X-Ray Visions: What is your radiation risk?

January 29, 2009

xraylogo.jpgCommentary by Michael Hanley MD PGY-3 and James D. Koonce MD PGY-3, Department of Radiology, Medical University of South Carolina

www.X-rayRisk.com - A Free Online Calculator that Estimates an Individual’s Additional Risk of Cancer as a Result of Medical Imaging

An estimated 62 million CT scans are obtained in the United States each year.(1) While debated, a recent study suggests that radiation exposure from medical imaging may be responsible for 1-3% of cancers worldwide.(2) With recent media coverage focusing on the risk of cancer from medical imaging, patients and physicians have become more concerned about the increased use of CT scans and x-rays. Patients are asking their primary care providers and emergency room physicians for information about their personal risk. In 2004, Lee et al concluded that “patients are not given information about the risks, benefits and radiation dose for a CT scan”. Additionally, this study found that both patients and physicians were “unable to provide accurate estimates of CT doses”.(3)

While the need for education in this area has clearly been established, there are no widely available resources that provide information to both patients and health care providers about the increased risk of cancer from medical imaging. X-rayRisk.com is a free educational website that caclulates this risk. The site is specifically designed for both patients and health care providers. One of the site’s main features is a web-based calculator that allows users to track their imaging history and estimate their personal risk. The site also provides basic information about radiation in the form of ‘Frequently Asked Questions’.

There are no published studies that prove the direct causality between medical imaging and cancer, however there is enough data to warrant precaution. Current data on radiation exposure and cancer risk is based on data from survivors of atomic bombs, nuclear accidents and the early use of x-rays. The assumed risk of cancer from medical imaging (primarily CT scans) is based on individuals exposed to atomic bombs and nuclear accidents. The theory that the increased risk of cancer holds true at these lower doses is called the linear no threshold model and is generally accepted in the scientific community.

From the Home Page of X-rayRisk.com, the user can select the ‘Calculate Your Risk’ icon. Next, the user enters his or her gender and age. There is an extensive list of different plain films (x-rays), CT scans, nuclear medicine studies, as well as fluoroscopic and interventional procedures. The user’s radiation dose, age and sex are used to calculate the additional risk of cancer based on that study. After a user builds their complete imaging history, they have the option to print, e-mail or save their report. The user can also log-in (optional) and save their report to revisit and update with subsequent studies.

Exposure data for the website was compiled from the National Academy of Sciences report on the Health Risks From Exposure to Low Levels of Ionizing Radiation in 2006, specifically the Biological Effects of Ionizing Radiation (BEIR) VII Phase 2 Report.(4) Average doses from common imaging studies were used, which were adapted from Mettler et al.(5) If exact dose is known, the user can convert their Dose Length Product (mGy • cm) to Effective Dose (mSv), then calculate additional cancer risk using conversion factors adapted from Huda, et al.(6) The website is also designed to assist in radiation exposure research and clinical record keeping.

Great effort has been made throughout the medical community to ensure patient safety while providing quality diagnostic images. It is important to realize that in a properly performed individual exam, the potential health benefits almost always outweigh the potential risks of radiation exposure. However, patients are expecting their physicians to address these risks. X-rayRisk.com provides accurate information for patients and health care providers to facilitate well-informed discussions about the increased risk of cancer from medical imaging.

1. Brenner DJ, Hall EJ. Computed Tomography – An Increasing Source of Radiation Exposure. NEJM 357: 2277-84, 2007. 2. Berrington de Gonzalez A, Darby S. Risk of cancer from diagnostic x-rays: estimates for the UK and 14 other countries. Lancet 363:345-51, 2004.
3. Lee CI, Haims AH, Monico EP, et al. Diagnostic CT Scans: Assessment of Patient, Physician, and Radiologist Awareness of Radiation Dose and Possible Risks. Radiology 231 (2): 393-398, 2004.
4. National Research Council. Health risks from exposure to low levels of ionizing radiation. BEIR VII Phase 2. Washington, DC: National Academies Press, 2006.
5. Mettler FA, Huda W, Yoshizumi TT, Mahesh M: Effective Doses in Radiology and Diagnostic Nuclear Medicine: A Catalog. Radiology 248: 254-263, 2008.
6. Huda W, Ogden KM, Khorasani MR: Converting Dose-Length Product to Effective Dose at CT. Radiology 248:995-1003, 2008.

One Response to X-Ray Visions: What is your radiation risk?

  1. Owen Hoffman on March 31, 2009 at 1:51 pm

    This risk calcualtor is a step in the right direction. However, I have a few suggestions to make:

    It would be best to base risks on the abosorbed dose (Gy) in specific tissues and to address risks based on all tissues exposed. The health physics concept of “effective dose” is not a good indicator of radiogenic cancer risk since many simplifying assumptions are made to adjust individual tissue doses to compare the risk per organ and tissue site as if the whole body had been exposed. Rather crude adjustments are made that account for differences in disease incidence, disease severity and the potential of years of life lost. Thus, very low weights are given to the incidence of bone, skin, and thyroid cancer, for example. By using the absorbed organ dose as the starting point for the calculation of risk, all other factors needed to translate dose to risk can be identified and addressed explicitly.

    The present risk calculator needs to account for uncertainty in risk estimation. This uncertainty will be largest when doses affect a single organs and somewhat smaller when multiple organs are irradiated.

    BEIR VII appears to underestimate uncertainty in risk to multiple organs exposed acutely at doses commensurate with those associated with CT scans. At present, almost all of the uncertainty in the BEIR VII risk to all solid cancers combined is due to the low dose and dose rate effectiveness factor. This factor accounts for the difference in risk between acture and fractionated and prolonged-chronic exposures.

    For all cancers combined, the uncertainty in the BEIR VII risk estimates at low doses and low dose rates is about a factor of two either side of a central estimate. This uncertainty would increase if differences in risk between exposure to x-rays and high energy gamma radiation were to be considered or when exposurs occur to only a few organ sites as compared with all organ sites.

    A distinction needs to be made in the risk calculator between acute exposures to high enery gamma radiation(experienced by the LSS cohort of Japan) and fractionated exposures to diagnostic x-ray procedures which are composed of lower energy photons. In general, exposure to lower energy photons should be associated with higher risks than exposure to high energy gamma rays.

    The risk of non-fatal basal cell carcinomas should be included with those of other solid tumors. The risk of basal cell carcinomas are currently absent from the BEIR VII risk estimates and thus absent from your overall risk reults.

    Lastly, in addition to an estimate of the excess lifetime risk of cancer incidence, an estimate should be provided of the probability of causation (with uncertainty) for individuals eventually diagnosed with cancer who were exposed previously to medical diagnostic forms of radiation. For individuals exposed to several CT scans, for example, a cancer later in life might be associated with a fairly high chance that past exposure to the series of CT scans contributed to the manifestation of disease.

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