiological Appearances of Fibrous Dysplasia and its Differential Diagnosis

Fibrous dysplasia (FD) is a non-inherited developmental disorder in which normal bone is replaced by fibro-osseous tissue. FD is categorized as mono-ostotic or poly-ostotic (1020%) affecting skeleton. The triad of poly-ostotic FD combined with abnormal skin pigmentation and various endocrine disturbances is identified as McCune Albright syndrome. Cherubism is an autosomal hereditary fibro-osseous neoplasm involving the mandible and maxilla with some resemblance to FD, but is a separate entity and unrelated to FD. FD is caused by a gene mutation of a cell surface receptor guanine nucleotide protein (GNAS 1). The gene is located at chromosome location 20q13.2. Mono- and poly-ostotic FD can occur anywhere in the skeleton, but the most common locations are the craniofacial region, femur, tibia and rib and may affect any bone in the body and are found in all age groups, but mostly in the age between 10-30 years, without sex prevalence. The clinical symptoms are pain, fractures, bone deformation and some neurological disorders in cases with craniofacial affection. Asymptomatic cases are incidentally found on radiographs. Imaging appearances of FD: FD may be assessed by different modalities. In general, conventional films are sufficient for diagnosis and especially in follow up of skeletal changes on axial skeleton. A sharply marginated and expansile lesion that is lucent relative to normal bone is seen on radiographs. FD affecting he craniofacial bone tends to be more variable. Radionuclide bone scanning of FD shows increased tracer accumulation on early perfusion and delayed bone imaging. These non-specific findings are only useful to assess the extent of skeletal involvement in poly-ostotic cases. CT imaging of FD reflects the macroscopic and microscopic features of expansile lesion with ground glass density, based on the medullary cavity of the affected bone. MRI is an excellent method for the assessment of complex cases of FD especially when affecting the craniofacial region, to observe the compression of neurological structures. This presentation is based on a retrospective study of 52 cases of FD affecting the extremities (mono-ostotic 16, poly-ostotic 2, craniofacial FD (mono-ostotic 18 and poly-ostotic 9) and 7 cases with McCune Albright syndrome (2 of them without bone affection). Two cases of cherubism and some other bone lesions with macroscopic similarity to FD will also be demonstrated. The different modalities used in evaluation of these presented cases will be shown. The various features of bone changes in FD will be an important subject of discussion in differential diagnosis with other skeletal diseases showing similar changes. The frequent incidence of FD and their MRI appearances will also be discussed.