Sentinel lymph node biopsy is a way to look at the lymph nodes without having to remove all of them first. For this test, a radioactive substance and a dye are injected near the tumor or under the nipple. They are carried by the lymph system to the first (sentinel) node to receive lymph from the tumor. This lymph node is the one most likely to contain cancer cells if the cancer has spread. Once this node is found, it is removed and examined. If the sentinel node contains cancer, more lymph nodes are removed. If it is free of cancer, further lymph node surgery might not be needed. This type of biopsy is complex, so it is best to have it done by experts who have significant experience in doing it.