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	X-rays have been the cause of more than 5,600 cancer cases in the United States according to the study.
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Diagnostic X-rays could be causing almost one of every 1,000 cases of cancer in the United States, according to the estimates of a new British study recently published in the Lancet.

Diagnostic X-rays and other radiodiagnostic devices are the largest man made sources of radiation. Unfortunately, the effects of radiation from these devices has not been previously studied in a reliable way.

The importance of this new study lies in the fact that there are no reliable data proving that radiation doses used in diagnostic X-rays do indeed contribute to the development of cancer. On the other hand, there is no threshold of radiation dose under which the absence of any cancer risk is proven.

X-rays are a form of electromagnetic radiation of extremely short wavelength. X-rays travel at the speed of light. On the electromagnetic spectrum they lie between gamma rays and ultraviolet radiation. Gamma rays have a high penetration ability, beta particles come next whilst alpha particles are the heaviest and hence have the lowest penetration ability.

To minimize the risks of developing caner, X-rays are given at a low radiation doses. A chest X-ray delivers three to ten days' worth of low, background radiation.

Background radiation comes from radioactivity in our own bodies and the other bodies in the cosmos. It amounts to approximately 3 millisievert (mSv) per year. Below is a list of other X-ray studies and their corresponding radiation doses.

The benefits of X-rays is beyond doubt as regards diagnosis and even in the industrial sector. However, fear arises as to the effects of the radiation emitted by them and the consequences this may have on the human body. In the lab radiation can penetrate through cells and damage DNA and in some lab tests this has led to cancer. Theoretically the same can be caused by the radiation effects of X-rays in real life.

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Previous attempts to quantify the risk of cancer from X-rays have been made before. The most recent previous study was conducted in 1981 and found that X-rays probably accounted for 0.5% of cancers in the United States. The new estimates are twice that high for the United States. This study represents a much needed update to the previous study as the use X-rays has increased dramatically in the last 20 years.

Fear of the risk arises because the number of CT exams done in recent years has increased steadily in comparison to conventional radiology. CT exams deliver larger doses of X-rays than conventional radiographs.

The study published in the Lancet used cancer-rate data which was collected between 1991 and 1996 from survivors of the Japanese atomic bombings as a model to study the risk of cancer from the X-rays. They used the survivor data because it was the only time in history in which a large population was exposed to ionizing radiation. The disadvantages of using this model include the fact that the survivors were not only exposed to X-rays and gamma rays but also to beta radiation and maybe even alpha radiation through contaminated water and food.

These researchers compiled their data on the incidence of cancers from tumor registers in the UK and 14 other countries. They compared these statistically with the numbers of X-ray procedures done in these countries and statistically analyzed the numbers of cancers induced by the radiation exposure from these procedures. The lifetime risk of developing cancer attributable to diagnostic X-rays was 0.6-1.8% in the countries investigated, except in Japan where the lifetime risk was 3.2%, where X-ray use is more common. In the US this was estimated to amount to 0.9% of all cancer cases. In plain English this means that X-rays have been the cause of more than 5,600 cancer cases in the United States.

It isn't possible to know who will develop cancer and which type of cancer on exposure to X-rays. To conclude, X-rays (and CTs) should be prescribed by doctors when they are absolutely necessary otherwise resorting to safer methods of investigation (e.g. ultrasonography, MRI) is always better. That should not undermine the benefits of X-ray screening such as routine mammograms to help early detection of cancer.

References:

Berrington de Gonzalez, A and Darby S. Risk of cancer from diagnostic X-rays: estimates for the UK and 14 other countries. The Lancet. 363, 345-351. January, 2004.

Article reviewed by:	Dr. Tamer Fouad, M.D.
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