Most breast cancers move in fairly predictable ways, and depending on the type and location of your tumor, your doctor may recommend a sentinel node biopsy as a means of determining whether your cancer has spread beyond your breast. It’s a diagnostic tool used fairly early in your breast cancer journey that can confirm and add detail to your diagnosis.

The sentinel nodes are lymph nodes adjacent to the breast where your tumor is located. The word “sentinel” means a soldier or guard that keeps watch over an area or person, and in this meaning, the sentinel node is the one keeping first watch over the breast. The Mayo Clinic explains that “the sentinel nodes are the first few lymph nodes into which a tumor drains.”

[See: A Tour of Mammographic Screenings During Your Life.]

Lymph and Lymph Nodes

According to the National Cancer Institute, “lymph nodes are small, round organs that are part of the body’s lymphatic system.” They’re scattered throughout the body and are connected to one another via a complex network of lymph vessels. These organs cluster in the neck, underarms, chest, groin and abdomen and help circulate lymph fluid throughout the body.

This lymph, the NIH reports, is “interstitial fluid that has diffused, or ‘leaked’ out of small blood vessels called capillaries.” The fluid contains proteins, glucose, oxygen and other substances and it “bathes most of the body’s cells, providing them with the oxygen and nutrients they need for growth and survival.” In short, lymph and lymph nodes are critical components of the body’s immune system and can trigger the first line of defense against bacteria, viruses and other foreign invaders into the body.

As such, the lymph nodes are typically the first places where cancer cells will travel to once a tumor has progressed to the stage where it begins to spread beyond its point of origin. In many breast cancer cases, doctors seek out the sentinel lymph node or nodes, which the NIH defines as “the first lymph node to which cancer cells are most likely to spread from a primary tumor.” For breast cancer patients, these sentinel lymph nodes are typically found under the arm.

Most people have anywhere from 20 to 40 lymph nodes clustered in this underarm, or axilla, region, and determining which of these nodes is the sentinel node, or the one most likely to receive cancer cells first if they’re on the move, requires the use of a tracer. Dr. Erica Mayer, senior physician at the Susan F. Smith Center for Women’s Cancers at the Dana-Farber Cancer Institute, explains that to find the sentinel node, the doctor will “inject a fluid into the breast that has two tracers. One is a color, a blue dye, and the other is a bit of radioactive dye.” That fluid then drains from the tumor “under the arm, and the idea is — and this was an amazing surgical breakthrough when it was developed — the sentinel node would be the first one to receive drainage from the breast.” Mayer says by looking for the lymph nodes that turn blue soon after injecting the dye, the doctor can tell which lymph nodes the breast drains to first.

[See: 10 Lessons From Empowered Patients.]

Dr. Mary Gemignani, attending surgeon at Memorial Sloan Kettering Cancer Center and the program director for surgical fellowship, says this process “maps the highway to the lymph nodes under the arm to see if the cancer has spread there.” It’s a clever way to see exactly how each individual patient’s body works, since each one of us has a unique drainage pathway.

During a sentinel node biopsy, which is often conducted in conjunction with a lumpectomy to remove the tumor, the surgeon will take the lymph node or nodes that turned blue from the tracer fluid. Breastcancer.org reports that in most patients, “the surgeon usually removes a cluster of two or three nodes — the sentinel node and those closest to it.” These nodes will be analyzed in a lab, and if cancer cells are found, that means the cancer has spread to the lymph nodes, which could also mean that it will be found in other organs or distant parts of the body. “This information can help a doctor determine the stage of the cancer (extent of the disease within the body) and develop an appropriate treatment plan,” the NIH reports.

Mayer says the use of sentinel node biopsies was pioneered in the 1990s as an alternative to axillary lymph node dissections, in which all of the lymph nodes from under the arm are removed. This radical procedure can result in serious side effects, such as lymphedema — a buildup of lymph fluid where the lymph nodes used to be that can be painful and disabling — so having a less invasive way of checking the region for the spread of cancer tends to reduce the severity of these potential side effects.

Although a sentinel node biopsy is typically less invasive than an axillary lymph node dissection, it can still have side effects. Some patients will experience pain, swelling, numbness or tingling in the area, infection, bruising or the development of lymphedema at the site.

[See: 16 Health Screenings All Women Need.]

Another potential problem with sentinel node biopsy is the possibility of a false negative, in which cancer cells are not detected in the sentinel lymph node but are still present elsewhere in other regional nodes or in other parts of the body. The NIH reports “a false-negative biopsy result gives the patient and the doctor a false sense of security about the extent of cancer in the patient’s body.”

Breastcancer.org also warns that sentinel node biopsies “must be done by a surgeon who has significant experience with the technique.” If you have any doubts about your surgeon, you should consider seeking a second opinion.

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What Happens During a Sentinel Node Biopsy for Breast Cancer? originally appeared on usnews.com