This procedure is done the day of surgery and is used to help identify the sentinel lymph node, which is the first lymph node to which cancer cells are likely to spread from the primary tumor. Radioactive tracer will be injected into the skin near the areola. This special dye will determine which lymph nodes are the main draining lymph nodes. The tracer leads your surgeon to your sentinel lymph nodes, which may help to determine if they need to be removed.