Estimating cancer risk from radiation

I appreciate Dr Vakil’s helpful summary of the attendant risks of radiologic imaging in the October issue of Canadian Family Physician.¹ Given the difficulty of determining the absolute risk of a given procedure from the radiation exposure alone, I usually find it helpful to use a calculator that takes into account a patient’s age and sex in addition to his or her radiation exposure. In this manner, it is possible to estimate a baseline cancer risk for a given patient, as well as the additional risk that might be expected from an x-ray or computed tomography scan. For example, using the calculator at X-RayRisk.com (www.xrayrisk.com/calculator/calculator.php), I can estimate that a 50-year-old male patient who is considering a computed tomography scan of the abdomen can expect his future probability of cancer to increase by 0.04%. This translates into a number needed to harm of 2472.

When considering that the additional cancer risk posed by a radiologic procedure is likely to only manifest many years in the future, without a clear causal link to the inciting event, the individual patient might be more likely to undergo a necessary imaging test if they know the absolute numbers involved.