Scientists estimate the risk of developing cancer from very low doses of radiation (such as from CT exams) by following the health of people exposed to small doses of radiation and comparing against the health of people not exposed to radiation. However, because the estimated increase in the number of cancers due to medical imaging exams is so low compared to the number of cancers in the general population, it would require following 5 million exposed persons and another 5 million unexposed persons over their entire lifetime to detect a clear increase in risk. And, the two groups would have to be similar in all other aspects (age, other health conditions, smoking history, etc.). Since we don’t have and are never likely to have such data, much smaller studies are used to estimate what the risk might be. Since scientists would prefer to be overly cautious rather than not cautious enough, we provide estimates of risk for low doses, even though at the dose levels associated with a CT scan, an increased risk of cancer has not been proven to exist. Data from various studies, all of which contain too few subjects and too many uncertainties, contradict each other, some finding no risk (or even benefit) and some finding a very small increase in risk.

However, because we can’t prove that there is no risk, medical exposures should be justified by the potential health benefit to the patient from the information gained, and the exam should be performed using the lowest reasonable doses. So if your exams were helpful in any way to your physician in managing your medical care, then the estimated radiation risk is negligible in comparison.

Still, patients and physicians want to have some idea of what the risk might be. We certainly know the risk is small! Otherwise it wouldn’t be so hard to demonstrate whether it does or doesn’t exist for these low doses. We may not know exactly how small, but we can give you a general idea using the following example.

The increase in risk associated with most CT procedures is generously considered to be about 0.05%. Therefore, the increase in risk for three CT scans within a single day would be about 0.15%. This increase is added to the risk of cancer in the general population, which is about 23%. That means that without the 3 CT scans, 23 out of 100 Americans die from cancer. If everyone had 3 CT scans, this might increase to 23.15 out of 100 Americans. So, you can see that the 3 CT scans make very little difference. You should also keep in mind that the information gained from those CT scans can help detect and manage cancers, as well as other serious medical conditions. That is, they may improve people’s health so that fewer than 23 out of 100 Americans die from cancer. In fact, since the availability of modern diagnostic and treatment technologies, like CT scans, the death rate from cancer continues to go down, even though the numbers of cancers that occur have stayed relatively the same.

In summary, stay focused on your health and make sure that any medical procedure you may be asked to undergo is warranted. And if one or more CTs are needed, be assured that the risk is very small and the diagnostic information is extremely valuable.