Sorry to hear of the pain issue, but it's good you're doing well otherwise. I had similar lately, but was caused by whiplash being a passenger in a car, which the driver suddenly stopped, jerking my head forward and back, and felt a twinge. I couldn't turn my head for months from that, but it's ok now. I see you're quite active. Do you think any of them could contribute to the problem, especially turning the head while swimming to breath, biking to check your sides, tje bouncing from running, etc. I used to swim 1 sometimes 2 miles at the Y, when I was healthy lol, and would have a sore neck afterwards, next day. 1 mile was 88 laps. There can be other causes from the radiation, as mentioned fibrosis, and does other things to muscles, nerves. Radiation has short term toxicities, less than 3 months, and long term toxicities after 3 months.

Good luck.