This patient presented with recurrent knee swelling and pain. AP and lateral radiographs of the knee joint showed a moderate knee joint effusion without evident bony lesion. He had undergone knee joint fluid aspiration, which showed chronic non-specific synovitis.

MRI of the knee was performed, which showed a well-defined PD FS hyperintense and T1W intermediate intensity lesion in the patella with a small cystic component. The lesion showed moderate homogeneous enhancement. There was thinning and focal breaks in the anterior & posterior cortices of the patella, as seen on CT. A diagnosis of patellar giant cell tumor was made on radiological basis and tissue diagnosis was advised; however, the patient was lost to follow-up. 

Associated moderate knee joint effusion with synovial enhancement was noted, which was consistent with synovitis. No nodular synovial thickening noted.

The patella is the largest sesamoid bone and primary tumors of the patella are rare. Among the benign tumors, giant cell tumor is the most common followed by chondroblastoma. It can be affected by metastasis also.

Imaging features of patellar giant cell tumor are typical on MRI. As in our case, the lesion can be missed on conventional radiographs. In some cases, patellar GCT can be seen as an ill-defined lytic lesion with faded margins. 

Secondary aneurysmal bone cyst (ABC) can develop within the GCT. However, in our case, the possibility of secondary ABC was not raised as there was only a small unilocular cystic component with no fluid-fluid level.