Hi HappyGirl, I was radiated in the neck and lower jaw area, and some on the shoulders. My pain was in those areas towards the end. If your getting your treatment in the chest area, it would stand to reason that you could be having some pain in your back associated with the radiation. Of course I can't say what it is, but I always told my Dr. about anything that was new or unusual. They will tell you if it's something to be concerned about.
Here is a link to our Beginner's Guide to Cancer with a specific guide to what to expect with radiation treatments. Take a look and see if there is anything in there to help you. http://bit.ly/10BQKCi