A PET scan is not dangerous for most people. It does involve radiation, but the dose from a standard scan is modest, roughly equivalent to about two to three years of natural background radiation you’d absorb just from living on Earth. For the vast majority of patients, the diagnostic benefit of catching cancer, heart disease, or other serious conditions far outweighs the small radiation exposure involved.
That said, “not dangerous” doesn’t mean “zero risk.” Here’s what you should actually know about the radiation, the tracer injection, and the specific situations where extra caution matters.
How Much Radiation You Actually Get
Radiation from medical imaging is measured in millisieverts (mSv). A standard PET scan delivers about 8 mSv from the radioactive tracer alone. To put that in perspective, a chest X-ray delivers about 0.1 mSv, and the average person absorbs roughly 3 mSv per year just from natural sources like radon, cosmic rays, and the soil.
Here’s where it gets important: most PET scans today are actually PET/CT scans, meaning a CT scan is performed at the same time to provide detailed anatomical images alongside the PET’s metabolic data. That CT portion adds its own radiation. A low-dose CT used for basic positioning adds around 7 mSv, but if your doctor orders a full diagnostic-quality CT alongside the PET, the combined dose can reach 30 mSv or more for a whole-body study. So the total exposure depends heavily on what type of CT is paired with your PET.
For comparison, a nuclear cardiac stress test delivers roughly 41 mSv, and a standard bone scan about 6.3 mSv. A PET/CT with a low-dose CT component falls somewhere in the middle of the medical imaging spectrum.
The Cancer Risk From a Single Scan
The concern most people have is whether the radiation from a PET scan could itself cause cancer. At the doses involved in medical imaging, any increase in lifetime cancer risk is extremely small and largely theoretical. Researchers estimate the risk using models built from populations exposed to much higher radiation levels (like atomic bomb survivors), then extrapolate downward. The result: for a single PET/CT scan, the estimated added cancer risk is a fraction of a percent over your entire lifetime.
That fraction gets even harder to detect against your baseline cancer risk, which is already around 40% over a lifetime regardless of imaging. In practice, doctors order PET scans when they suspect or are tracking a serious disease. The information gained from the scan, catching a tumor early or confirming that treatment is working, almost always provides more benefit than the tiny theoretical risk from the radiation.
Where this calculus shifts is with repeated scans. If you’re undergoing cancer treatment and getting PET/CTs every few months for years, the cumulative dose adds up. Your care team tracks this and weighs each scan’s necessity against the running total.
Side Effects From the Tracer Injection
The most common PET tracer is a mildly radioactive sugar called FDG. It’s injected into a vein before the scan, and your body processes and eliminates it within hours. Allergic reactions to FDG are rare enough that their incidence hasn’t been reliably quantified. When reactions do occur, they can include hives, rash, itching, nausea, or difficulty breathing, but these are genuinely uncommon.
At the injection site, some people experience minor irritation: redness, soreness, or swelling. This typically resolves on its own. The tracer itself doesn’t affect how you feel during the scan. You won’t feel warm, dizzy, or nauseous from the FDG in most cases.
One thing to note: if your PET/CT includes a contrast dye for the CT portion, that’s a separate substance from the PET tracer. Iodine-based CT contrast can pose a risk to people with reduced kidney function, particularly those with advanced chronic kidney disease, diabetes, or dehydration. If both are being used, your team will typically check your kidney function beforehand and may give you IV fluids to protect your kidneys.
Precautions After the Scan
You’ll be mildly radioactive for several hours after a PET scan. The tracer decays quickly, with a half-life of about two hours, so the radiation fades fast. Still, Memorial Sloan Kettering and other major cancer centers recommend that for at least one hour after your scan, you avoid skin-to-skin contact with pregnant people and anyone under 18. You should also avoid sleeping in the same bed with them that night.
Drinking plenty of water after your scan helps flush the tracer from your body faster through urination. Most facilities will give you these instructions before you leave.
Why Children Face Higher Risk
Children are considerably more sensitive to radiation than adults. Their cells are dividing more rapidly, which makes them more vulnerable to radiation-induced DNA damage. They also have decades more life ahead of them, giving any damage a longer window to potentially develop into cancer. The National Cancer Institute notes that the risk of developing a radiation-related cancer can be several times higher for a young child compared to an adult receiving the same scan.
Because of this, doctors take extra steps before ordering any radiation-based imaging for kids. They’ll first consider whether ultrasound or MRI could answer the diagnostic question without radiation. When a PET or CT scan is truly needed, the machine settings are adjusted for the child’s smaller body size to deliver the lowest effective dose. Scans are also limited to the smallest body area necessary rather than defaulting to a whole-body study.
Pregnancy and Breastfeeding
Radiation exposure during pregnancy raises understandable concern, but guidelines from the American College of Obstetricians and Gynecologists clarify that the doses used in most diagnostic imaging, including CT and nuclear medicine scans, fall well below levels associated with fetal harm. If a PET scan is genuinely needed for a pregnant patient’s diagnosis and no radiation-free alternative will provide the same information, guidelines say it should not be withheld.
For breastfeeding mothers, the radioactive tracer can be present in breast milk in small amounts. Some facilities recommend pumping and discarding milk for a few hours after the scan, though specific guidance varies. The bigger concern with PET scans and breastfeeding is close physical contact with the infant while you’re still emitting radiation, which is why the same one-hour distance precaution applies.
How PET Compares to Other Imaging
If you’re weighing whether a PET scan’s radiation is worth worrying about relative to other tests you’ve had, here’s a quick comparison of approximate doses:
- Chest X-ray: 0.1 mSv
- Bone scan: 6.3 mSv
- Chest CT: 7 mSv
- PET scan (tracer only): 8 mSv
- PET/CT (with low-dose CT): roughly 15 mSv
- PET/CT (with diagnostic CT): up to 25 to 38 mSv
- Nuclear cardiac stress test: about 41 mSv
A PET/CT lands in the moderate-to-high range for medical imaging, but it remains one of the most powerful tools available for detecting cancer spread, evaluating treatment response, and distinguishing scar tissue from active disease. The scan exists because there are situations where no other test provides the same information, and in those situations, the radiation is a reasonable tradeoff.