What is a “sentinel lymph node” and why does it need to be biopsied if my tests show that there is no spread to the lymph nodes?

The sentinel lymph node is the first lymph node(s) that drains the breast. Therefore, if the cancer has spread beyond the breast, chances are that the first place it will go to will be the sentinel lymph node, which is usually located in the armpit on the same side as the cancer. Sometime small deposits of cancer can be missed on ultrasound or needle biopsy, so even though there has been no proof of spread there, the breast surgeon will usually remove 1-3 sentinel lymph nodes at the time of the breast cancer surgery to more definitively exclude spread of cancer cells there.

There are usually a couple of different ways to identify which lymph nodes are the sentinel lymph nodes. One method is a special test called a lymphoscintigraphy will be done usually the day before the planned surgery. A special dye which emits a very faint radioactive signal will be injected into the breast, usually around the cancer, then special scans will be done to “see” the lymph nodes, then at the time of surgery, the surgeon will use a probe which detects the signal and identifies the correct lymph nodes for removal. Another method is injecting patent blue dye into the breast just before surgery, when the patient is already under the general anaesthetic, and then looking for a blue coloured lymph node to remove. Sometimes both methods are utilized.