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The radioactive tracer injection: is it safe?
The word radioactive can sound alarming, but the tracer used to find the sentinel lymph node is a tiny, short-lived dose. This page explains why it is considered safe, how it compares with other radiation, what the injection is like and what to know about pregnancy, breastfeeding and family.
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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
- Is the radioactive tracer for sentinel node biopsy safe?
- Four reasons the tracer carries little risk
- How the tracer compares with other radiation
- The vocabulary, in plain language
- The honest picture on tracer risks
- Having the tracer injection
- Pregnancy, breastfeeding and children at home
- What people assume about the radioactive tracer
- Common questions about tracer safety
The short answer
Is the radioactive tracer for sentinel node biopsy safe?
Yes, for almost everyone the radioactive tracer used to find the sentinel lymph node is considered very safe. The word radioactive understandably worries people, but the amount used is tiny, far smaller than a dose of radiotherapy and broadly similar to the radiation from some routine X-ray or nuclear medicine scans. The tracer is a small quantity of a short-lived radioactive substance attached to particles that travel through the lymph channels and settle in the sentinel nodes. It loses most of its radioactivity within a day, and much of what is left is removed with the nodes during surgery. The injection is given into the breast by a nuclear medicine team, either the day before surgery or a few hours ahead, and it can sting for a short time. After that, a handheld probe helps the surgeon locate the nodes. Side effects from the tracer itself are very uncommon, and allergic reactions are rare. You do not become a danger to family members, and normal contact, including hugging children, is fine. Some centres suggest that pregnant visitors and small babies avoid long close contact for the rest of the day, simply as a cautious step. If you are pregnant or breastfeeding, tell your team, as the plan may be adjusted, but the tracer can still usually be used.
The dose is very small
It is used only to find nodes, not to treat cancer.
It fades quickly
Most of the radioactivity is gone within a day.
Family contact is safe
You can be close to loved ones as usual.
This page gives general information only. Your nuclear medicine team can answer questions about your dose.Why it is considered safe
Four reasons the tracer carries little risk
Safety comes from the tiny dose, the short life of the substance and careful handling.
A tiny amount
The quantity injected is just enough for a sensitive probe to detect. It is thousands of times smaller than the radiation used to treat cancer.
Short-lived
The substance loses half its radioactivity in a matter of hours, so very little remains by the next day.
It does not build up in the body.Much is removed
The tracer gathers in the sentinel nodes and at the injection site, and a large share leaves the body with the tissue removed at surgery.
Trained handling
Nuclear medicine staff prepare and inject the tracer following strict radiation safety rules.
Tell the team if you are
- Pregnant or might be
- Breastfeeding
- Allergic to any injection
Not sure whether this applies to you?
Ask an oncologistPutting it in context
How the tracer compares with other radiation
Source
Rough comparison
Sentinel node tracer
A very low dose, used once, fading within a day
Mammogram
Also a low dose, broadly in the same range
Natural background radiation
Everyone receives some every day from soil, air and food
CT scan of the chest
Usually a higher dose than the tracer
Radiotherapy for breast cancer
A far larger, targeted dose designed to destroy cancer cells
Words you may hear
The vocabulary, in plain language
- Radioactive tracer
- A substance giving off a small, detectable signal, used to show where it travels in the body.
- Half-life
- The time it takes for a radioactive substance to lose half of its radioactivity.
- Nuclear medicine
- The hospital department that uses small radioactive doses for scans and tests.
- Gamma probe
- A handheld detector the surgeon uses to find the node holding the tracer.
- Lymphoscintigraphy
- A scan taken after the injection to show where the tracer has gathered.
- Radiation dose
- The amount of radiation energy the body receives.
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Being straight with you
The honest picture on tracer risks
Low risk does not mean no risk at all, and it is fair to understand the small uncertainties.
Any radiation carries a theoretical risk
Experts believe the tracer dose adds an extremely small, theoretical lifetime risk. For people who need the test, the benefit of accurate staging clearly outweighs it.
The injection can hurt briefly
The sting is short, but some people find it uncomfortable. Ask whether numbing cream or a slow injection is possible.
Allergic reactions are rare but possible
They are far less common than with blue dye, and staff are trained to deal with them.
What this page cannot tell you
It cannot give the exact dose your centre uses. Your nuclear medicine team can explain that.
What happens
Having the tracer injection
Knowing what to expect makes the injection much less daunting.
Where and when
You will usually go to the nuclear medicine unit either the afternoon before surgery or on the morning of the operation.
The injection itself
You lie down and a small needle is used to inject the tracer into the skin or tissue near the nipple or the cancer. You may be asked to massage the area gently.
A scan may follow
Some centres take pictures with a special camera to see where the tracer has gathered. You lie still for a short while.
Then surgery
The tracer stays in the nodes long enough for the operation, even if it happens the next day.
Special situations
Pregnancy, breastfeeding and children at home
These situations need a little more planning, but the tracer can usually still be used.
If you are pregnant
The dose reaching the baby is very small, and a tracer alone is generally regarded as the safer marker in pregnancy. Your team may use a lower dose or a same-day injection.
If you are breastfeeding
You may be advised to express and discard milk for a short time after the injection. Your team will give specific advice.
Young children at home
Normal care and cuddles are fine. Some centres suggest avoiding long periods holding a baby close to the chest for the rest of that day.
Commonly believed
What people assume about the radioactive tracer
The tracer is a form of radiotherapy.
It is a tiny diagnostic dose that locates nodes and does not treat cancer.
You must stay away from your family afterwards.
Normal contact is safe; any precautions are brief and simple.
The radioactivity stays in your body for months.
It fades within about a day and much of it is removed at surgery.
The tracer can spread the cancer.
There is no evidence that injecting the tracer causes cancer to spread.
Questions we are asked
Common questions about tracer safety
Will the tracer make me feel unwell?
Most people feel nothing apart from a brief sting at the injection site. The tracer does not cause nausea, tiredness or hair loss. Any sore or bruised feeling in the breast is usually mild and fades within a day or two. Tell the team if you feel faint or itchy after the injection.
Can I hug my children after the injection?
Yes. The radiation from your body is very low and falls quickly. Hugs and normal family life are fine. Some centres suggest that pregnant relatives and small babies avoid spending many hours pressed close to you on the day of the injection, but brief contact is not a concern.
Is the tracer safe if I am pregnant?
Sentinel node biopsy with a radioactive tracer is generally considered acceptable in pregnancy, because the dose reaching the baby is very low. Your team may lower the dose or inject it on the same day as surgery. Always tell them if you are, or might be, pregnant.
Do I need to stop breastfeeding?
Usually only briefly. You may be advised to express and throw away breast milk for a short period after the injection, and to avoid long close holding of your baby that day. Your nuclear medicine team will tell you exactly what to do based on the tracer used.
Can I travel home after the injection?
Yes. If the tracer is given the day before surgery, you can normally go home and return the next morning. The dose is too low to affect people near you on a bus, train or car journey. Follow any fasting instructions for your operation.
Will it set off airport or security scanners?
Very sensitive radiation detectors at some borders can pick up tiny amounts for a short time after nuclear medicine tests. This is rarely an issue with the sentinel node tracer, which fades quickly. If you plan to travel within a day or two, ask for a letter explaining the test.
Does the tracer cause cancer?
The tracer dose is so small that any added cancer risk is theoretical and extremely low. Experts agree that the value of knowing whether cancer has reached your lymph nodes, and avoiding unneeded armpit surgery, is far greater than this tiny risk.
Is there a way to avoid radioactivity altogether?
Some hospitals offer alternatives such as blue dye alone, a fluorescent dye called ICG, or a magnetic tracer. Each has its own advantages and drawbacks. Ask your surgeon which options are available and whether they suit your situation.
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Speak to a breast cancer specialist
Call the helpline or leave your details, and someone will help you arrange a consultation at the CION centre nearest you. One helpline serves every CION centre.
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Sources
- National Cancer Institute — Sentinel lymph node biopsy
- RadiologyInfo — General nuclear medicine
- Cancer Research UK — Surgery to remove lymph nodes
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.