Sorry I missed this post when I got back the other day. The aero digestive tract encompasses what you would expect, those portions of the anatomy associated with breathing and digestion. So for example the esophagus, the trachea, the pharynx, the larynx, the lungs, the stomach, etc. all would be considered components of the aero-digestive tract. Most are lined with the same types of tissue as the oral environment, and therefore are not only close to where the primary cancer has occurred, but are also made up of the typical type of tissue that SCC (the most common oral cancer type) is associated with.