There are several types of benign tumors which grow on the sheaths of tendons - usually involving the tendons of the hand, though others can be affected.

The most prevalent types are Giant Cell Tumors, and a less common sub-type called Tendon Sheath Fibromas. Despite more than a century of study, neither has had its root cause(s) explained.

Giant cell tumor of the tendon sheath (GCTTS) is a benign tumor, presenting as the second most common mass of the hand after ganglion cysts. It was first described by Chassaignac in 1852 as fibrous xanthoma and has since been referred to by multiple names, including localized nodular tenosynovitis, pigmented villonodular proliferative synovitis, sclerosing hemangioma, benign synovioma, proliferative synovitis, xanthoma, xanthogranuloma, xanthosarcoma, myeloid endothelioma, fibrohemosideric sarcoma, giant cell fibrohemangioma, pigmented villonodular tenosynovitis, fibroma, myeloma, myeloxanthoma, and fibrous histiocytoma.
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No consensus exists on the etiology, prognostic factors, or recurrence rate of giant cell tumors of the tendon sheath.

Source : Eplasty. 2012; 12: e50.

Note: The large number of different names for the condition probably reflects its common mis-diagnosis and relatively unclear definition.