Regional lymph nodes are frequently the first location where cancer regionally spreads. Clinically, they are also called “sentinel lymph nodes”. In the breast those nodes are located in the armpit or collarbone near the area where the cancer was originally detected. In certain conditions, your doctor will check to see if you have cancer in your regional lymph nodes. In specific cased your doctor may perform regional lymph node radiation on top of the local radiation to the breast.