How Much Radiation Is in — a PET-CT Scan?

Yes, for the clinical reason it is being ordered. The dose is real and measured, but your oncologist has weighed it against the diagnostic value the scan provides. International radiology bodies, including the IAEA, regard PET-CT as an established and justified procedure when indicated. Any imaging dose carries a small theoretical long-term risk, and your team is factoring that against the greater risk of an undiagnosed or poorly staged cancer.

A PET-CT delivers several hundred times the dose of a chest X-ray, and falls in a similar range to a diagnostic CT covering the chest, abdomen and pelvis combined. By IAEA reference data, a chest X-ray delivers about 0.02 millisieverts, a CT of the chest alone around 7 millisieverts, and a combined chest-abdomen-pelvis CT in the range of 15 to 20 millisieverts. A PET-CT sits at 14 to 25 millisieverts, because it includes both a CT component and the radioactive tracer. Your team has already weighed whether the diagnostic benefit justifies that dose.

At diagnostic imaging doses, any added risk is very small and is outweighed, in most situations, by the benefit of an accurate diagnosis. The IAEA and UNSCEAR note that the relationship between low-dose radiation and cancer involves theoretical extrapolation from higher exposures — it is not directly measured at diagnostic dose levels. What is not theoretical is the risk of an inaccurate diagnosis or incorrect staging. Your team is balancing both sides.

After six to eight hours, the tracer's radioactivity has fallen to a small fraction of its starting level, and by the following morning it is negligible. This follows from the half-life of fluorine-18 — around 110 minutes — which means the radioactivity halves roughly every two hours. The CT component does not make you radioactive at all. X-ray radiation passes through the body and leaves no lasting activity.

Brief contact — a hug, sitting together, sharing a meal — is not restricted. What nuclear medicine departments recommend avoiding for the first six hours is prolonged, very close contact with young children, particularly those who might sit in your lap or sleep beside you. Tell the nuclear medicine team the ages of your children before your scan and they will give you specific advice.

You do not need to leave home. Keep more than an arm's length from your partner for the first six hours after injection, and avoid sharing a bed during that window. After that, the tracer has decayed significantly and normal contact resumes. Tell the nuclear medicine team before your scan that your partner is pregnant — they can give advice specific to your household.

Yes. There is no restriction on public or shared transport after a PET-CT. The dose rate from your body is low enough that brief proximity to other passengers is not a meaningful exposure for them. The precautions about pregnant women and young children apply to prolonged close contact at home, not to ordinary travel. Many patients use public transport to get home after their scan.

Not automatically — what matters is whether the clinical need justifies each scan, and your oncologist has made that judgement. Each scan does add to your cumulative dose, and your team is aware of this. Accurately knowing whether treatment is working is not a small consideration. If you want to understand the reasoning, ask what the second scan is expected to change in your management — that is a reasonable question to put to your team.