The patient proceeded to CT guided core biopsy

MACROSCOPIC DESCRIPTION: "L popliteal mass Bx": Three 2mm diameter pale tan core biopsies 5-20mm long. 

MICROSCOPIC DESCRIPTION: The core biopsies contain a densely cellular infiltrate, comprising many mononuclear cells as well as some multinucleated giant cells. They all have similar morphology with mildly enlarged nuclei and occasional conspicuous nucleoli. Many of the mononuclear cells contain refractile haemosiderin deposits in the cytoplasm. There are  intermixed lymphocytes, foamy macrophages and occasional eosinophils in the background. No abnormal mitoses are seen. There is no necrosis. The overall features are those of pigmented villonodular synovitis. There is no evidence of malignancy.

DIAGNOSIS: Left popliteal mass biopsy: Pigmented villonodular synovitis.