Rogz, my understanding is that radiation is so sophisticated and focused (literally) now, that the chance for heart/lung problems just isn't what it used to be. My advice: Talk with the radiation oncologist,and tell him/her your concerns, sparing no details of your history, your fears, etc. Have a discussion; don't let yourself be lectured to. And then make your decision. A mastectomy, of course, is much more difficult than a lumpectomy. It can involve long-term shoulder issues, pain, tingling, numbness... and then there's the decision to get reconstruction, have an implant, a prosthesis, or just go without. It's just complicated, and if you could avoid it, I would.
On the other hand - yes, if you're VERY fearful about radiation, consider a mastectomy seriously. But I'd still advise talking to the radiation oncologist first.
Good luck - PJH