A locally aggressive, diffusely infiltrating tumor, arising in the tendon sheath. It is composed of synovial-like mononuclear cells, hemosiderin-laden macrophages, foam cells, and inflammatory cells. Multinucleated osteoclast-like giant cells are usually present, although in a minority of cases they may be absent or rare. It predominantly affects young adults. Symptoms include joint swelling, pain, and joint effusion.