Pediatric Distal Femoral Surface Tumor With Extraosseous Lesion Mimicking Parosteal Osteosarcoma: A Diagnostic Challenge.

Background/Aim: Parosteal osteosarcoma is a low-grade surface osteosarcoma that most commonly arises from the posterior aspect of the distal femur. In pediatric patients, a surface-based sclerotic lesion accompanied by an extraosseous component may closely resemble parosteal osteosarcoma, creating a diagnostic dilemma. Case Report: A six-year-old boy was referred with right knee discomfort and progressive difficulty in deep flexion. Imaging revealed a broad-based sclerotic surface lesion on the posterior distal femur with an extraosseous component, closely resembling previously reported pediatric parosteal osteosarcoma, although an open biopsy revealed no malignant features. A hemicortical excision without internal fixation was performed, resulting in a cortical defect involving <25% of the femoral circumference. Histopathological examination confirmed a sessile-type osteochondroma. Postoperatively, the patient achieved full weight-bearing and unrestricted knee motion. Radiological remodeling was observed at six months without fracture or recurrence. Conclusion: Sessile-type osteochondroma of the pediatric distal femur can radiologically and clinically simulate low-grade parosteal osteosarcoma by presenting as a heavily ossified exophytic surface mass with an extraosseous component. Histopathological validation coupled with the absence of MDM2 amplification provides definitive differentiation between these entities, guiding appropriate bone-conserving hemicortical excision without the need for internal fixation.