Malignant Transformation of Giant Cell Tumor of the Distal Femur into Telangiectatic Osteosarcoma: A Rare Case Report

Giant cell tumor (GCT) is a benign but locally aggressive lesion with rare malignant transformation. Telangiectatic osteosarcoma (TOS) is an uncommon aggressive osteosarcoma variant that can arise secondary to GCT, posing diagnostic challenges. A 36-year-old male presented with pain and swelling in the distal femur. He underwent curettage, cementing, and fixation for biopsy-proven GCT. Despite surgery, symptoms persisted, and radiographs at 6 months showed progressive destruction and cortical breach. Repeat biopsy confirmed malignant transformation to TOS, supported by histopathology and immunohistochemistry (H3.3G34W, SATB2 positivity, Ki-67 40%). Positron emission tomography-computed tomography showed a destructive fluorodeoxyglucose-avid distal femoral lesion with intramuscular extension, lymphadenopathy, and pulmonary metastases. The patient was planned for neoadjuvant chemotherapy, limb-salvage surgery, and adjuvant chemotherapy. Malignant transformation of GCT into TOS is rare but aggressive. Persistent symptoms after curettage warrant re-evaluation with biopsy and staging for timely management.