How Many PET-CT Scans — Are Safe in a Year?

For most people having cancer treatment, yes — the radiation dose from one scan a year is small relative to the clinical information it provides and the risk of the disease going unmonitored. Your oncologist makes this assessment for your specific situation each time a scan is requested, taking into account your age, cancer type, and how many scans you have already had. If you are concerned about a particular scan, ask your team to explain why it is being requested and what it will change about your care.

A PET-CT combines two sources of radiation: the radiotracer injected before the scan, and the CT used to create the anatomical map. According to ICRP estimates, the combined effective dose from a standard whole-body FDG PET-CT falls roughly in the range of seven to fourteen millisieverts, depending heavily on whether a low-dose or full diagnostic CT protocol is used. For comparison, annual natural background radiation is approximately two to three millisieverts per year globally, according to UNSCEAR. Your team can tell you which protocol is planned for your scan.

At the doses used in diagnostic imaging, the theoretical risk of radiation-induced cancer is very small. ICRP and WHO guidance consistently notes that this small risk is far outweighed by the clinical benefit for someone who already has cancer and needs accurate staging or response assessment. Your oncologist weighs this each time a scan is requested. If you have concerns about cumulative dose over time, raise them — your team can review your scan history and explain their reasoning.

Standard guidance is to avoid prolonged, close contact with young children and pregnant women for the rest of the day following your scan. The radiotracer decays quickly — fluorine-18 has a half-life of approximately 110 minutes — so the level of radioactivity drops substantially within a few hours. You will be given specific written instructions by the nuclear medicine team before you leave the imaging centre. These are a precaution, not an emergency measure, and for most tracers they end by the evening of the scan day.

Children's tissues are more sensitive to radiation than adult tissues, which is why paediatric PET-CT protocols use weight-adjusted tracer doses and reduced CT settings wherever possible. If your child is having a scan, the nuclear medicine team will calculate the dose based on their weight and age. The same principle applies as in adults — each scan must be clinically justified — but applied with additional care. Ask the imaging team what dose adjustments are being made before the scan proceeds.

Yes, and it is a reasonable question to raise. The CT component of a PET-CT can often be performed at a reduced dose — sometimes called a low-dose or attenuation-correction CT — which lowers overall exposure without significantly affecting PET image quality. Whether this is appropriate depends on what the scan is being done for. If your oncologist also needs a detailed anatomical CT for surgical planning, a full-dose CT may be necessary for that purpose. Ask before the scan, not after.

You have every right to ask your oncologist to explain the purpose of each scan before you agree to it. A scan should not be ordered unless it will change a clinical decision. If you feel the frequency is not being explained clearly, ask what specific question each scan is answering and what would happen if it were delayed. It is also reasonable to seek a second opinion on whether the monitoring frequency is appropriate for your cancer type and stage.

CION coordinates PET-CT for its patients through partner imaging centres across Telangana and Andhra Pradesh. The scan is performed at the imaging centre; your results are reviewed by your CION oncologist as part of your treatment planning. Your care team will arrange the referral and give you instructions on where to go, how to prepare, and what to bring.