Lymph nodes are tiny, bean-shaped organs that fight infection and disease and filter cancer cells. A sentinel lymph node biopsy (often shortened to sentinel node biopsy, or SNB) is a procedure where one or a few lymph nodes are removed from under the arm and examined for evidence of cancer. When cancer spreads from the breast, it travels through the lymphatic system. The first lymph node or group of lymph nodes encountered is called the sentinel node. In most patients, if no evidence of cancer is found in the sentinel node, it can be expected that no cancer will be found in the remaining axillary (underarm) lymph nodes. Knowing whether the cancer has spread helps determine the stage and approach to treatment. That is, cancer that has spread to the lymph nodes may be treated differently than cancer that has not spread to the lymph nodes.
To locate the sentinel node, the surgeon injects a harmless, radioactive substance and/or a blue dye into the breast near the tumor. Then, the surgeon makes an incision under the arm and either follows the radioactive signal or finds the lymph nodes that are stained blue and removes them. Usually, between one and three nodes are removed. A pathologist (a doctor who specializes in interpreting laboratory tests and evaluating cells, tissues, and organs to diagnose disease) then carefully examines the sentinel node(s) for evidence of cancer.