American Melanoma Institute · 5 min read
What a sentinel lymph node is, who is usually offered the biopsy, how mapping works, the risks, and what a positive result leads to today.
Lymph fluid drains from every patch of skin through tiny channels to a nearby cluster of lymph nodes. When melanoma cells leave the skin, they usually travel the same route. The sentinel node is the first node on that path, and sometimes 2 or 3 nodes share the role.
A sentinel lymph node biopsy finds those first nodes, removes them, and checks them for melanoma under the microscope. The biopsy is a staging test. It tells doctors whether a melanoma that looks confined to the skin has already sent cells to the nodes.
The decision rests mostly on the thickness and ulceration reported on the biopsy. The joint guideline from the American Society of Clinical Oncology and the Society of Surgical Oncology (Wong and colleagues, Journal of Clinical Oncology, 2018) lays it out by T category:
National Comprehensive Cancer Network guidance frames the same idea as a probability. When the estimated chance of a positive node is under 5%, the procedure is generally discouraged. Between 5% and 10%, doctors discuss and consider it, and above 10% they usually offer it.
Other findings can push a borderline case one way or the other, including a high mitotic rate, melanoma cells in lymphatic vessels, a very young age, or a biopsy that cut through the base of the tumor. General health matters too, since the procedure usually involves general anesthesia.
On the day of surgery, or the afternoon before, a nuclear medicine team injects a small amount of radioactive tracer into the skin around the melanoma site. A special camera follows the tracer as it drains and shows which node group receives it. This matters most on the trunk, head, and neck, where drainage can go to more than one region.
In the operating room, many surgeons also inject a blue dye at the same spot. The surgeon makes a small incision over the node area, uses a handheld probe to find nodes giving off a radioactive signal, and looks for nodes stained blue. Those nodes are removed. Most people have between 1 and 5 sentinel nodes.
The wide excision of the melanoma site is usually done during the same operation. Surgeons prefer this order because a large excision done earlier can disturb the lymph channels and make mapping less accurate. Most patients go home the same day.
The pathologist slices each node thinly and uses immunostains that highlight melanoma cells, so even a small cluster can be found. Results typically take about a week or longer.
Sentinel node biopsy is a smaller operation than a full lymph node dissection, and its complication rates are lower. The National Cancer Institute lists these possible harms:
On that last point, a pooled analysis of 71 studies covering about 25,000 patients found that melanoma returned in the same node region in 5% or fewer of people whose sentinel nodes were negative.
If the sentinel nodes are clear, the chance that other nodes in that region contain melanoma is very low, and no further node surgery is needed. The stage stays at I or II. Follow-up consists of regular skin and lymph node exams.
Melanoma in a sentinel node makes the cancer stage III. Until 2017, the standard next step was a completion lymph node dissection, an operation to remove all remaining nodes in that region. One large trial changed that.
The MSLT-II trial (Faries and colleagues, New England Journal of Medicine, 2017) followed about 1,900 patients with a positive sentinel node. Half had an immediate completion dissection. The other half kept their remaining nodes and had regular ultrasound exams of the area, with surgery only if melanoma showed up later.
At 3 years, melanoma-specific survival was 86% in both groups. Dissection did lower the rate of melanoma returning in that node region, and it gave extra information about outlook. It also caused lymphedema in 24.1% of patients, compared with 6.3% in the observation group.
Because survival was the same, most patients with a positive sentinel node are now followed with exams and ultrasound, and completion dissection is kept for selected cases. The American Cancer Society describes observation as the more common path today.
A positive node also opens a separate conversation about adjuvant treatment, meaning immunotherapy, or targeted therapy for tumors with a BRAF gene change, given after surgery to lower the chance of recurrence. The size of the deposit in the node, measured in tenths of a millimeter, is one of the details oncologists use in that discussion.
