OXFORD, UNITED KINGDOM. It is estimated that about 14% of total radiation exposure worldwide is attributable to medical x-rays. This figure is undoubtedly significantly higher in developed countries where modern diagnostic facilities are more abundant. It is known and accepted that any exposure to ionizing radiation (x-rays) increases the risk of cancer. What is less clear and still somewhat controversial is just how significant the risk increase actually is. Researchers at the University of Oxford have just completed a major study to attempt to shed light on this question. Their report makes sobering reading indeed.
The researchers estimated cancer risk based on known cancer rates and exposure to medical x-rays in 14 developed countries including Canada, Australia, Germany, Sweden, Switzerland, the UK, and the USA. They used exposure-based risk estimates based on populations exposed to known doses such as Japanese atomic bomb survivors. All exposure data are expressed in milliGray (mGy), which is a measure of the energy deposited in the tissue by the radiation beam. One mGy is equivalent to 1 milliSievert (mSv) in the case of medical x-rays. The researchers estimated organ-specific radiation doses by type of diagnostic x-ray. These varied from 0.00 mGy in the case of bladder exposure from a chest x-ray to 43.9 mGy in the case of thyroid exposure from a CT scan of the cervical spine. Table 1 depicts organ exposures in excess of 10 mGy caused by different x-ray techniques. NOTE: The Canadian Safety Board has set an upper limit on overall annual radiation exposure of 1 mSv and US authorities have set an annual limit of 50 mSv for specific organ exposure.
Organ Exposure, mGy
A mammogram produces a breast radiation exposure of 2.0 mGy (per view) and a dental x-ray (wing bite) yields a radiation exposure of about 0.1 mGy. It is clear that the radiation exposure involved in diagnostic x-rays can be substantial and should be avoided whenever possible. In some cases it may also be possible to substitute a high radiation procedure (eg. barium enema) with an alternative low-risk procedure (eg. colonoscopy).
The researchers found that radiation exposure contributed 0.6% to the overall cancer incidence in the 14 countries surveyed. However, for Japan, where more procedures are done, the risk contribution was 3.2% while it reached 0.9% in the US due to higher doses used per procedure in an attempt to get clearer images.
Table 2 shows the estimated lifetime risk (to age 75 years) attributable to diagnostic x-rays.
Cumulative Risk by Organ, %
The researchers estimate that coronary angiography accounts for 280 cases of cancer per million examinations, cerebral angiography for 180 cases, and barium enema for 170 cases. German radiologists, in commenting on the study, conclude that, "Up to 30% of chest x-rays may not be indicated; unnecessary CT examination can lengthen hospital stay as well as causing radiation exposure. In everyday practice, those ordering radiological procedures should think carefully about the benefit for and the risk of their patients for each examination."
Editor's comment: While the overall cancer risk attributable to medical x-rays is fairly low it is certainly clear that patients who undergo several procedures can accumulate a substantial risk. It should be kept in mind that the tissue damage induced by x-rays does not diminish with the passage of time, but remains for life. It is possible to significantly reduce x-ray-induced radiation damage by supplementing with 30 mg/day of beta-carotene for at least a week prior to x-ray exposure.