Kelly got the idea and answer right, but you have two parotid glands, they are the major producers of all the saliva in your mouth and they are damaged because they are in your cheeks (not in the back of your throat) in front of your ears, and all the way down near the edge of your mandible, right where a lot of the radiation has to pass through. Remember that the people programing IMRT and other targeted software controlled radiation delivery techniques these days, have to know anatomy like a surgeon. They are doing their best in each patient to deliver the maximum radiation for effect while mapping around vital structures that the consequences of damage to are long term QOL issues. They map as much away from your spinal column, major nerve bundles, salivary glands, etc. But at the end of the day, there are thousands of angle to approach something from, and not all of those angles will cover the needed area and still COMPLETELY spare something like the salivary glands. They not only map areas, from a pin point to a brush stroke - but intensity and diffusion of the beams, so there is a lot to consider when the personal map for any patient is drawn.