Osteosarcoma of the patella mimicking giant cell tumor: imaging features with histopathological correlation.

Patellar tumors represent an uncommon etiology of anterior knee pain and their diagnosis is often delayed. We present an unusual case of conventional osteosarcoma arising in the patella of a 47-year-old man. The patient presented with a 1-year history of increasing anterior knee pain and swelling. Plain radiographs revealed a multi-locular lytic lesion in the inferolateral side of the patella. Computed tomography scans demonstrated an intraosseous lytic lesion with cortical thinning/breakthrough anteriorly. On magnetic resonance imaging, the lesion exhibited low signal intensity on T1-weighted images and heterogeneous high signal intensity on T2-weighted images. Fluid-fluid levels were also observed on T2-weighted images. Contrast-enhanced fat-suppressed T1-weighted images demonstrated strong enhancement of the lesion. These imaging features were suggestive of a benign condition; however, the diagnosis of osteosarcoma was confirmed by histopathology. After neoadjuvant chemotherapy, a wide resection with a free anterolateral thigh flap was performed. The patient subsequently underwent adjuvant chemotherapy and had no evidence of local recurrence or distant metastasis six months after surgery. Our case highlights the difficulty in the diagnosis of patellar osteosarcoma and the importance of performing a biopsy before definitive treatment.