Given it's epiphyseal location, what is the differential diagnosis?
Chondroblastoma, giant cell tumour (GCT), clear cell chondrosarcoma, aneurysmal bone cyst (ABC) (usually metaphyseal, but if it occurs after the growth plate is obliterated then it may extend into the epiphysis).
What features help favour GCT over chondroblastoma?
GCTs are more common, and have a predilection for this location (chondroblastomas are more frequently seen around the knee). Chondroblastomas tend to have a thin sclerotic narrow zone of transition, whereas GCT usually do not have sclerosis and often (when agressive) have a wider zone of transition.
X-rays of the right forearm and wrist demonstrate a lytic expansile lesion involving the distal epiphysis of the radius. It abuts the radiocarpal joint (involving the epiphysis) and results in dorsal subluxation of the distal ulna. There is no matrix calcification. The margins are irregular with an aggressive appearance and relatively broad zone of transition. There is no convincing periosteal reaction. The carpus and ulnar appear unremarkable other than osteopaenic, presumably due to disuse.