Cancer myth: Medical imaging (x-rays) and cancer

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Origin of the misconception

High dose radiation, such as that from nuclear explosions, atomic bombs and nuclear power plant disasters causes birth defects and cancers.  It is not surprising that this makes people particularly cautious of other sources of radiation, such as x-rays and other forms of medical imaging.

Current evidence

X-rays are a type of ionising radiation.  Ionising radiation is capable of breaking molecular bonds and causing genetic mutations, and can lead to cancer and genetic defects.  We are exposed to ionising radiation every day from our environment – from the sun, outer space, rocks, plants, water and soil.  In fact, the environment accounts for the greatest average amount of population exposure.  We are also exposed to additional ionising radiation when we have an x-ray or fly in a plane.1, 2

The dose of x-rays we receive, in energy per kilogram, is measured in Sieverts (Sv).  We absorb around 2 milliSieverts (mSv) from the environment every year.  People who are exposed to nuclear radiation from nuclear weapons or nuclear power plant disasters are exposed to high-dose radiation.  High-dose radiation is considered to be exposure of 200 mSv or more. The amount of radiation which the general population is exposed to is in the low-dose range.1

Studies have associated x-ray exposure with thyroid, breast, and lung cancer and leukaemia.  The US Report on Carcinogens (RoC) is a list of known or reasonably anticipated human carcinogens (cancer causing substances).  X-rays are classified as a known human carcinogen in the RoC.3

The International Agency for Cancer Research (IARC), an agency of the World Health Organization, classifies x-rays as carcinogenic to humans.4

Risk-versus benefit

X-ray screening is a powerful tool for detecting diseases in soft tissue and for imaging bones and teeth, to check for breakage or decay.  Many x-rays, including diagnostic mammography or having an x-ray to detect a broken bone or decayed tooth result in immediate health benefits.

Equally, it is important not to have x-rays that are unnecessary.  An important question to ask yourself and your doctor is, “will this x-ray change my diagnosis or treatment?”.  Needless x-rays are an unnecessary source of radiation and should be avoided.

CT (computed tomography) scans form three dimensional (3D) pictures of the inside of the body which can help to monitor health conditions and detect cancer, disease or inflammation.  CT scans deliver a much higher radiation dose, with some delivering up to 10.6 mSv (Figure 1).  People who undergo multiple investigations can be exposed to doses of more than 50 mSV in one single CT session.5

There is no need to be reluctant to take x-rays that are necessary for diagnosis or for screening.  The radiation dose from most x-rays is well below the range absorbed from the natural environment (1.5 - 2.0 mSv).

Typical values of effective dose for various medical x-rays

Figure 1: Typical Values of Effective Dose for Various Medical X-rays6

Alternatives to x-rays

Many medical imaging techniques are based on the use of x-rays, such as mammography, CT (or CAT) scans and fluoroscopy.  Positron emission tomography (PET) scans, another form of medical imaging, use gamma rays, also a form of ionising radiation.  Ultrasound and magnetic resonance imaging (MRI) are medical imaging techniques that do not use ionising radiation. 

Ultrasound uses high frequency sound waves to produce a real-time (moving) image.  They are commonly used to examine internal organs, blood vessels and foetuses in pregnant women.

MRI uses magnets and radio waves to produce two and three dimensional images of soft tissues in the body.  These images can be used to detect abnormalities, the exact site of tumours and for screening.  MRI is not suitable for people with metal implants, pacemakers and cochlear hearing aids. 

In some cases, either or both of these techniques can be used instead of x-rays.  For some conditions and in some cases however use of these techniques may be impractical, inappropriate or too expensive.

Summary

The use of x-rays can be extremely beneficial in the detection and treatment of many diseases, cancers, and bone problems.  Human exposures to radiation should follow the ALARA principle; that is, be as low as reasonably achievable.7  This ensures that the benefits balance the risks.  Pregnant women are not advised to have abdominal x-rays due to the effects they may have on the development of the unborn child.

Further reading

References

1. Boice Jr, J., Ionizing Radiation, in Cancer Epidemiology and Prevention, D. Schottenfeld and J. Fraumeni Jr, Editors. 2006, Oxford University Press: New York, NY. p. 259-293.
2. Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). Cosmic Radiation Exposure. Radiation Protection  2006  [cited 08/08/2014]; Available from: http://www.arpansa.gov.au/radiationprotection/Factsheets/is_cosmic.cfm.
3. U.S. Department of Health and Human Services, 12th Report on Carcinogens. 2011, Public Health Service - National Toxicology Program.
4. International Agency for Research on Cancer (IARC), Volume 75: Ionizing Radiation, Part 1: X- and Gamma (g)-Radiation, and Neutrons, in IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, IARC, Editor. 2000, IARC: Lyon, France.
5. Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). X-rays. Radiation Protection  March 2014  [cited 08/08/2014]; Available from: http://www.arpansa.gov.au/radiationprotection/Basics/xrays.cfm.
6. Australian Radiation Protection and Nuclear Safety Agency (ARPANSA), Typical values of effective dose for various medical x-rays. 2004.
7. International Commission on Radiological Protection (IRCP), P103: The 2007 Recommendations of the International Commission on Radiological Protection. Annals of the ICRP, 2007. 37(2-4): p. 1-332.

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