Usually, the surgeon will also perform a sentinel lymph node dissection, which means that 1-3 lymph nodes under the arm on the side of the tumour will be removed to check whether the cancer has spread there. The sentinel lymph nodes are the first lymph nodes to which cancer might spread. A procedure called lymphatic mapping is done either the day before, the morning of, or during the surgery to help the surgeon locate the sentinel nodes. This involves injecting a radioactive liquid, a blue dye, or both, underneath the nipple or near the tumour site.
