2025, Vol. 7, Issue 2, Part B

Background: Giant cell tumor of bone (GCTB) is a benign but locally aggressive tumor that usually affects the epiphysis of long bones in young adults. Although most cases are effectively treated with intralesional curettage and local adjuvants, malignant transformation is a rare but devastating complication. Secondary transformation is typically associated with prior radiotherapy, while post-surgical sarcomatous change without radiation is exceptionally uncommon.

Case Presentation: We present the case of a 40-year-old male with progressive left knee pain and swelling. Imaging demonstrated a lytic lesion in the distal femoral condyle, and histopathology confirmed a benign GCTB. The patient was treated with curettage and defect filling using polymethylmethacrylate (PMMA) cement. Six years later, he experienced the onset of progressively worsening knee pain. Imaging studies showed suspicion of a malignant lesion. Biopsy confirmed a high-grade osteosarcoma arising in the previously treated site, consistent with secondary malignant transformation of GCTB. After multidisciplinary discussion, the patient received neoadjuvant chemotherapy, followed by en bloc resection of the distal femur and total knee arthroplasty with a tumoral prothesis. Histology confirmed a grade III osteosarcoma with clear margins, and adjuvant chemotherapy was completed. Two years later, routine surveillance detected a solitary pulmonary metastasis, which was biopsy-confirmed and managed with palliative radiotherapy. The patient remains under close oncological follow-up.