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Sentinel node biopsy of the groin: how it works and what the result means | CION Cancer Clinics
A sentinel node biopsy finds the first lymph node that drains a tumour, removes it through a small cut and checks it for cancer. If it is clear, the rest of the groin nodes are very likely clear, and a full dissection can be avoided. It is used for vulval cancer, penile cancer and melanoma of the leg when no node can be felt. This page explains the tracer, the operation and what each result leads to. CION Cancer Clinics’ surgical oncologists in Hyderabad can talk this through with you.
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Medically reviewed by Dr. Muralidhar MuddusettyConsultant Surgical Oncologist · MBBS (AIIMS), MS (Surgery, AIIMS), DNB (Surgical Oncology), MRCS (Edinburgh) · last reviewed September 2026, next review due September 2027
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On this page
- What is a sentinel node biopsy of the groin?
- What actually happens on the day?
- Who is it usually offered to, and who does it not suit?
- Sentinel node biopsy and full groin dissection, compared
- What happens if the sentinel node is clear, or not?
- Three things families ask about the tracer and the test
- Common questions about sentinel node biopsy of the groin
The short answer
What is a sentinel node biopsy of the groin?
A sentinel node biopsy finds the first lymph node (a small gland that filters fluid) that drains the tumour, removes that one node, and checks it under the microscope for cancer. If the sentinel node is clear, the rest of the groin nodes are very likely clear too, and a full dissection can be avoided.
Why it is done instead of removing every node
A full groin dissection removes the whole pad of nodes and carries a real risk of wound problems and lasting leg swelling. For most people with an early vulval, penile or skin cancer, those nodes turn out to be clear, so the operation would have been done for nothing. The sentinel test answers the same question with one or two nodes and a small cut.
How the surgeon knows which node is first
A small amount of radioactive tracer, and often a blue dye, is injected into the skin around the tumour. It travels along the same channels that cancer cells would use and collects in the first node it reaches. A hand-held probe picks up the tracer during the operation, and the blue colour makes the node easy to see.
What this page cannot tell you
It cannot tell you whether a sentinel biopsy is right for your cancer, or what a positive result would mean for you. Both depend on the type of cancer, the size of the tumour and what the scans show. Ask your surgeon which of the situations below applies to you.
Step by step
What actually happens on the day?
The tracer injection
Either the afternoon before or the morning of surgery, a tiny dose of radioactive tracer is injected into the skin next to the tumour. The needle stings briefly. The dose is very small and is not a treatment.
The mapping scan
A scan follows the tracer to show which groin it has gone to, and whether it has reached one node or more. For tumours near the middle of the body it may show both groins, which changes the plan.
In theatre
Under a general anaesthetic, blue dye may be injected as well. The surgeon makes a small cut in the groin, follows the probe to the node that is "hot" or blue, and removes it. Sometimes two or three nodes are taken.
The laboratory examination
The node is sliced thinly and stained so that even a tiny deposit of cancer can be found. This takes days. Some centres do a rapid check during the operation, but the final report is the one that counts.
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Who is it usually offered to, and who does it not suit?
The test only works when there is no sign of spread already. Once a node can be felt or is proven on a needle test, the question it answers has already been answered.
Vulval cancer
Usually offered for a single tumour below a certain size, with no enlarged nodes on examination or scan. Larger tumours, more than one tumour, or an enlarged node point towards a full dissection instead.
Penile cancer
Offered when the nodes cannot be felt but the tumour is deep or aggressive enough that spread is possible. It replaces the older practice of clearing the groin "just in case" in these men.
Melanoma of the leg or lower body
Offered once the melanoma is thicker than a set threshold, or thinner with worrying features. It is the main way melanoma is staged when the nodes cannot be felt.
Who it does not suit
Anyone with a node that can be felt or has tested positive. Anyone whose groin has had previous surgery or radiotherapy, because the drainage may no longer follow the usual route.
Also check with your team if
- You are pregnant
- You have had a reaction to blue dye before
- The tumour has already been fully removed
Side by side
Sentinel node biopsy and full groin dissection, compared
Sentinel node biopsy
Full groin dissection
Removes one to three nodes through a small cut
Removes the whole pad of nodes and fat through a longer cut
Often a day-case or overnight stay, usually no drain
Several days in hospital, with a drain for a while
Low chance of lasting leg swelling
Higher chance of lasting leg swelling
Done to find out whether cancer has spread
Done when spread is proven or strongly suspected
A small chance of missing a deposit in another node
Removes all the nodes, so nothing is left to miss
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The result
What happens if the sentinel node is clear, or not?
A clear node means no further groin surgery in most cases, and follow-up continues with examination and scans. A positive node means cancer has reached the groin, and the plan changes.
If it is positive
For vulval and penile cancer, a positive sentinel node usually leads to a full dissection of that groin, sometimes with radiotherapy afterwards. For melanoma, many centres now offer close ultrasound follow-up of the groin instead of an immediate dissection, together with drug treatment. Which applies to you depends on the cancer and on how much cancer was found in the node.
What a clear result does not promise
The test is very good, but not perfect. In a small number of people cancer sits in a node the tracer did not reach, and shows up later as a lump in the groin. That is why follow-up visits continue and why any new groin lump should be reported promptly, even after a clear result.
Ask your surgeon what your follow-up will look like after a clear result, and how often the groin will be examined.
Did you know
The word "sentinel" means a guard who stands watch. The sentinel node is the guard at the gate of the groin: whatever passes from the tumour has to go through it first. That is the whole idea behind the test.
Commonly believed
Three things families ask about the tracer and the test
"The radioactive injection is dangerous for the family."
The dose is tiny and it fades within hours. Nobody around you is exposed to anything meaningful, and no special precautions are needed at home. The blue dye may turn your urine green or blue for a day, which is harmless.
"If they only take one node, they are not being thorough."
Taking one node is the point. It answers whether the cancer has spread without the wound and swelling problems of removing all of them. If that node is positive, the rest are removed. If it is clear, the others were almost certainly clear too.
"A clear sentinel node means the cancer is finished with."
It means the groin nodes are very likely clear, which is important news. It says nothing about whether the original tumour has been fully removed, and it does not end follow-up. Treatment of the tumour itself continues as planned.
Questions we are asked
Common questions about sentinel node biopsy of the groin
Is it done at the same time as removing the tumour?
Usually yes. The tracer is injected before you go to theatre, the sentinel node is removed first, and the tumour is then removed in the same anaesthetic. Occasionally the node is checked at a separate sitting, for example if the tumour was already removed elsewhere before the plan was made.
How long does it take to get the result?
The node is examined in thin slices with special stains, so the full report takes days rather than hours. If your centre does a rapid check during the operation, you may be told a provisional result the same day, but the final report can still change it. Wait for the final one.
Will I need a drain or a long stay?
Usually not. The cut is small and the amount of tissue removed is little, so most people go home the same day or the next, without a drain. The wound is checked in clinic. A longer stay happens only if the tumour operation itself is large, or if a rapid check turns positive and a dissection is done.
Can the test miss cancer?
Rarely, yes. If the tracer does not reach the node that actually holds cancer, that node is not removed and the result looks clear. This is uncommon but it is why follow-up continues after a clear result, and why any new lump in the groin should be reported straight away rather than watched at home.
Why was the tracer found on both sides?
The vulva and the penis sit in the middle of the body, and tumours near the midline can drain to either groin. When the mapping scan shows both sides, the surgeon takes a sentinel node from each. If the tracer reaches only one side despite a midline tumour, your team may advise clearing the other side another way.
What if the sentinel node cannot be found?
Sometimes the tracer does not show a clear node on the scan or with the probe. When that happens the surgeon cannot rely on the test, and the usual fallback is a full dissection of that groin so that the nodes are not left unchecked. You should be told before surgery that this is a possibility.
Is it available for melanoma on the leg in Hyderabad?
The test needs a tracer, a mapping scan and a surgeon trained to use the probe, so not every hospital offers it. Ask your centre directly whether they do, and if not, whether they can refer you. Call the helpline and we will tell you what is available and where.
Is it covered by Aarogyasri or insurance?
When it is part of an agreed cancer treatment plan, it is usually covered in the same way as the main operation. Aarogyasri, CGHS, ECHS and EHS are accepted, and most cashless insurers are empanelled. Call the helpline with your card details and we will check your cover before you travel.
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Sources
- National Cancer Institute — Sentinel Lymph Node Biopsy
- Cancer Research UK — Surgery for vulval cancer
- American Cancer Society — Surgery for penile cancer
- Cancer Research UK — Surgery for melanoma
- Macmillan Cancer Support — Lymph node removal (lymphadenectomy)
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
Which Cancers Need Groin Node SurgeryThe cancers that drain to the groin, and those that do notGroin Lymph Node Dissection: What It InvolvesWhat happens if the sentinel node is positiveSuperficial vs Deep Inguinal DissectionHow far a completion dissection goesThe Pathology Report After SurgeryReading the node report when it arrivesBiopsy guideHow tissue is examined and why it takes days
Talk to us
Been offered a sentinel node test, or wondering why you were not?
Send us the biopsy or scan report, or call the helpline. A surgical oncologist will explain whether the test fits your situation. One helpline serves every CION centre.