Fat tissue certainly can die as a result of radiation - either due to injury to the tissue from the radiation itself, or having the blood supply to the tissue cut off because of the radiation.
Some who have necrosis of adipose suffer no ill effects - it eventually liquefies and reabsorbs, and the area fills in with new healthy tissue. The big risk, however, is of abscess and/or infection - as a nurse I took care of a few people who had to deal with this, and it wasn't fun. Thus you are wise to keep seeking until you find help.
Also, ask your dr about the pathology report on the core biopsy. Are they absolutely sure that the necrotic tissue is adipose, and not dead cancer cells? Necrotic cancer cells also will usually reabsorb over time, without ill effects, but it's important to differentiate whether these are dead normal cells or part of a tumor that died. If there's any doubt that they might be tumor cells, you will want to make sure there is no evidence of live cells in the surrounding area. (PET scan might or might not be a good choice for this - if there is inflammation or infection, the scan might show a false positive.)
OK - I've not seen any messages about this subject on Cancer Compass in the past 8 - 9 months, but then I haven't seen every message on the board, either. If you don't get any other responses to your query, you might try doing an internal search on CC to see if there are any past message threads that would have answers for you. Check for cancers other than breast cancer - radiation can kill off adipose tissue in ANY part of the body, and necrotic adipose tissue might actually be more common in radiation of stomach and colon cancers. I imagine subsequent treatment would be much the same regardless of location.
Other resources you might try are www.drsusanloveresearchfoundation.org and http://ww5.komen.org/ (the Susan Komen foundation). They might have information on this, or you might try sending them an email asking if they can direct you to someone for treatment. I believe there is a "contact us" link on both sites.
I wish you good luck in getting answers, and hope that this turns out to be nothing more than a temporary concern!
Sincerely,
Tre