You do not necessarily destroy all your glands permanently with IMRT. What exactly gets nuked depends on radiation field and strength and the person. Although IMPT sounds great, if it delays the regular treatment for a long time it is not advisable. The other concern is how much data there is for such a complex combination treatment.
If you want to use IMRT on the whole neck you need a significant amount of radiation to get a desired effect, otherwise what was the point. In that case there will be radiation damage the severity again depending on the field/strength.

That said let us know what you are going to do and what your treatment plan is.

M