If you’ve been diagnosed with breast cancer or melanoma, your doctor may have told you that the cancer could spread to nearby lymph nodes and from there, to other, more distant, parts of the body.

It’s important for your doctor to know if the cancer has spread so that he or she can formulate the best course of treatment for you. To do this, your doctor might recommend that you undergo a surgical procedure called a sentinel lymph node biopsy (SLNB).

In this procedure, a surgeon removes one, or in some cases, a few sentinel lymph nodes—the lymph nodes to which cancer first spreads—near the site of the original tumor. For a patient with breast cancer, for instance, the surgeon will remove lymph nodes in the armpit closest to the affected breasts. The lymph nodes are then examined to check for the presence of cancer cells.

In the past, before SLNB was in use, to determine whether cancer had spread to the lymph nodes, surgeons typically removed nearly all of the lymph nodes near the site of the cancer. This subjected patients to a major surgery with a substantial risk for long-term complications.

But today, doctors can use SLNB to accurately determine if cancer has spread without the need for major surgery and the risks that come with it.

Adults have hundreds of small, bean-shaped structures called lymph nodes spread throughout the body. Among other places, clusters of lymph nodes are found in the armpit, neck, abdomen, groin, and chest.

Each of these lymph nodes is part of the lymphatic system, a network of vessels that drains a colorless fluid called lymph away from the body’s tissues and into the bloodstream. The system is crucial to maintaining fluid balance in the body.

Lymph is made up of various substances including water, cells of the immune system, waste products, and sometimes outside invaders like viruses or bacteria. It can also contain cancer cells.

Before lymph reaches the bloodstream, it passes through the lymph nodes. The lymph nodes play a critical role: They filter out waste products, viruses, bacteria, and if they are present, cancer cells. The immune system kills many of the cancer cells that reach the lymph nodes. But some cancer cells may escape and migrate to other lymph nodes or even to distant parts of the body.

When cancer cells spread from some types of tumors, they migrate to nearby lymph nodes. For example, when certain types of breast cancer spreads, cancer cells frequently move to the lymph nodes in the armpit (where there are 20 to 30 lymph nodes) before spreading to other parts of the body.

But the cancer cells do not reach each of these lymph nodes simultaneously. That’s because the lymph nodes are organized in such a way that the cancer cells reach one lymph node, or sometimes a few lymph nodes, before moving on to the others. The lymph nodes to which cancer cells first spread are known as sentinel lymph nodes.

Doctors make decisions on how to best treat cancer patients based on several factors, including whether the cancer has spread to the lymph nodes.

One way of determining whether certain types of cancer have begun to spread is via SLNB. Before performing this surgical procedure, the surgeon must first identify the sentinel lymph nodes. 

To do this, a small amount of radioactive material, a blue dye, or both are injected into the tissue near the primary tumor. These substances drain through the lymphatic vessels and into nearby lymph nodes. The first node into which the injected substances drain is the sentinel lymph node.

If radioactive dye was injected, the surgeon will use a radiation-detecting device (like a Geiger counter) to locate the sentinel lymph node. If blue dye was injected, the sentinel lymph node will be stained blue, which allows the surgeon to locate it. 

The surgeon then removes the sentinel node through a small incision. (He or she may remove more than one lymph node if there are two or more sentinel lymph nodes.) After the sentinel lymph nodes are removed, a pathologist examines them under a microscope to see if they contain cancer cells.

If cancer cells are present (a “positive” biopsy result), then the surgeon may remove the remaining lymph nodes from the area and further testing is necessary to determine whether the cancer has spread beyond the lymph nodes. In some cases, instead of removing all remaining lymph nodes, doctors may proceed with radiation therapy, chemotherapy, and other cancer therapies or opt to monitor the status of the remaining lymph nodes. Monitoring may involve physical exams, imaging studies, or other tests.

If no cancer cells are detected (a “negative” biopsy result), then it is unlikely that the cancer has spread to the lymph nodes. In this case, it is not necessary to remove the remaining lymph nodes.

In most cases, side effects are minor and short-lived. They may include bruising, pain, numbness, discomfort, swelling, or bleeding at the incision site. In some cases, SLNB can cause a seroma, which is a buildup of fluid at the site of the surgery. If the surgeon injected blue dye for the procedure, it may be visible under the skin for two to three months (or longer) and urine may look bluish for the first day or two after the procedure.

In some cases, after a SLNB, the ability of the remaining lymph nodes to effectively drain fluid from nearby tissues is impaired. This may result in a long-term condition called lymphedema, which is characterized by the swelling of the tissues around the site of surgery. 

For instance, if SLNB is performed on the lymph nodes in an armpit, the lymphedema may cause the tissues in the arm to swell. Around 6% of women with breast cancer who undergo SLNB of lymph nodes in an armpit and 1% of people with melanoma who undergo SLNB develop lymphedema.

Other long-term complications that may result from SLNB include numbness near the biopsy site. People who undergo SLNB of the lymph nodes in the armpit, may have problems with shoulder movement and a reduced range of motion.

Last medically reviewed by Melanie Lynch, MD in March 2022.