The purpose of this research study is to measure acid reflux into the throat both before and after medical treatment in people who have had radiation therapy to their head and neck for the treatment of cancer. Many people who have received head and neck radiation therapy develop a dry mouth as a result of the radiation damage to their saliva glands. In addition to the discomfort associated with a dry mouth, the decrease in saliva may increase the severity of gastro-esophageal reflux disease (acid reflux). Acid reflux occurs when acid escapes from your stomach into your throat. You may not have any symptoms of acid reflux, but often it can cause symptoms of heartburn or chest discomfort.