A newer procedure performed in order to determine whether breast cancer has spread to auxiliary (underarm) lymph nodes. A blue radioactive tracer and/or blue dye is injected into the area of the breast tumor. The lymphatic vessels carry the dye or radioactive material, to a ”sentinel_node”. This sentinel_node is thought to be the first lymph node receiving fluid from the tumor and the one most likely to contain cancer cells if the cancer has spread. Only if the sentinel node contains cancer cells are more lymph nodes removed.