Development of a Non-ossifying Fibroma in the Same Femur 6 Years after Limb-Salvage Surgery for Osteosarcoma: A Case Report

Abstract

Abstract Introduction: A new osteolytic lesion detected during surveillance after osteosarcoma treatment usually raises concern for local recurrence, a skip lesion, metastasis, or a treatment-related secondary process. Non-ossifying fibroma (NOF) is a common benign fibrous lesion in children and adolescents, but its delayed appearance in the same surgically reconstructed femur is unusual and may create a major diagnostic dilemma. Case Report: A 12-year-old child presented with a 4-month history of pain and swelling in the right thigh. Imaging and core needle biopsy confirmed osteosarcoma of the femur. The patient received three cycles of doxorubicin- and cisplatin-based neoadjuvant chemotherapy, followed by wide resection, extracorporeal radiotherapy of the resected segment, reimplantation, and plate fixation. The patient remained symptom-free with good limb function for 6 years. Routine surveillance then identified a new, well-defined cortically based osteolytic lesion in the distal femoral metaphysis. Computed tomography (CT) demonstrated sclerotic margins without cortical breach or a soft-tissue component, while positron emission tomography-CT showed only low-grade metabolic activity. Serial imaging and the absence of symptoms supported a radiological diagnosis of NOF, and observation was continued. At 8 years of follow-up, the patient remained asymptomatic with satisfactory knee motion and no evidence of osteosarcoma recurrence. Conclusion: Benign lesions should remain in the differential diagnosis when a new lytic lesion is detected during long-term osteosarcoma surveillance. Correlation of lesion morphology, metabolic activity, clinical findings, and interval behavior is essential to avoid misdiagnosis and overtreatment.