Another reason to avoid that barium enema

It looks as though cancers caused by x-ray exposure
could be higher than we thought

By Henry Peters

Roughly 784 cases of cancer diagnosed annually in Canada could be attributable to exposure to diagnostic x-rays. A new study in the January 31 issue of The Lancet offers the most detailed calculation of its kind to estimate the risk of cancer from exposure to diagnostic x-rays. For the cancer sites included in the study, that amounts to 1.1% of all cases.

Diagnostic x-rays, including imaging techniques such as computed tomography, are the largest man-made source of radiation exposure to the general population, contributing about 14% of the total annual exposure worldwide from all sources. It's generally accepted that the use of these technologies is associated with small increases in cancer risk, a risk that almost everyone agrees is outweighed by its diagnostic benefits.

In the study, Dr Amy Berrington de Gonzçlez of Oxford University and Dr Sarah Darby of Cancer Research UK estimated the likely extent of the cancer risk based on the annual number of diagnostic x-rays undertaken in the UK and in 14 other developed countries. In addition to Canada, the other countries included in the study were Australia, Croatia, the Czech Republic, Finland, Germany, Japan, Kuwait, the Netherlands, Norway, Poland, Sweden, Switzerland and the US.

Using data on the frequency of diagnostic x-ray use, radiation risk models, population-based cancer incidence rates and mortality rates for all causes of death, the researchers calculated site-specific radiation-induced risks for eight types of solid tumour and for leukemia. The solid tumours were cancers of the esophagus, stomach, colon, liver, lung, bladder, breast and thyroid. In the UK, for people up to age 75, these eight cancer sites account for 56% of total cumulative risk of all solid cancers in men and 61% in women.

Of the nine cancers studied in detail, bladder cancer accounted for the largest number of radiation-induced cases per year in men, followed by colon cancer and leukemia. In women, colon cancer made the greatest contribution to the annual total, followed by cancers of the lung and breast. Given its poor prognosis, lung cancer might well account for the most radiation-induced cancer deaths in both sexes.

As it happens, the likelihood of being x-rayed increases with age in a similar fashion to the likelihood of getting cancer, so the burden of x-ray induced cancers is unlikely to fall on the young any more than getting cancers.

Canada fell roughly in the middle of the field for both frequency of x-rays and frequency of x-ray-induced cancers. The only Canadian data used, however, was the overall number of x-rays. Types of x-rays, ages of patients and organ-specific radiation doses were all extrapolated from British data. The radiation risk model itself was mostly based on UN studies of Japanese atomic bomb survivors.

Japan, incidentally, has by far the highest number of diagnostic x-rays, radiation doses and diagnostic radiation induced cancers. The authors estimate that x-rays account for 3% of cancers in Japan. Britain and Poland had the smallest percentage of radiation induced cancers, at 0.6%, essentially because they are less likely to use x-rays.

The diagnostic procedures most likely to cause cancers were coronary and cerebral angiography, and barium enema, with a risk rate of 160-280 cases per million examinations. CT scans, not surprisingly, also produced considerably more cancers than low-dose scans, at about 60 cases per million examinations.

The authors had to make a number of assumptions to arrive at their conclusions. They assumed that individuals who receive diagnostic x-rays have mortality rates equal to those of the general population; that low doses of radiation are as harmful per unit dose as higher doses; and that radiation-induced risks persist indefinitely. The authors freely admit that "If any of these assumptions is incorrect, the radiation-induced cumulative risks will be lower than those estimated, possibly by up to 50%." The authors cautiously conclude: "The possibility that we have overestimated the risks cannot be ruled out, but it seems unlikely that we have underestimated them substantially."