If the mole or growth turns out to be melanoma, your doctor will need to learn the extent, or stage, of the disease. The stage of the disease depends on how thick the tumor is, how fast the tumor cells are dividing, whether it is cracked or bleeding (ulcerated), and how far it has spread.
The best way to find out how far melanoma may have spread is through a sentinel lymph node (SLN) biopsy.
The lymph nodes are glands that are part of the body's immune system—the system that helps protect against disease. Your body has many lymph glands (or nodes), especially in the armpit, neck, and groin.
A fluid called lymph flows through the lymph glands. Lymph is similar to blood and contains white blood cells, proteins, fats, and other substances. The lymph system helps drain bacteria and other harmful substances from the body. Sometimes, cancer cells can break off from a tumor and enter the lymph. As the fluid flows through the lymph nodes that are near the tumor, the cancer cells can get trapped in the lymph node and begin to grow there.
The first lymph nodes that the fluid from the tumor flows into are called the "sentinel lymph nodes." A sentinel is someone who stands guard. You can think of the sentinel lymph node as the gatekeeper to the rest of the lymph nodes. If the sentinel lymph node has cancer cells in it, there is a chance that the cancer has spread. If the sentinel node does not have cancer cells, the other lymph nodes in that area are probably also cancer-free, and the cancer probably has not spread.
The surgeon will inject a blue dye and a radioactive tracer around the tumor site. The lymph fluid carries the blue dye and tracer away from the tumor, to the nearest lymph nodes. The surgeon looks for the lymph node that has blue dye in it (or uses a detector to find the lymph node that has the highest amount of tracer). This is the sentinel lymph node. The surgeon removes this node and a pathologist will test it to see whether it contains cancer cells.
If the sentinel node is cancer-free, the remaining lymph nodes in that area should also be cancer-free. If the sentinel node has cancer, then other lymph nodes in the area may be removed to stop the disease from spreading further.
The SLN biopsy is an important step in diagnosing melanoma. It is the most accurate way of finding out how far the melanoma may have spread. This helps your doctors select the best treatment for you.
An SLN biopsy can also help you avoid unnecessary surgery. For example, if the sentinel node is cancer-free, you will probably not need to have any other lymph nodes removed.