Answer to Question #12489 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues — Diagnostic X Ray and CT

The following question was answered by an expert in the appropriate field:

Q

Over the last few years, there have been several studies on the effects of low-level radiation exposure. A study by Mathews (2013) published in the British Medical Journal is alarming. The study seems to indicate a 24 percent increase in cancer attributable to medical computed tomography (CT) screening. Are these results as alarming as they appear to be? How do these results square with the Health Physics Society statement on low-level radiation exposure?

A

Unfortunately, the study you cited, and others like it, has a major flaw—they confuse correlation with causation. They observe that children and adolescents who have received CT scans have a higher incidence of cancer and they leap to the conclusion that the CT scans were the cause of this increase. However, it appears far more likely that the children studied had underlying health problems that increased their risk of cancer and this was the reason that they had the CT scans in the first place.

For example, children frequently get CT scans when they have a head injury (from falling down or bumping into things). Head injuries alone have been shown to increase the risk of developing brain cancer. So the real cause of the increase in cancer risk is the problem that brought the child to the hospital, not the CT scan that may have saved his or her life. This is known as reverse causation and it is a major problem with recent epidemiological studies that claim to show increased risk of cancer from CT scans.

Multiple studies published since the one you cited have convincingly demonstrated this problem. As I explained in a 2015 article (Ulsh 2015), several other limitations in the Mathews et al. (2013) study have been identified. These include (1) the authors misclassified patients with unknown radiation exposures at the start of the study as unexposed, which the authors assert would have a small impact, but no supporting evidence for this assertion was provided, (2) the authors didn't capture some of the radiation exams the patients received, so they underestimated radiation doses, which led them to overestimate the radiation risk, and (3) often, there is a problem with the CT image that requires a retake. These were not considered by the authors, so again they underestimated radiation doses and overestimated radiation risk.

There are also several results that are inconsistent with other studies of radiation effects and which raise concerns that something is amiss with this study. So if you put all this together, it doesn’t give a lot of confidence in this study’s results. The bottom line continues to be that if your doctor recommends a CT scan and gives you a plausible reason for it, take the advice, get the CT scan, and don’t lose any sleep over it.

Brant Ulsh, PhD
Certified Health Physicist

References
Mathews JD, Forsythe AV, Brady Z, Goergen SK, Byrnes GB, Giles GG, Wallace AB, Answerson PR, Guiver TA, McGale P, Cain TM, Dowty JG, Bickerstaffe AC, Darby SC. Cancer risk in 680,000 people exposed to computed tomography scans in childhood or adolescence: data linkage study of 11 million Australians. BMJ 346:f2360; 2013.

Ulsh BA. Are risks from medical imaging still too small to be observed or nonexistent? Dose-Response 13:1–27; 2015.

Kent Lambert
Certified Health Physicist
Fellow, Health Physics Society
Topic Editor, Medical and Dental Patient Issues

Answer posted on 6 July 2018. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.