X-rays and CT scans use radiation to see inside the body, and radiation can cause cancer. Of course, we’re exposed to the sun’s radiation all the time, and about 25% of us wind up dying of cancer. So it’s important to understand comparative risks, without becoming too emotional or exaggerated about it all. I once explained to my 14-year-old daughter that the lifetime risk of fatal cancer from a certain CT scan was 1 in 2,000, although it’s 1 in 4 from getting out of bed in the morning. She replied, “Then I’m not getting out of bed.”
When considering the risks of getting a test, we also need to consider the risk of not getting it. A missed diagnosis may be lots riskier. I got annoyed with a neurologist once, who already had done 2 invasive angiograms (with radiation) on a 26-year-old woman with a small brain hemorrhage, without finding a source. Instead of doing a third, he ordered an MRI to “avoid radiation,” even though the angiogram was a much better test. By pure probabilities, the chance of causing a stroke or cancer with the angiogram would be around 1 in 300; while the chance of a bigger hemorrhage was 1 in 20-30. But he’d gotten swayed by fear of “radiation.”
In general , people are usually willing to assume risks on the order of 1 in 1,000. The main point is for medical providers, who order lots of radiological studies, to avoid unnecessary ones so as to prevent extra cancers among the general population. Since risks of a procedure are defined over the course of a lifetime, we can minimize them by trying to avoid radiation in younger persons, who have longer life spans ahead of them (cancer usually takes 20-30 years to develop).
Below are estimated lifetime risks of fatal cancer from various radiation exposures. They’re derived from studies of atom bomb survivors in Japan. It should be stressed that these are very general estimates, which may vary greatly for a particular person.
- Chest X-ray = 1 in 100,000
- Abdominal + Pelvic CT Scan = 1 in 1,000
- Dental X-ray = 1 in 2,000,000
- Low-back x-ray = 1 in 7,500
- Head CT Scan = 1 in 5,000
- CT Angiogram of the brain (rule out stroke) = 1 in 750
- Mammogram = 1 in 25,000
- DEXA Scan (for osteoporosis) = 1 in 10,000,000
- Whole-body CT = 1 in 850
The above are based on the assumption that an exposure of 10 mSv (milliSieverts) confers a 1 in 1,000 risk. Radiation risk calculators are available from the American Society of Radiologic Technologists (http://www.xrayrisk.com/calculator/calculator.php) and the University of California at San Diego (https://ehs.ucsd.edu/Radiation_Risk/request/home).