Giant cell tumor: patella

H/oHistory of recurrent swelling of the right knee joint. Previous knee joint fluid aspiration showed features of chronic non-specific synovitis.

23 year old maleThis 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 joint was done for the first timeperformed, which showed a well defined PDFS-defined PD FS hyperintense and T1wT1W intermediate intensity lesion in the patella with a small cystic component. LesionThe lesion showed moderate homogeneous enhancement. There was thinning and focal breaks in the anterior & posterior cortices of the patella, as seen on CT. DiagnosisA diagnosis of patellar giant cell tumor was made on radiological basis and tissue diagnosis was advised tissue diagnosis; however, the patient was lost to follow-up. 

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

PatellaThe 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 absentno fluid-fluid level.

Large intraosseous T2wT2W slightly hyperintense, T1wT1W intermediate and PDFSPD FS hyperintense lesion is noted, involving most of the patella with sparing of thin rim of bone on lateral side. Well defined T2w-defined T2W hyperintense cystic component is noted within the lesion without fluid-fluid levels. There are multifocal areas of discontinuities in both anterior and posterior cortices of patella with focal areas of grade IV chondromalacia. LesionThe lesion shows moderate homogeneous enhancement of the solid component with thin peripheral enhancement of cystic component. No pre-, para- or retropatellar soft tissue component noted. These imaging features are suggestive ofcompatible with a giant cell tumor of the patella.

ModerateThere is a moderate knee joint effusion is noted extending into suprapatellar recess with smooth, diffuse enhancement of the synovium -, consistent with synovitis. No evidence of intraarticularintra-articular loose bodies.

No major ligamentous or meniscal pathology is identified.

Axial CT images of the knee joint inin a bone window show a well defined-defined lytic lesion involving most of the patella with cortical thinning and focal cortical breaks -, compatiblewith consistent with giant cell tumor.

Moderate knee joint effusion is also noted.

1. What are the differentialsis a differential for a lytic patellar lesionslesion?

Benign lesions: Giant cell tumor (GCT), Chondroblastoma, Aneurysmal bone cyst (ABC). Malignant lesions: Osteosarcoma, metastases.