Lymph node mapping

A procedure known as sentinel lymph node mapping is recommended when there is an increased risk the melanoma may have spread to nearby lymph nodes. Also known as lymphoscintigraphy, this procedure tracks the exact path of lymph (the bodily fluid that carries white blood cells) as it drains from the skin surrounding the melanoma to the nearest lymph node. The draining lymph node closest to the melanoma is called the sentinel lymph node (SLN). Identifying which lymph node is the SLN is important because the decision to remove lymph nodes often depends on whether melanoma has spread to an SLN.

How is lymph node mapping done?

Lymphoscintigraphy is a special type of imaging technique done in a hospital’s nuclear medicine department on the day before or the day of surgery to remove the melanoma. A radioactive tracer is injected into the skin around the site of the melanoma, and an imaging device that detects radioactivity makes a series of images that show the path of the radioactive material as it travels to the nearest group of lymph nodes.

What is an SLN biopsy?

In the operating room, the surgeon will inject a blue dye into the skin around the site of the melanoma to visually identify the SLN (see Figure 1). The surgeon will then make a small incision in the area of the lymph nodes and remove the SLN, which can be identified by the blue dye and the presence of the radioactive tracer as detected by a hand-held device. This procedure is called an SLN biopsy. The node is then carefully examined by a pathologist for the presence of melanoma cells. Because the SLN is the first place to which lymph drains from the site of the melanoma, it’s highly unlikely the melanoma has spread to any other lymph nodes if no cancer cells are found in the SLN.

This procedure involves a team of experienced physicians: A radiologist who specializes in nuclear medicine imaging reviews the images; a surgeon who injects the blue dye and performs the biopsy; and a pathologist who evaluates tissue from the SLN to see if melanoma cells are present.

Who should have lymph node mapping and SLN biopsy?

Two major organizations of cancer experts – the American Society of Clinical Oncology (ASCO) and the Society of Surgical Oncology (SSO) – both recommend that the procedure be performed for all people who have a melanoma of the skin (at any place on the body) that is 1 to 4 millimeters thick.

Although there’s not enough evidence of treatment benefit for an SLN biopsy for melanomas less than 1 mm thick, the procedure should be considered if a thin melanoma has certain aggressive features, such as:

  • Positive deep surgical margins (making the true thickness uncertain)
  • Ulceration (the melanoma has broken the skin covering it)
  • Lymphovascular invasion (melanoma cells are found in lymph vessels)
  • Mitotic rate of 1/mm² or more and a thickness of 0.7 mm or greater

An SLN biopsy should also be considered as part of the staging of thicker melanomas (more than 4 mm) if the information will help guide additional surgery and other treatments, and the risks of the procedure are acceptable.

What are the benefits and risks?

The primary benefit of lymph node mapping and SLN biopsy is that they accurately identify people at low risk for recurrent disease; these people can then be spared a lymph node dissection and additional treatment after surgery. Additionally, recent evidence has shown a substantial survival benefit for patients with intermediate-thickness melanomas (1.2 to 3.5 mm thick) with a positive SLN biopsy who then had a complete regional lymph node dissection. The rate of complications associated with the procedure alone is low, especially compared with the rate of complications after a lymph node dissection.

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