Most PET scans use a radiotracer called FDG, labeled with fluorine-18, which has a physical half-life of about 110 minutes. That means the radioactivity in your body drops by half roughly every two hours, and within a day it is essentially gone. Your main job afterward is straightforward: drink plenty of water, urinate frequently, and keep a reasonable distance from pregnant women and small children for several hours. Beyond those basics, there are a handful of situations that call for more specific attention.
Flushing the Tracer Out
FDG is eliminated through your kidneys, with the urinary bladder receiving the highest radiation dose of any organ. About a fifth of the injected FDG is cleared in urine within roughly two hours of administration.1Journal of Nuclear Medicine Technology. Reducing Radiation Exposure from PET Patients That is why nuclear medicine departments almost universally tell you to drink extra fluids and use the bathroom often in the hours after your scan. Each time you empty your bladder, you are removing a portion of the remaining radioactivity from your body and reducing the dose your bladder wall absorbs.
There is no magic number of glasses of water you need to hit. The goal is simply to stay well hydrated so your kidneys keep working efficiently. If you already drink a reasonable amount of water throughout the day, adding a few extra glasses in the first four to six hours is sufficient. Avoid holding your urine for long stretches. Wash your hands thoroughly after each bathroom visit, since trace radioactivity can be present in urine. If you are caring for young children and changing diapers, it is worth washing your hands with extra care during this window, though the risk is genuinely small.
Protecting the People Around You
After a PET scan you are, briefly, a low-level source of gamma radiation. The dose anyone near you receives drops sharply with distance. Staff radiation exposure from PET patients comes mainly from close contact, and it can be substantially limited simply by increasing the distance between the patient and other people.2Radiation Protection Dosimetry. Radiation exposure of patients and personnel from a PET/CT procedure with 18F-FDG This is the same inverse-square principle that makes a campfire feel warm up close and barely noticeable across a field.
In practical terms, most imaging centers recommend that for the rest of the day after your scan you avoid prolonged close contact with pregnant women and young children. “Prolonged close contact” means things like holding a baby on your lap for an extended period or sitting pressed against someone for hours. A quick hug or passing someone in a hallway is not a concern. By the following morning the radioactivity will have decayed to a negligible level, and no special precautions are needed.
You do not need to sleep in a separate room or avoid your partner overnight unless your facility specifically tells you otherwise. Some centers are more conservative than others, particularly for patients who received higher-than-standard doses, but for a typical diagnostic FDG-PET scan the exposure to a healthy adult sharing your bed is well within safe limits.
Side Effects and Complications
PET tracers at diagnostic doses are remarkably well tolerated. FDG itself is basically a sugar molecule with a radioactive tag, and it does not produce the nausea, hives, or allergic-type reactions that iodinated contrast dye sometimes does. Most people feel nothing at all from the injection. Rarely, someone will notice a slight metallic taste or a warm sensation at the injection site, both of which resolve in minutes.
The more realistic concern is extravasation, which is when some of the tracer leaks out of the vein into the surrounding tissue during injection. Extravasation of diagnostic radiopharmaceuticals is actually fairly common, but with tracers like FDG, no specific medical intervention is needed.3PubMed Central. Consequences of radiopharmaceutical extravasation and therapeutic interventions: a systematic review The area may look swollen or feel sore for a few hours. The main consequence is that the leaked tracer does not distribute normally through the body, which can affect image quality. If your technologist noticed an extravasation during the injection, they will have documented it and may recommend a repeat scan if the images are affected.
For therapeutic radiopharmaceuticals (used in treatment rather than imaging), extravasation is a more serious issue that can occasionally cause soft tissue damage. In those cases, the recommended steps include stopping the infusion immediately, elevating the affected limb, performing gentle massage to encourage lymphatic drainage, and monitoring the site over time.4Journal of Nuclear Medicine. Extravasation of Therapeutic Radiopharmaceuticals: A Systematic Review and Management Proposal But for a standard diagnostic PET scan, extravasation is an image-quality problem, not a health emergency.
If your PET/CT included intravenous iodinated contrast for the CT portion, the risk profile changes slightly. Contrast reactions range from mild (hives, itching, mild nausea) to rare but serious (difficulty breathing, severe drop in blood pressure). These reactions typically happen during or shortly after the injection, so you will usually still be in the department when they occur. If you have a known contrast allergy, your team will have pre-medicated you or chosen an alternative protocol. If you develop hives, swelling, or shortness of breath after leaving the facility, seek medical attention promptly.
Breastfeeding After a PET Scan
If you are breastfeeding and had an FDG-PET scan, the reassuring news is that very little FDG actually makes it into breast milk. A study measuring radioactivity in lactating women found that the calculated maximum dose to an infant from ingesting the milk, with no interruption of breastfeeding at all, was about 0.085 mSv, well below the recommended limit of 1 mSv.5Journal of Nuclear Medicine. Pattern of Uptake and Excretion of 18F-FDG in the Lactating Breast Interestingly, that same study found that the infant actually receives a higher radiation dose from being held close to the breast (external exposure from the radioactivity in the mother’s body) than from drinking the milk itself.
Current guidance reflects this finding. For FDG specifically, no interruption of breastfeeding is considered necessary.6Radiation Protection Dosimetry. X-RAY AND MOLECULAR IMAGING DURING PREGNANCY AND BREASTFEEDING—WHEN SHOULD WE BE WORRIED? That said, some facilities still recommend pumping and discarding one feeding’s worth of milk as a precaution, or waiting a few hours before nursing, mainly to reduce the close-contact external dose. If your center gives you specific instructions, follow those. But if you accidentally nurse your baby before checking with anyone, the dose is negligibly small, and there is no reason to panic.
The story is quite different for other nuclear medicine tracers. Certain radioiodine compounds require a complete cessation of breastfeeding (iodine-131 therapy) or an interruption of 12 hours to 3 weeks, depending on the agent.7PubMed. Excretion of radionuclides in human breast milk after nuclear medicine examinations Always confirm with your nuclear medicine team which tracer was used and what their breastfeeding guidance is, because the answer varies dramatically from one radiopharmaceutical to another.
What If You Are Pregnant
PET scans are generally avoided during pregnancy unless there is a compelling medical reason, such as staging a newly diagnosed cancer where the scan will directly change treatment. When a PET/CT is medically necessary in a pregnant patient, the fetal radiation dose from FDG is low. The total dose from the combined PET and CT components falls in a range of roughly 1 to 8 mGy depending on the trimester and the specific CT protocol used.8Journal of Nuclear Medicine. Fetal Dose from PET and CT in Pregnant Patients This is well below the threshold at which deterministic effects (meaning direct, predictable harm to the fetus, like organ malformation) have been observed in animal studies, which is around 100 mGy.9PubMed Central. Use of Positron Emission Tomography for Pregnancy-Associated Cancer Assessment: A Review
Fetal doses tend to be higher in early pregnancy and lower as the pregnancy progresses, with considerable variation from patient to patient. Even so, the total absorbed dose remains well below the threshold for noncancer health effects throughout pregnancy.10Journal of Nuclear Medicine. Fetal Radiation Dose from 18F-FDG in Pregnant Patients Imaged with PET, PET/CT, and PET/MR The principle applied is to minimize fetal exposure as much as possible while still getting the diagnostic information needed. In practice, this might mean using a lower CT dose, limiting the scan area, or skipping the contrast agent.
If you discover you are pregnant after having had a PET scan, speak with your obstetrician and the nuclear medicine physician. They can estimate the fetal dose based on the specific protocol used and your gestational stage, which will almost certainly be reassuring.
Dealing with Scan Anxiety While Waiting for Results
For many people, the hardest part of a PET scan is not the procedure itself but the period between the scan and hearing the results. This experience has been given a name in the medical literature: scanxiety. A prospective study of over 400 oncology patients found that roughly seven in ten experienced clinically significant scan-related anxiety, and the period of waiting for results was the peak stress point, with a mean anxiety score of 7.1 out of 10.11Clinical Imaging. Prevalence, severity, and modifiable predictors of scanxiety in patients undergoing routine oncologic imaging: a prospective longitudinal study
The factors that made scanxiety worse included a longer wait between the scan and the results, more advanced disease, and having had more prior scans. That last point is worth noting: scanxiety does not necessarily get easier with experience. It can actually worsen as the number of scans accumulates. On the flip side, the same study found that certain things were genuinely protective. Patients who received clear procedural explanations beforehand, who felt the staff were friendly and communicative, and who had strong social support all reported lower anxiety.11Clinical Imaging. Prevalence, severity, and modifiable predictors of scanxiety in patients undergoing routine oncologic imaging: a prospective longitudinal study The waiting period between the scan and the results is consistently identified as the most stressful phase.12PubMed Central. Scanxiety among Adults with Cancer: A Scoping Review to Guide Research and Interventions
If you struggle with scanxiety, there are a few practical things that help. Ask your team before the scan how long results typically take and how they will be communicated. Knowing whether to expect a phone call in two days or a portal message in a week removes some of the uncertainty that fuels the anxiety. Plan something engaging for the day or two after the scan so you are not sitting by the phone. And if the wait becomes genuinely distressing, it is completely reasonable to call your care team and ask for an update.
Making Sense of Your Results
PET scan reports describe how avidly different areas of your body took up the tracer, and they typically express this using a metric called the standardized uptake value, or SUV. The SUV is essentially a number that reflects how much tracer accumulated in a given spot, adjusted for your body size and the dose you received.13Nuclear Medicine Communications. The use of standardized uptake values for assessing FDG uptake with PET in oncology: a clinical perspective Higher values generally indicate more metabolic activity, which in the context of cancer imaging can point to malignancy, but infections, inflammation, and normal physiologic activity (like brain metabolism or bladder filling) also produce high uptake.
A common misconception is that a specific SUV number acts as a clean dividing line between cancer and not-cancer. It does not work that way. The usefulness of the SUV depends on the type and stage of the disease, the location of the finding, and how it fits with all your other clinical information.13Nuclear Medicine Communications. The use of standardized uptake values for assessing FDG uptake with PET in oncology: a clinical perspective Your doctor interprets the number in context, not in isolation.
Where SUV becomes particularly powerful is in comparing serial scans. If you are being treated for cancer, your oncologist may track the SUV of a known tumor across multiple scans to see whether the treatment is working. A drop in SUV after chemotherapy or radiation generally signals a favorable response, while a rise may prompt a change in strategy.14PubMed Central. Positron emission tomography-computed tomography standardized uptake values in clinical practice and assessing response to therapy For some cancers, such as head-and-neck squamous cell carcinoma, higher pretreatment SUV values have been associated with poorer outcomes, which can factor into treatment planning.15International Journal of Radiation Oncology*Biology*Physics. Analysis of Pretreatment FDG-PET SUV Parameters in Head-and-Neck Cancer: Tumor SUVmean Has Superior Prognostic Value
Timing of Follow-Up Scans After Treatment
If you have just finished cancer treatment, you may wonder how soon a follow-up PET scan can reliably tell you whether the treatment worked. Scanning too early risks false positives: radiation therapy and chemotherapy cause inflammation that lights up on FDG-PET in a way that can mimic residual tumor. In head-and-neck cancer, researchers found that a PET scan performed as early as one month after completing radiotherapy was a valuable and reliable tool for evaluating the response. Patients with negative findings at that one-month scan had excellent outcomes, while positive findings triggered further evaluation.16Journal of Nuclear Medicine. The Feasibility of 18F-FDG PET Scans 1 Month After Completing Radiotherapy of Squamous Cell Carcinoma of the Head and Neck
Timing guidelines vary by cancer type. For lymphomas, many protocols call for an interim PET scan partway through chemotherapy and then a post-treatment scan a few weeks to a couple of months after the last cycle. For solid tumors treated with radiation, the window is often six to twelve weeks, though some centers push the first scan earlier when the clinical question is urgent. Your oncologist will set the schedule based on your specific situation, balancing the need for early information against the risk of misleading results from post-treatment inflammation.
Cumulative Radiation from Repeat Scans
One PET/CT delivers a radiation dose in the neighborhood of 10 to 25 mSv depending on the protocol, with most of that coming from the CT component rather than the tracer itself. For perspective, that is roughly equivalent to a few years of natural background radiation compressed into a single session. A single scan poses very little risk, but patients being monitored for cancer may accumulate many scans over years of surveillance.
A large retrospective study looked at cumulative effective doses across more than eight thousand PET/CT patients and found that only a tiny fraction, about 0.2 percent, reached a total of 100 mSv or more across all their scans. The patients most likely to reach that threshold were those with conditions requiring frequent long-term monitoring, such as malignant lymphoma or cardiac sarcoidosis.17PubMed Central. Cumulative radiation doses from recurrent PET–CT examinations The 100 mSv mark matters because it is the level at which epidemiologic studies begin to detect a slight, measurable increase in lifetime cancer risk.
Pediatric patients deserve special mention. Children are more sensitive to radiation, and their longer expected lifespan gives any radiation-related risk more time to manifest. A five-year review of children with malignancies found that cumulative radiation from serial PET/CT studies was considerable, though the authors emphasized that lower doses can be used for both the PET and CT portions without sacrificing diagnostic quality.18PubMed Central. Estimated cumulative radiation dose from PET/CT in children with malignancies: a 5-year retrospective review The guiding principle, known by the acronym ALARA (as low as reasonably achievable), pushes imaging teams to use the lowest dose that still produces a useful image, especially for younger patients.
If you are concerned about cumulative exposure, it is worth asking your physician two questions before each scan: will the result change my treatment, and is there a lower-dose protocol available? A scan that will not alter clinical decisions is a scan that may not need to happen.
Newer PET Tracers and What They Mean for You
FDG is by far the most widely used PET tracer, but the field has been expanding rapidly. One important area is prostate cancer, where PSMA-targeted tracers have changed the landscape. These tracers bind to a protein found on the surface of prostate cancer cells, making them far more specific for prostate cancer than FDG, which lights up any metabolically active tissue. Several PSMA tracers are already in clinical use, and newer candidates continue to be evaluated.
A recent first-in-human trial of a gallium-68-labeled PSMA tracer found that it cleared rapidly from blood and soft tissue, showed prominent uptake in expected organs like the salivary glands and kidneys, and produced no adverse events in any of the ten participants.19PubMed. First-in-Human Evaluation of [(68)Ga]Ga-HTK03149, a PSMA-Targeted Tracer for PET Imaging in Prostate Cancer A separate multicenter trial of an F-18-labeled PSMA tracer called INR101 reported a mean effective radiation dose of about 7.6 mSv, no serious adverse events, and favorable pharmacokinetics with low urinary tract interference, which matters because urinary activity near the prostate can obscure the very lesions you are trying to find.20PubMed Central. Evaluation of INR101, a PSMA-Targeted (18)F PET Tracer, for suspected prostate cancer: A multicenter phase I/IIa trial with histopathologic confirmation
From a post-scan standpoint, these newer tracers follow the same general principles as FDG: hydrate, urinate frequently, maintain some distance from vulnerable individuals for a few hours. Gallium-68 has an even shorter half-life than fluorine-18 (about 68 minutes versus 110), so the radioactivity clears your body faster. The side-effect profile of PSMA tracers at diagnostic doses is similar to FDG, which is to say almost nonexistent. If you undergo a PSMA-PET scan, the post-scan routine will feel familiar to anyone who has had an FDG scan before. The real difference is in what the images reveal, not in how you feel afterward.