My differentials - Ganglion cyst – I agree with u guys, 'Ganglion cyst' is the most likely etiology - Giant cell tumor aka Pigmented villonodular synovitis is the 2[SUP]nd[/SUP] most common hand tumor and can arise from synovial membrane of joints and tendon sheaths ( acc to both Schwartz and Sabiston ) - Lipoma - Traumatic inclusion cyst aka implantation dermoid aka epidermal inclusion cyst ex. in gardeners - Synovial cell sarcoma