Bill, I may be all wet, and not knowing anything about your case, it's only a guess. I worked in the dental field 30 years and know that not all Dr's are equal. It's hard for them to think outside the box at times. Has your Dr. suggested cutting the bridge and saving at least one pontic with the front anchor crown/s?(Even if temporarily).
I only once saw it, but there exists a type of implant that is a framework over the bone, rather than into the bone. I wonder if that might be an option for post-cancer patients. I must say, the one case I saw was remarkable. she had no lower teeth,little bone and dentures she couldn't use and was very skinny. a few weeks after treatment, she was eating even corn nuts (not recommended) and gained 50 pounds in a few months. It certainly doesn't help the loss to date, but if it would work, might be a way to replace the bridge. Wish you good luck whatever comes and thanks. Your sharing may help a LOT of people.