Sentinel Lymph Node Biopsy for Melanoma: What to Expect

The Bottom Line

A sentinel lymph node (SLN) biopsy is a minimally invasive procedure that checks whether melanoma has started to spread to your lymph nodes. It is the most accurate way to find out if cancer cells have traveled beyond the original tumor. For melanomas between 1-4 mm thick, this procedure identifies hidden spread in about 5-40% of patients and directly shapes your treatment plan. The procedure is low-risk and is usually done at the same time as the main melanoma removal surgery.

What Is a Sentinel Lymph Node Biopsy?

Lymph nodes are small glands scattered throughout your body that act as filters for your immune system. When melanoma spreads beyond the original tumor, it typically travels first through lymphatic vessels to the nearest group of lymph nodes. The very first lymph node it reaches is called the sentinel lymph node.

A sentinel lymph node biopsy (SLN biopsy) is a procedure where surgeons identify and remove that specific first lymph node (or a small cluster of 1-3 nodes) and send it to a pathologist for detailed examination. If cancer cells are found in the sentinel node, that tells your doctors cancer has begun to spread. If no cancer cells are found, it is very likely the remaining lymph nodes are also clear.

This is the most sensitive and accurate staging tool available for intermediate-thickness melanoma. It identifies hidden (occult) spread in 5-10% of patients with melanomas 1-4 mm thick, and up to 40-50% of patients with melanomas thicker than 4 mm.

Who Needs This Procedure?

Your dermatologist or oncologist will recommend SLN biopsy based primarily on how thick your melanoma is (called Breslow depth):

  • Standard recommendation: Melanomas 1-4 mm thick. This group has the most to gain from knowing lymph node status.
  • May be considered: Thin melanomas (less than 1 mm) that have high-risk features—such as ulceration, a mitotic rate of 1 or more per mm², deep invasion, or patient age under 40.
  • Variable recommendation: Very thick melanomas (over 4 mm) have a high chance of nodal spread (30-50%), but the staging benefit is debated since additional testing is often needed regardless.

The procedure is not recommended if you are pregnant (it can be safely deferred until after delivery) or if prior surgery or radiation in the area makes lymph node identification technically difficult.

How the Procedure Works

The SLN biopsy is typically performed under general or local anesthesia at the same time as your wide local excision (the surgery to remove the melanoma with clear margins). Here is the step-by-step process:

Step 1: Lymphatic Mapping (Day Before or Day of Surgery)

A small amount of radioactive tracer (technetium-99m sulfur colloid) is injected around the area where your melanoma was. Nuclear imaging is then used to trace where the tracer flows in your lymphatic system, identifying which lymph node basin (groin, armpit, or neck) is the draining zone for your melanoma.

Step 2: Dye Injection in the Operating Room

Just before surgery begins, your surgeon injects a blue or green dye around the melanoma site. This dye travels through the lymphatic vessels and visually marks the sentinel node(s).

Step 3: Finding and Removing the Sentinel Node

The surgeon uses a handheld radiation-detection device (gamma probe) to locate the radioactive sentinel node(s), confirmed by their blue dye staining. Typically 1-3 sentinel nodes are removed through a small incision. The surrounding lymph nodes are left in place.

Step 4: Pathology Examination

The removed nodes go to the pathology lab, where they are cut into very thin serial sections and examined under a microscope. Special staining techniques (immunohistochemistry using markers like S100, Melan-A, and HMB-45) are used to detect even very small clusters of melanoma cells that regular staining might miss.

What Do the Results Mean?

Negative Result (No Cancer Found)

Your melanoma has not spread to the regional lymph nodes as far as testing can detect. Depending on your primary tumor's thickness, your 5-year survival rate for node-negative disease is:

  • Stage I (thinner tumors): Over 90%
  • Stage II (thicker tumors): 70-90% depending on characteristics

A negative SLN is very reassuring. You will typically continue with surveillance appointments rather than systemic treatment.

Positive Result (Cancer Cells Found)

If melanoma cells are detected in the sentinel node, your stage is updated to Stage III. The amount and size of cancer in the node influences your specific substage:

  • Stage IIIA (micrometastasis, tiny cancer deposits): 5-year survival approximately 78%
  • Stage IIIB (more than 1 involved node or larger deposit): approximately 59%
  • Stage IIIC (4 or more involved nodes): approximately 40%

A positive result changes your treatment plan. Your oncologist will discuss adjuvant (additional) therapy options that can reduce your recurrence risk by 50-70%.

What Happens After a Positive SLN Biopsy?

If your sentinel node is positive, you and your care team will make decisions about next steps:

  • Observation: In many cases, especially when metastasis is small (micrometastasis), surgeons now watch the remaining lymph nodes with imaging rather than removing them all. This approach avoids lymphedema and other complications while not compromising survival.
  • Complete lymph node dissection: If there is obvious nodal disease or extensive sentinel node involvement, your surgeon may remove the remaining nodes in the affected region.
  • Adjuvant immunotherapy: Drugs like pembrolizumab (Keytruda) or nivolumab (Opdivo) are now recommended for Stage III melanoma after SLN biopsy, reducing recurrence risk by 25-30% compared to observation alone.
  • Targeted therapy: If your melanoma has a BRAF V600E mutation (present in about 60% of melanomas), dabrafenib plus trametinib is an additional option for adjuvant treatment.

When to See a Dermatologist or Surgeon

  • You have been diagnosed with melanoma and your doctor has recommended staging
  • Your melanoma biopsy showed a Breslow depth between 0.8 mm and 4 mm
  • You have a thin melanoma with concerning features (ulceration, high mitotic rate)
  • You want to understand your staging options before surgery
  • You have questions about whether adjuvant therapy is appropriate for you

Frequently Asked Questions

How is the SLN biopsy different from removing all my lymph nodes?

A complete lymph node dissection (CLND) removes all lymph nodes in an entire region (e.g., all nodes in your armpit or groin). An SLN biopsy removes only the 1-3 first-draining nodes. Because CLND removes many more nodes, it carries higher risks of lymphedema (swelling), nerve injury, and infection. SLN biopsy provides accurate staging information with much lower complication rates. CLND is now reserved for cases with obvious nodal disease rather than being done routinely after a positive SLN.

Is the radioactive tracer dangerous?

No. The amount of radiation used for lymphatic mapping is very small—far less than a standard CT scan. The tracer clears from your body quickly and poses no meaningful radiation risk to you or your family.

What does it mean if the SLN is positive but the tumor is small?

Even small deposits (called micrometastases—less than 2 mm in size) in the sentinel node officially change your stage to Stage III. However, micrometastases generally carry a better prognosis than larger (macroscopic) deposits. Your survival outlook depends on the size and number of affected nodes, as well as your primary tumor's characteristics. Your oncologist will tailor adjuvant therapy recommendations to your specific situation.

Can I still have an SLN biopsy if I had the melanoma removed at another facility?

Yes, in most cases. The procedure can be performed separately from the wide local excision, though it is more technically straightforward when done together. If the wide excision has already been done, your surgeon can still inject tracer and dye to map the draining nodes, though the mapping may be slightly less precise depending on how much tissue was removed.

References

  1. Morton DL, Thompson JF, Cochran AJ, et al. Final trial report of sentinel-node biopsy versus nodal observation in melanoma. N Engl J Med. 2014;370(7):599-609.
  2. Swetter SM, Tsao H, Bichakjian CK, et al. Guidelines of care for the management of primary cutaneous melanoma. J Am Acad Dermatol. 2019;80(1):208-250.
  3. Gershenwald JE, Scolyer RA, Hess KR, et al. Melanoma staging: Evidence-based changes in the American Joint Committee on Cancer eighth edition cancer staging manual. CA Cancer J Clin. 2017;67(6):472-492.
  4. Faries MB, Thompson JF, Cochran AJ, et al. Completion dissection or observation for sentinel-node metastasis in melanoma. N Engl J Med. 2017;376(23):2211-2222.
  5. Weber JS, Mandala M, Del Vecchio M, et al. Adjuvant nivolumab versus ipilimumab in resected stage III or IV melanoma. N Engl J Med. 2017;377(19):1824-1835.

Trusted Resources

Always consult a board-certified dermatologist or surgical oncologist for personalized advice about your melanoma treatment and staging options.