Sentinel lymph node biopsy (SLNB) for melanoma

What is a sentinel lymph node biopsy?

A sentinel lymph node biopsy (SLNB) is a way of checking the lymph nodes closest to the melanoma. You may have this test even if your lymph nodes do not look or feel swollen. It can help to find very small amounts of melanoma that have spread to the lymph nodes.

The sentinel node is the first node lymph fluid drains to from the melanoma. If the melanoma has spread, the sentinel node is the most likely to be affected. There may be more than 1 sentinel node.

This test is not suitable for everyone. Your doctor or specialist nurse may talk to you about having an SLNB if the melanoma is:

  • 1mm or thicker
  • thinner than 1mm, but there are other risk factors.

You usually have an SLNB at the same time as surgery to remove a surrounding area of healthy tissue from where the melanoma was (called wide local excision).

An SNLB provides more accurate information about:

Your doctor will ask you to decide whether to have an SLNB. They will explain the possible advantages and disadvantages. It may help you and your doctor to plan the best treatment for you. If you decide not to have an SLNB, you still have the wide local excision.

If there are no melanoma cells in the sentinel lymph nodes, it is unlikely the melanoma has spread to other lymph nodes. You will not need further tests or treatment.

What happens if the SLNB is positive?

If there are melanoma cells in the sentinel lymph node, your doctor will talk to you about the best way of managing this. This may depend on:

  • the amount of melanoma cells that have spread (the melanoma’s size)
  • whether there are melanoma cells in more than 1 lymph node.

You and your doctor may decide that having regular ultrasounds of your lymph nodes is the best option for you. This monitors whether the cancer is growing in that area and whether you need treatment.

Or your doctor may ask you to think about further treatment with targeted or immunotherapy drugs. This aims to reduce the risk of the melanoma coming back. It is called adjuvant treatment.

Your doctor may talk to you about surgery to remove all the nearby lymph nodes. It is not clear how helpful this operation is after a positive SLNB, so this is less common. The operation also has long-term side effects.

Before an SLNB

You need a scan to find out which lymph nodes are the sentinel lymph nodes. You usually have the scan on the day of your surgery or the day before.

Having the scan

You will be asked to lie down. You may be given a local anaesthetic cream to rub onto the skin to numb the area. A harmless amount of radioactive liquid is injected into the area where the melanoma started. The dose is very small, and it quickly loses its radioactivity.

The radioactive liquid travels through the skin’s lymphatic vessels. It drains into the lymph nodes closest to the melanoma. These are the sentinel lymph nodes.

You usually then have a scan using a special camera. It finds the nodes that picked up the radioactivity and shows them on a screen. The skin is marked over the sentinel node or nodes to help the surgeon find them easily and remove them. The scan finds the sentinel nodes but cannot show whether melanoma cells are in the lymph nodes.

Radioactive liquid is injected where the melanoma started

Sentinel lymph node biopsy: Radioactive liquid is injected where the melanoma started  

What happens during an SLNB?

You have the SLNB at the same time as the wide local excision. You have it under a general anaesthetic. Your nurse will explain how to prepare for this operation.

Blue dye is injected

Sentinel lymph node biopsy: Blue dye is injected  

The surgeon removes the blue and radioactive nodes which are the sentinel nodes through a cut or cuts they make on the skin. The nodes are then looked at under a microscope to check for melanoma cells. It may be up to 2 weeks before you get the results of the SLNB.

Side effects of an SLNB

Your doctor and nurse will explain the side effects and any possible complications of an SLNB.

Because you only have the sentinel nodes removed, there are fewer side effects than having all the lymph nodes removed. For example, you are less likely to have an infection or bruising or bleeding. Lymphoedema (swelling) is a possible complication of having all the lymph nodes removed. But the risk of lymphoedema after an SLNB is very low.

You usually have self-dissolving stitches. You will have a scar where the nodes were removed.

The blue dye may cause the following side effects:

  • Blue or green urine (pee) – you may notice this for up to 2 days after the SLNB. This is the body’s way of flushing out the blue dye. It is not harmful.
  • Allergic reaction – there is a small risk of this after you have the dye. Your surgeon will monitor you closely. If you have a reaction, you will be given drugs to treat it straight away.

About our information

  • References

    Below is a sample of the sources used in our melanoma information. If you would like more information about the sources we use, please contact us at

    Michielin O, van Akkooi ACJ, Ascierto PA, et al. Cutaneous melanoma: ESMO Clinical Practice Guidelines for diagnosis, treatment and follow-up. Annals of Oncology. 2019; 30, 12, 1884-1901 [accessed May 2022].

    Michielin O, van Akkooi ACJ, Ascierto PA, et al. ESMO consensus conference recommendations on the management of locoregional melanoma: under the auspices of the ESMO Guidelines Committee. Annals of Oncology. 2020; 31, 11, 1449-1461 [accessed May 2022].

    Peach H, Board R, Cook M, et al. Current role of sentinel lymph node biopsy in the management of cutaneous melanoma: A UK consensus statement. Journal of Plastic, Reconstructive & Aesthetic Surgery. 2020; 73, 1, 36-42 [accessed May 2022].

  • Reviewers

    This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert medical and health professionals and people living with cancer. It has been approved by Senior Medical Editor, Dr Samra Turajlic, Consultant Medical Oncologist.

    Our cancer information has been awarded the PIF TICK. Created by the Patient Information Forum, this quality mark shows we meet PIF’s 10 criteria for trustworthy health information.

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Date reviewed

Reviewed: 01 October 2022
Next review: 01 October 2025
Trusted Information Creator - Patient Information Forum
Trusted Information Creator - Patient Information Forum

Our cancer information meets the PIF TICK quality mark.

This means it is easy to use, up-to-date and based on the latest evidence. Learn more about how we produce our information.