Diagnostic Criteria for Fibrous Dysplasia
Fibrous dysplasia is diagnosed through the integration of clinical presentation (painless bone swelling, deformity, or incidental finding), characteristic radiographic features (ground-glass appearance on imaging), and histopathological confirmation showing replacement of normal bone and marrow by fibrous tissue. 1, 2
Clinical Presentation
The clinical diagnosis begins with recognizing the pattern of bone involvement:
- Monostotic form (single bone involvement) is most common, occurring in approximately 70-80% of cases and frequently identified incidentally as asymptomatic lesions 1, 2
- Polyostotic form (multiple bone involvement) affects multiple skeletal sites and may be associated with endocrine dysfunction and café-au-lait spots (McCune-Albright syndrome) 3
- Craniofacial fibrous dysplasia involves the skull base and facial bones, with long bones, ribs, and craniofacial bones being the most commonly affected sites 2, 4
Key clinical features include bone pain, bone deformities, pathologic fractures, and painless swelling that develops gradually over months to years 1, 3, 4
Radiographic Criteria
Imaging is essential for diagnosis and should include:
- CT scanning is the primary imaging modality, revealing the characteristic "ground-glass" appearance of lesions with ill-defined borders 4, 5
- MRI with gadolinium enhancement is useful for differentiating fibrous dysplasia from other fibro-osseous lesions, identifying neoplastic foci within dysplastic tissue, and distinguishing benign from malignant bone lesions 5
- Radiographic findings show expansion of affected bone with a ground-glass or hazy appearance on plain films and CT 2, 4
A critical caveat: MRI enhancement patterns must be carefully interpreted, as irregular or intense enhancement may indicate malignant transformation (fibrosarcoma) or alternative diagnoses such as multiple myeloma or ossifying fibroma 5
Histopathological Confirmation
Definitive diagnosis requires biopsy showing:
- Replacement of normal bone and marrow by fibrous tissue 1
- Characteristic histological pattern of fibro-osseous tissue 2, 3
- Biopsy is indicated for confirmatory diagnosis, especially when radiographic findings are atypical or when malignant transformation is suspected 2, 5
Genetic Basis
The underlying etiology involves:
- Activating mutation in the GNAS gene (encoding the alpha subunit of stimulatory G protein) located at chromosome 20q13.2-13.3 2, 3
- This is a post-zygotic, non-inheritable mutation resulting in osteoblastic differentiation defects 3
Differential Diagnosis Considerations
When evaluating suspected fibrous dysplasia, exclude:
- Ossifying fibroma 5
- Multiple myeloma (particularly if irregular MRI enhancement is present) 5
- Low-grade fibrosarcoma (malignant transformation, identified by intense enhancement on MRI) 5
- Myeloproliferative disease 5
The combination of CT and contrast-enhanced MRI is recommended for comprehensive evaluation, as MRI proves particularly useful in defining clinical behavior and identifying potential malignant foci within dysplastic tissue 5