Malignant Transformation of Monostotic Fibrous Dysplasia after Surgery: A Case Report

Introduction: Fibrous dysplasia (FD) is a benign lesion characterized by replacement of normal bone with abnormal connective tissue. It occurs in monostotic or polyostotic forms, with a rare but proven potential for malignant transformation. Symptoms of acute pain, rapid swelling, or an enlarging mass should increase suspicion for possible sarcomatous change. Complete surgical resection is the mainstay of treatment, and chemotherapy is recommended to improve survival. Case Report: A 52-year-old male presented with a painful, enlarging mass on the right proximal thigh 2 years after undergoing plate fixation of the distal femur for a pathologic fracture secondary to monostotic FD. Diagnostic imaging revealed signs of recurrence, and core needle biopsy revealed aggressive features suggestive of malign ant transformation. Following surgical resection and chemotherapy, the outcome has been uneventful without evidence of recurrence or metastasis at 4-year post-operation. Conclusion: Malignant transformation in monostotic FD is rare. Symptom exacerbation should increase the suspicion for sarcomatous change and prompt the need for diagnostic imaging as well as histologic confirmation.