He acknowledges the there is a mass though. I've been doing some reading online and from my findings edema does show up with a mass and can prevent them from finding it in an mri.
Edema is a fluid build up, generally lymphatic fluid.
An MRI can easily distinguish between tissue (a mass) and edema.
Your physician is advising you very appropriately.
The question is why isn't the area draining? That is the puzzle.
There may be an old injury.
Sauna treatment ans massage are sometimes helpful, as well as a mild anti-inflammatory such as motrin.