Hi Xue,
For a GCT to develop you don’t necessarily need a tendon sheath.
All you need is synovium.
This means it can occur within a joint capsule like PVNS of the knee, or be associated with a bursa.
Given that synovium is so prolific in the human body the potential to gave a GCT in almost any location seems possible.
Obviously tendon sheaths are the most common location but anywhere synovium exists is a potential site.
Hence an extensor tendon at a MCP joint seems a reasonable location for synvium to exist and hence potentially a GCT.
Steve
Steve