Monarch Disease Ontology entry MONDO_0024686 (tenosynovial giant cell tumor, diffuse type) is 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. Also known as: TGCT, TSGCT, Tenosynovial Giant Cell Tumor, diffuse giant cell neoplasm of Tenosynovium, diffuse giant cell neoplasm of tendon sheath, diffuse giant cell neoplasm of the Tenosynovium, diffuse giant cell tumor of Tenosynovium, diffuse giant cell tumor of tendon sheath.