Hi Amanda, I had an appointment ealier on in my treatment with the radiation Oncologist & his registrar who both went into great detail as to why they do radiation. The registrar in particular was very god at explaining this. She said that it is to mop up any stray cells like in between the scar tissue where I had my mastectomy & the lymph nodes near my neck & another area that I can't remember as they can't be removed/tested like the ones under the arm. I was very impressed with her & she explained my bc to me better than any of the others! This would not be an easy decision for you to make, I'm glad they advised me to have rads so I wasn't faced with that decision. Good luck.
Loretta xx