What to Know About Sentinel Lymph Node Mapping

Medically Reviewed by Poonam Sachdev on November 12, 2021

Sentinel lymph node mapping or lymphatic mapping is an important step in invasive breast cancer detection. It's used to identify whether breast cancer cells are spreading to your other body parts. Lymphatic mapping is followed by a sentinel lymph node biopsy for accurate diagnosis. 

Lymph nodes are small bean-shaped structures present throughout your body. They are connected through lymphatic vessels and are a part of the immune system that filters foreign particles like bacteria. Breast cancer cells can travel through the lymphatic vessels toward the lymph nodes in your underarm area. 

Lymphatic vessels from the breast connect to these lymph nodes and drain into one or more nodes before moving to others. The first node into which breast cancer cells spread or are drained into is called the sentinel or guarding lymph node.

The sentinel lymph node can be identified by lymphatic mapping. A radioactive agent or isotope called technetium 99 and a blue dye called isosulfan blue are injected into the breast tissue close to the site of cancer. This helps identify the sentinel node or nodes into which breast cancer cells get drained. If cancer has not spread the dye won’t be present. But if it has spread, it will be detected in the sentinel node or nodes.

The sentinel lymph node mapping procedure lasts for 20 minutes. Your doctor or nurse will inject a small amount of the radioactive liquid under the areola of your affected breast. It may sting or burn.

After this, you’ll have to massage near the injection site for 10 minutes to allow the injected liquid to travel to the sentinel node. Your doctor will then measure the radioactivity in your breast, underarm, and thigh on the affected side to check if the liquid drained in your lymph vessels. This can be done using a handheld device or full-body imaging.

After lymphatic mapping, your sentinel nodes will be identified and surgically removed to analyze whether the cancer cells are present. This procedure is called sentinel lymph node biopsy. It helps your doctor make an informed decision about your treatment and predict treatment outcomes. 

For a couple of days after sentinel lymph node mapping, your body will continue to expel blue dye through urine. The dye is safe for your body, so don’t worry if your urine appears blue or green. Make sure you drink plenty of liquids to eliminate the dye faster.

Show Sources


JAMA Oncology: “Lymphatic Mapping and Sentinel Lymph Node Biopsy for Breast Cancer.”

Memorial Sloan Kettering Cancer Center: “Lymphatic Mapping with Sentinel Node Biopsy.”

National Cancer Institute: “Sentinel Lymph Node Biopsy.”

Torrance Memorial: “Sentinel Lymph Node Mapping.”

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