Low-grade Central Osteosarcoma with High-grade Transformation in the Distal Tibia: A Rare Case Report.

INTRODUCTION: Low-grade central osteosarcoma (LGCO) is a rare subtype of osteosarcoma, accounting for approximately 1-2% of cases. It typically exhibits indolent behavior but poses a diagnostic challenge due to its close radiologic and histologic resemblance to benign fibro-osseous lesions. Although LGCO generally has a favorable prognosis, it may undergo high-grade transformation, leading to more aggressive clinical behavior. CASE REPORT: We report a case of a 36-year-old male with type 2 diabetes mellitus who presented with recurrent right ankle pain following a remote history of trauma. Imaging revealed an expansile lytic lesion in the distal tibia. Initial biopsy suggested a benign fibroblastic lesion, favoring non-ossifying fibroma; however, repeat biopsy raised suspicion for LGCO based on spindle cell proliferation, irregular woven bone, mild nuclear pleomorphism, and focal osteoid formation. Positron emission tomography-computed tomography demonstrated a metabolically active lesion confined to the distal tibia with no evidence of metastasis. The patient underwent wide local excision followed by reconstruction with fibular tibialization and ankle arthrodesis. Final histopathology confirmed LGCO with high-grade transformation, demonstrating increased mitotic activity, marked nuclear atypia, vascular invasion, and osteoid production. Surgical margins were negative. DISCUSSION: This case is notable for its uncommon presentation in an adult patient, distal tibial involvement, and early high-grade transformation. The initial misdiagnosis highlights the critical importance of clinico-radiologic-pathologic correlation and the role of adjunct immunohistochemistry (MDM2/CDK4) in differentiating LGCO from benign mimickers. Limb salvage with fibular centralization proved to be an effective reconstructive strategy, achieving oncologic clearance while preserving function. CONCLUSION: LGCO can present diagnostic difficulties and may exhibit unexpected aggressive behavior due to high-grade transformation. Early recognition, accurate diagnosis, complete surgical excision, and vigilant long-term follow-up are essential to optimize outcomes. Distal tibial reconstruction using fibular tibialization and ankle arthrodesis remains a viable limb-salvage option.