Locally aggressive fibrous dysplasia mimicking malignancy: a report of four cases and review of the literature.

BACKGROUND: Fibrous dysplasia is a benign fibroosseous bone tumor that accounts for 5% to 10% of benign bone tumors. It can present as monostotic fibrous dysplasia (70% to 80%), polyostotic fibrous dysplasia (20% to 30%), McCune-Albright syndrome (2% to 3%), or Mazabraud's syndrome in rare cases. Bone lesions in fibrous dysplasia arise in the medullary canal and usually are confined to the bone. Cortical destruction and extension into soft tissue usually indicates malignant transformation or secondary aneurysmal bone cyst formation. Locally aggressive fibrous dysplasia with cortical destruction and extension into soft tissue in the absence of these two possibilities is extremely rare. It is important for the treating physician to distinguish this entity from more aggressive or malignant tumors to avoid overtreating the patient for a benign condition or inattention to a malignant tumor. CASE DESCRIPTIONS: We report four unusual cases of fibrous dysplasia with an aggressive radiographic appearance. They occurred in the rib (1), ilium (2), and distal femur (1). Two patients had pain and two had swelling. Radiologically, all were associated with cortical destruction and an associated soft tissue mass, and initially they were interpreted as potentially malignant. Three patients underwent biopsy and one patient did not have a biopsy. Histopathologic analysis by an experienced bone pathologist confirmed fibrous dysplasia in all patients. Two patients were treated surgically; one patient with zoledronic acid and one patient currently is being followed by observation alone. LITERATURE REVIEW: There are only a few reports in the literature that describe the locally aggressive variant of fibrous dysplasia that presents with pain and progressive swelling clinically and with cortical destruction and soft tissue extension on imaging which suggest malignancy. We could not find any article that describes the use of bisphosphonates in such lesions or the response to bisphosphonates clinically, on laboratory parameters or imaging. To our knowledge, this is the largest case report published regarding locally aggressive fibrous dysplasia arising outside the craniofacial skeleton. CLINICAL RELEVANCE: The locally aggressive variant of fibrous dysplasia may be confused with a malignant tumor or malignant degeneration of fibrous dysplasia. It is important to properly evaluate these lesions to ensure that a proper diagnosis is made, especially with respect to a malignant versus benign mass.