Malignant Tenosynovial Giant Cell Tumor Presenting as an Extra-Articular Superficial Soft-Tissue Mass in a Knee.

Malignant tenosynovial giant cell tumor (TsGCT) is a rare disease that can arise as a recurrent lesion or co-exist with a benign TsGCT lesion. Here we report a rare case of malignant TsGCT in a 73-year-old male with a history of lymphoma. The tumor appeared as a superficial soft-tissue mass in the subcutaneous fat tissue of the left knee.