Hi there... I'm going to leave the mandible information to Christine and Eric as they definitely know what they are talking about as they have both been through it.

The opening of your mouth may have something to do with the damage to you mandible, but it may also be caused by radiation damage. I would suggest physio, and possibly acupuncture to help with it. There are others here who use a therabite, or something home made, but usually radiation damage has to do with shortening and tightening of the muscles and improved blood flow to these areas helps soften the tissue. That's why I suggested acupuncture. I have been doing it for a year now. It really does help and my naturopath also suggested a form of massage to the inside of my mouth that helps stretch the right muscles.

hopefully this helps but definitely have your jaw looked at by an ENT.

good luck