I had radiation 3 times on the same side of the left neck for 5 recurrences. First round was 35 IMRT with 60-70 Grays. 75 grays is usually the max due to unacceptable tissue failure, at least in the H&N, and salivary glands with with even less. There are ways they increase dosage with less toxicity with split sessions, accelerated dosage, etc. The 2nd radiation I had was done during surgery for a neck dissection with the neck open (3rd time) using HD-IORT...high dose Intraoperative Radiation Treatment, which is also external beam radiation at 15Grays. Anything above 20 Grays with IORTwas found to have to have too high of a failure rates, including death. A pectoral flap was then done during surgery, after radiation, to protect the neck, especially the carotid artery, to assist in healing, and In order to do IMRT again after healing with new tissue protecting the neck. The RO who did the surgery with the ENT, said this was the safest way to do radiation again, and my only chance to survive after my recurrences. I started IMRT in October for 5 weeks, 25 daily sessions, with a total of 50 Grays.

In all, I had 135 Grays, which is almost double the max usually given, so there are ways to do radiation again, like I did, and the way others mentioned.

After my 3rd and 4th recurrence, my original RO did not want to do any radiation after surgery, no chemo either, saying I already received the max 70gy and it
was too risky to do more, saying, "Radiation can always be done, but once given, you can't take it back." Only after the 5th Cancer is when they agreed to do it again, but I did not like the risks involved, and sought out the treatment described above, which reduced the risk, and increased my odds for survival.

You are about right with the percentages, and it's a real concern when it's happening to you, so look into everything you can.

With all the chemo, radiation, surgeries I still eat by mouth, and mostly everything, except spices. I did lose my taste, and have difficulty swallowing due to just completing treatment this past Monday, but expect to get that back.

Good luck with everything.