Chondromyxoid Fibroma-Like Osteosarcoma in a 16-Year-Old: A Rare Case Report

Introduction: Chondromyxoid fibroma-like osteosarcoma (CMF-like OS) is a rare subtype of osteosarcoma that exhibits low-grade histological features but high-grade aggressive behaviour. It shares overlapping characteristics with other benign and malignant bone lesions, often complicating diagnosis and management. Case Presentation: We report a case of a 16-year-old male presenting with a painful, progressively enlarging mass on the proximal tibia over three years. Initial imaging suggested a giant cell tumor or aneurysmal bone cyst. Histopathological examination revealed proliferative stellate and spindle cells in a chondromyxoid stroma, with cellular atypia and osteoid matrix areas. Immunohistochemical analysis showed positive staining for MDM2, S-100, and low P53. Based on clinical, radiologic, and histologic findings, CMF-like OS was diagnosed. The patient was started on neoadjuvant chemotherapy with plans for surgical resection. Discussion: CMF-like OS is often misdiagnosed due to its resemblance to benign lesions such as chondromyxoid fibroma. Unlike benign lesions, CMF-like OS exhibits aggressive features, including osteoid production and potential for metastasis. Radiologic findings, though non-specific, may indicate malignancy when there is cortical destruction and soft tissue extension. Histological and immunohistochemical analyses are essential for distinguishing CMF-like OS from other bone tumors. Conclusion: CMF-like OS is a diagnostic challenge requiring meticulous histologic and immunohistochemical evaluation alongside clinical and radiologic correlation. Early diagnosis and prompt treatment are crucial, but further research is needed to establish definitive management guidelines and improve prognosis for this rare entity. Keywords: aggressive bone malignancy, chondromyxoid fibroma-like osteosarcoma, chondromyxoid stroma, neoadjuvant chemotherapy, osteosarcoma subtype, tumor diagnosis