Lymph nodes are small clumps of immune cells that act like filters, catching viruses and damaged cells in the lymphatic system. The lymphatic system carries fluid and cells throughout the body, similar to the blood stream.

If breast cancer spreads beyond the breast, the first place it usually goes is the axillary lymph nodes in the underarm area. Whether these lymph nodes contain cancer affects breast cancer staging and prognosis. If the axillary lymph nodes contain cancer, it’s called lymph node-positive. If they don’t, it’s lymph node-negative. Prognosis is better when breast cancer has not spread to the lymph nodes. Whether or not axillary lymph nodes need to be removed depends on certain factors, such as the breast cancer diagnosis, type of breast surgery, age, menopausal status and if the lymph nodes showed signs of cancer on an ultrasound. If axillary lymph nodes will be removed, a sentinel node biopsy is the most common way to check them for breast cancer, and it’s usually done during breast surgery. Before or during the procedure, a radioactive substance called a tracer, or a blue dye, is injected into the breast to help the surgeon detect any cancer in the lymph nodes. The first axillary lymph nodes to absorb the radioactive tracer, or dye, are called the sentinel nodes. During the procedure, the surgeon finds these nodes by identifying which lymph nodes have absorbed the tracer or dye. This is done by using a special device called a gamma probe or by spotting the lymph nodes that have turned blue from the dye. The radioactive tracer, or blue dye usually identifies 1 to 5 nodes as sentinel nodes. The surgeon removes these nodes and sends them to a pathologist for review. The removal doesn’t mean cancer is present; it just allows the pathologist to check the nodes for cancer.

A pathologist examines the removed nodes under a microscope. If cancer is present, the pathologist counts the number of nodes that have cancer and includes this information in a pathology report. If there’s a positive sentinel lymph node, the surgeon may perform an axillary lymph node dissection. Axillary dissection is more invasive and removes more lymph nodes than a sentinel node biopsy. Some women with 1 to 2 positive sentinel nodes who have a lumpectomy followed by whole breast radiation therapy may not need axillary dissection.

Most people with one or more positive sentinel nodes who have a mastectomy will need either an axillary dissection or radiation therapy to the axillary lymph nodes.

An axillary dissection may also be done in certain cases like when the lymph nodes are already known to contain cancer.

For more information on lymph node status, visit the Diagnosis section on komen.org.