My mri suggests possible 5mm synovial cyst on tmj joint. my neurologist says not to worry they are common. what are the chances it's cancer?

I would not worry: 5 mm sized cysts are benign. Unless it grows in size to over one inch, I would not do anything right now. Your doctor can monitor it once every 6 to 12 months and tell you if it is growing or not. if it does not grow in size, you can leave it alone.

Chances of it: being cancer is very small. Cancer in this area is not common. It usually occurs in children or teenagers. Adults may have symptoms as a result of a cancer elsewhere in the body.