Osteosarcoma of the mandible arising de novo in fibrous dysplasia. Case report and review of the literature

SUMMARY: Introduction: Malignant transformation of fibrous dysplasia is rare, accounting 4% of cases with McCune-Albright syndrome and 0,5% of monostotic type.Case presentation: An 11-year old boy presented with hard swelling and disfigurement of his mandible in conjunction with alterations of his dental occlusion. Following biopsy, fibrous dysplasia was diagnosed. The patient underwent surgical contouring of the affected bone. Four years later a fast growing mass developed at the same site of the operated mandible. After an incisional biopsy the diagnosis of low-grade osteosarcoma was established. The patient underwent wide mandibulectomy and reconstruction of the defect with bonegraft and is free of the disease for the last 14 years.Conclusion: Most cases of malignant transformation of FD are associated with irradiation of the fibrous dysplasia.The spontaneous transformation of the monostotictype is extremely rare. The development of a soft tissue mass or elevation of serum alkaline phosphatase in a patient with fibrous dysplasia should be considered alarming features and require further investigation to exclude sarcoma.