Procedure: Left knee arthroscopy and synovial biopsy (5 months after the MRI). Microscopic description: Focal chronic arthritis and nodular collection of mononuclear cells admixed with multinucleated giant cells & hemosiderin pigment deposition. Diagnosis: Pigmented villonodular tenosynovitis. Negative for malignancy.

Procedure: Arthroscopic synovectomy (9 months after the synovial biopsy).  Diagnosis: Tenosynovial giant cell tumor (diffuse type). Tumor size: 6.5 cm (ex vivo). Tumor involves inked/cauterized margins. Negative for malignant transformation.