There is no legal limit, as far as I know, how much radiation a patient may receive, and never heard or read of a lifetime radiation limit exposure many speak about, although I believe I've read "whole body" radiation at once, 10Gy, would likely cause death, and there are toxicity limits for certain structures that above such would cause unacceptable toxicities such as to the brain stem, spinal cord, brachial plexus, etc, which are usually limited in Grays, and daily fractions. Non cancer involved Parotid Gland may receive less than 30Gy, around 50% volume, so it can recover, produce saliva, as is one reason for dose sparing IMRT, but your case is probably different being it was involved.

For radiation workers there are set exposure limits, but as far as patients, they're pushing the limits each year with different types of radiation, Grays, fractions, volume, flaps, etc, and it's becoming more common to possibly reirridate in select patients at experienced centers as I have had radiation 5 times in three years. Twice was with IMRT, twice with IORT during surgery, and my last time was with Proton therapy, totaling over 200Gy to the same area, which is more than any of my doctors know of.

As far as recurrence rate, it seems to be on par with some non HPV head and neck cancer, but many factors can be involved, type of cancer, and stats may be outdated, and as one ENT said, "We don't have a crystal ball." As someone who has had 7 recurrences in four years, I try not to think much of recurrences, at least now the further I'm out from treatment, but surgery is often the preferred treatment with recurrences, which I did twice alone, and others minus or plus radiation, possibly with chemo, but I do think I may have reached my own radiation limit, at least in specific areas, which resulted structures removed, like carotid, veins, nerves, and side effects, which may seem unacceptable, like arm paralysis, muscle atrophy, neuropathy, due to the brachial plexus radiation exposure, but I've been through worse.

Best I can say is to enjoy yourself, exercise, eat healthy, maintain a healthy weight, avoid infections to maintain healthy immune system, have adequate sleep, no smoking, limit or no alcohol, do self monitoring, educate yourself to this disease, and see your doctors as scheduled for follow-up, exams, and scans.

I hope this helps.