Jeff is right on.
You need to consider the irradiation volume and location with respect to the submandibular, sublingual and parotid (major) saliva glands. If they are fully in the irradiation field the probably are going to be nuked regardless (your RO should know more). If they are not in the radiated volume then they are fine anyway. In my case the submandibular an sublingual were in the radiation field and were fully toasted while one of the parotids was totally spared and the other received partial rads (from which at least a portion if not all was expected to recover. Which it did.
M