A Rare Clinical Presentation With a Difficult Imaging Diagnosis of an Intra-articular Clear Cell Sarcoma of the Knee.

Clear cell sarcoma, a highly aggressive cell sarcoma with melanotic differentiation, typically occurs in the tendon and aponeuroses of the lower extremities and rarely develops in the intra-articular region. Herein, we present an extremely rare case of clear cell sarcoma originating from the intra-articular knee and suspected as benign tumors due to the benign-looking findings on MRI and PET/CT. The image results in our case were completely consistent with a tenosynovial giant cell tumor, resulting in the misdiagnosis. For differential diagnosis, especially malignant melanoma, histopathology, including IHC and FISH, was indispensable.