Sentinel Lymph Node Biopsy for Melanoma Staging

Lymph fluid drains from your skin in a predictable direction. The sentinel node is the first node along that path. It is the first place melanoma cells would land if any had left the original site.

To find it, the surgeon injects a small amount of radioactive tracer near the melanoma, usually along with a blue dye. A camera and a handheld probe then locate the node or nodes that take up the tracer. Those specific nodes, commonly one to three, are removed through a small incision and examined under a microscope. This is normally done during the same operation as the wide local excision, which is the surgery that removes the melanoma itself.

Thickness drives the conversation. The ASCO and Society of Surgical Oncology guideline puts it this way:

• Thin, non-ulcerated melanoma under 0.8 mm (T1a) — routine sentinel node biopsy is not recommended.
• 0.8 to 1.0 mm, or under 0.8 mm with ulceration (T1b) — may be considered, after a full discussion of benefits and harms.
• Over 1.0 to 4.0 mm (T2 or T3) — recommended.
• Over 4.0 mm (T4) — may be recommended, again after discussion.

Other pathology features can shift the advice within those bands. So can your age and general health. Those features include the mitotic rate, meaning how fast the cells are dividing, and lymphovascular invasion, meaning cancer cells seen inside lymph or blood vessels.

This is mainly a staging test. It gives the single strongest piece of prognostic information available after the melanoma itself. It also decides whether you are a candidate for adjuvant drug therapy, meaning treatment given after surgery, or for clinical trials.

• Negative — no melanoma cells found. Most people fall here. The risk of regional nodes later becoming involved is about 5% or less.
• Positive — melanoma cells were found. This raises the stage and opens the conversation about treatment after surgery.

Be clear about what it is not. Removing the sentinel node is not itself expected to cure the melanoma.

For years, a positive sentinel node was followed almost automatically by completion lymph node dissection. That means removing every remaining node in that basin. The MSLT-II trial tested that assumption. It randomised 1,939 patients with a positive sentinel node to one of two paths: immediate completion dissection, or observation with regular ultrasound of the node basin.

• Three-year melanoma-specific survival was 86% in both groups (P=0.42).
• Disease-free survival was slightly better with dissection (68% versus 63%), driven by better control of disease within the nodes.
• Lymphedema occurred in 24.1% after dissection, versus 6.3% with observation.

Lymphedema is lasting swelling caused by damage to lymph drainage. In plain terms: taking out all the nodes gave better regional control and more staging detail. But it did not help people live longer, and it caused roughly four times as much lymphedema.

That result changed practice. Many people with a positive sentinel node are now offered careful observation with scheduled ultrasound of the node basin instead of completion dissection. Dissection is kept for higher-risk situations, or if nodal disease appears later. The trial did not settle what to do for people who cannot reliably attend frequent follow-up. That is a legitimate part of the discussion to have openly.

These are uncommon but real. They include a seroma, which is a collection of fluid at the site; numbness or tingling; bruising; and wound infection. There is some risk of lymphedema, lower than after full dissection but not zero. The blue dye can also cause reactions, and it can briefly tint urine or skin. False negatives happen too: the node can be clear while cells have gone elsewhere.

Your pathology report is the document that matters here. Ask for the Breslow thickness, which is how deep the melanoma goes in millimetres. Ask whether the melanoma is ulcerated, and ask for the mitotic rate. Then ask how those specific numbers shape the recommendation you are being given. If a sentinel node biopsy is being offered rather than recommended, that wording is deliberate. It means the balance is close enough that your own view of the trade-off belongs in the decision.

• Call 911 or go to an emergency department if breathing becomes difficult, or your face or throat swells, in the hours after the dye injection. A severe reaction to the blue dye is uncommon and cannot wait.
• Call your care team the same day if the wound edges look inflamed, feel warm, or ooze, or your temperature reaches 100.4°F (38°C) or above. Wound infection after a node biopsy is treatable and should not be watched at home.
• Call your care team within a day or two if fluid collects into a lump at the biopsy site, or the nearby limb feels heavier or tighter than before. Also report any new lump in the node basin you are having watched by ultrasound.

• Completion Dissection or Observation for Sentinel-Node Metastasis in Melanoma (MSLT-II, NEJM 2017)
• MSLT-II full text (PubMed Central)
• NCI — Melanoma Treatment (PDQ®) Patient Version
• American Cancer Society — Tests for Melanoma Skin Cancer