Medical Oncologist
Treatment options
How the sentinel node is found: dye, tracer and ICG
Before the sentinel node can be removed, it has to be found. Surgeons do this by injecting a marker that follows lymph fluid to the first nodes. This page explains radioactive tracer, blue dye, ICG and magnetic methods, how they compare and what each feels like for you.
MM
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
- How does the surgeon find the sentinel node?
- The main ways of marking the sentinel node
- How the methods compare
- The vocabulary, in plain language
- What these methods cannot always do
- Your experience of each method
- Pregnancy, allergies and chemotherapy first
- What people assume about finding the sentinel node
- Common questions about locating the sentinel node
The short answer
How does the surgeon find the sentinel node?
The sentinel node is found by following the natural path that lymph fluid takes from the breast to the armpit. A marker is injected into the breast, and within minutes to hours it travels along the lymph channels and collects in the first node or nodes it reaches. The surgeon then looks for that marker during the operation. Three main kinds of marker are used, alone or together. The first is a radioactive tracer: a very small dose is injected, often a few hours or the day before surgery, and a handheld probe that clicks or beeps near radioactivity guides the surgeon to the right node. The second is a blue dye, injected once you are asleep, which stains the lymph channels and nodes so they can be seen. The third is a newer option called ICG, a green fluorescent dye that glows under a special near-infrared camera, letting the surgeon watch the channels on a screen. Some centres also use a magnetic tracer detected with a magnetic probe. Using two methods together finds the node more often than one alone. The choice depends on what your hospital has, whether you are pregnant or allergic, and your surgeon's experience. All of these methods are well established and generally safe.
The marker follows lymph flow
It naturally gathers in the first nodes that drain the breast.
Tracer, dye or both
Combining a tracer with a dye improves the chance of finding the node.
ICG is a newer choice
A glowing green dye seen through a camera avoids radioactivity.
This page gives general information only. Methods differ between hospitals.The options
The main ways of marking the sentinel node
Each method has strengths, and many centres use two together.
Radioactive tracer
A tiny radioactive dose is injected near the nipple or tumour. A probe in the operating theatre picks up the signal through the skin, pointing the surgeon to the node.
Blue dye
Injected at the start of the operation, it turns the lymph channels and sentinel nodes blue, so the surgeon can see them directly.
It can tint urine and skin for a while.ICG fluorescence
ICG is a green dye that gives off light under a near-infrared camera. The surgeon sees glowing channels on a monitor and follows them to the node.
Magnetic tracer
A liquid containing tiny iron particles is injected, sometimes days ahead, and a magnetic probe detects it during surgery.
What decides the choice
- Equipment at the hospital
- Pregnancy or allergies
- Surgeon experience
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How the methods compare
Method
Points to know
Radioactive tracer
Widely used and reliable; needs a nuclear medicine unit and careful timing
Blue dye
Simple and cheap; can stain skin for months and rarely causes allergic reactions
Tracer plus blue dye
Often thought of as the standard pairing, finding the node in most people
ICG fluorescence
No radioactivity and good detection; needs a special camera
Magnetic tracer
No radioactivity and flexible timing; can affect later MRI pictures of the breast
Words you may hear
The vocabulary, in plain language
- Lymphatic mapping
- Tracing the route lymph fluid takes from the breast to find the sentinel node.
- Gamma probe
- A handheld device that detects the radioactive tracer and makes a sound near it.
- Lymphoscintigraphy
- A scan taken after the tracer injection that shows where it has collected.
- ICG
- Short for indocyanine green, a dye that glows under near-infrared light.
- Near-infrared camera
- A camera that shows the glow of ICG, invisible to the naked eye, on a screen.
- Detection rate
- How often a method succeeds in finding at least one sentinel node.
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Being straight with you
What these methods cannot always do
These techniques are very good at finding the sentinel node, but there are situations where they struggle.
The marker may not travel
Earlier surgery, radiotherapy, a large tumour or cancer blocking the channels can stop the marker reaching a node. The surgeon may then remove a few armpit nodes instead.
Not every hospital has every method
ICG cameras and magnetic probes are not available everywhere. A well-run tracer and dye service works very well.
Finding the node is not the same as a clear result
The marker shows which node to remove. Whether it holds cancer is decided later in the laboratory.
What this page cannot tell you
It cannot say which method your surgeon will choose. Ask them what they use and why.
What you will notice
Your experience of each method
From your side, the methods feel quite similar, because most of the work happens while you are asleep.
The tracer injection
If a radioactive tracer is used, you may have the injection while awake in the nuclear medicine unit. It stings briefly, and a scan may follow.
Dye while you sleep
Blue dye and ICG are usually injected after the anaesthetic, so you do not feel them. The breast may be gently massaged to help the dye move.
Waking up
After blue dye, your breast may look blue and your urine may be blue-green. ICG leaves little visible trace. A magnetic tracer can leave a faint brown mark on the skin.
When plans change
Pregnancy, allergies and chemotherapy first
Certain situations make one method more suitable than another. Tell your team about any of these well before surgery.
During pregnancy
A low-dose radioactive tracer alone is generally regarded as safe for the baby. Blue dye is usually avoided because of the small risk of allergic reaction.
Known allergies
If you have reacted to dyes, iodine or contrast before, your surgeon may choose a tracer that avoids that risk.
After chemotherapy
Treatment before surgery can make lymph channels harder to follow. Surgeons often use two markers and remove a few nodes to keep the check accurate.
Commonly believed
What people assume about finding the sentinel node
The tracer is the same as radiotherapy.
The tracer dose is tiny and used only to locate nodes, not to treat cancer.
Newer methods are always better.
Tracer with blue dye remains very reliable; newer methods mainly avoid radioactivity.
Blue staining means the dye went wrong.
Staining shows the dye did its job and fades gradually.
If the node cannot be found, the operation failed.
The surgeon has a backup plan, usually removing a few nodes to check them.
Questions we are asked
Common questions about locating the sentinel node
Which method is most accurate?
Using two methods together, such as a radioactive tracer with blue dye, or ICG with a tracer, finds the sentinel node more often than any single method. Studies suggest ICG and magnetic tracers perform about as well as the traditional tracer. The skill of the surgical team matters as much as the marker used.
Why is the tracer sometimes given the day before?
The tracer needs time to travel to the node, and it stays detectable for many hours. Giving it the day before lets the nuclear medicine unit fit it around surgery schedules, especially for early morning operations. Your team will tell you where to go and when.
Is ICG safe?
ICG has been used in medicine for many years, including for eye and heart tests, and serious reactions are rare. It contains iodine, so tell your team if you have an iodine or shellfish allergy or thyroid problems. It leaves the body within a day or so.
Will I feel the dye injection?
Blue dye and ICG are usually injected after you are asleep, so most people feel nothing. If a tracer is given while you are awake, there is a brief stinging feeling that passes within minutes. Some centres use a numbing cream or a slow injection to reduce discomfort.
Does the magnetic tracer affect future scans?
The iron particles can remain in the breast for a long time and may cause dark areas on later MRI scans of that breast. They do not affect mammograms or ultrasound. If you may need breast MRI in future, discuss this with your surgeon before choosing this method.
What happens if no marker reaches a node?
This happens in a small number of people. The surgeon will usually remove the lower group of armpit nodes so they can still be checked. It is worth asking before surgery what your surgeon plans to do if the node is not found, so the consent covers it.
Can the sentinel node be outside the armpit?
Occasionally the tracer shows drainage to nodes behind the breastbone. These internal chain nodes are usually not removed during routine surgery, but the finding may be considered when planning radiotherapy. Your team will explain what it means for you.
Is the radioactive tracer harmful to my family?
The dose is very small and fades quickly. You can hug your family and be around children normally. Some centres advise pregnant visitors and small children to avoid long close contact for the rest of the day, simply as a cautious step.
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Sources
- National Cancer Institute — Sentinel lymph node biopsy
- Cancer Research UK — Surgery to remove lymph nodes
- American Cancer Society — Lymph node surgery for breast cancer
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