Answer to Question #9489 Submitted to "Ask the Experts"

A presentation was made at the 2010 AHA (American Hospital Association) meeting that concluded: "For every 10 millisievert (mSv) increase in Low Dose Ionizing Radiation (LDIR), the risk for cancer increased by 4.4% in women and 2.1% in men." This was a Canadian respective study* that did not consider cohort cancer risk vs. control risk (i.e., patients referred for cardiac studies might also be at higher-than-average cancer risk).

Do you think this is true or do you have any comment?

Is it not time to properly do the extremely difficult task of long-term prospective studies and is the Health Physics Society (HPS) calling for studies to be done? The HPS seems to be fair minded.

(1) Does the HPS agree with the conclusions of a reported study regarding the quantitative risk of LDIR at 10 mSv, (2) does the HPS think it is time to launch on a long-term prospective human epidemiological study to look for risk from low-dose ionizing radiation, and (3) is the HPS calling for such studies?

*Footnote:  Afilalo J, Lawler PR, Eisenberg MJ, Richard H, Pilote L. Age and sex differences in risk of cancer with low dose ionizing radiation from cardiac imaging procedures after acute myocardial infarction [abstract]. Circulation. 2010;122 (21 Suppl Nov 23): A17486.

The Scientific and Public Issues Committee of the HPS offers the following response to your questions, in order:

1. The HPS does not take positions on individual reported studies. The HPS relies on the system of "consensus science" to develop positions. The system of consensus science includes having reports published in the scientific peer-reviewed literature that is responded to or judged by scientific colleagues. This also includes acceptance of scientific consensus reports by independent scientific committees such as the United Nations Scientific Committee on the Effects of Atomic Radiation, the National Academy of Sciences Committee on the Biological Effects of Ionizing Radiation, and the National Council on Radiation Protection and Measurements. As for the general question of radiation risk attributed to exposures of 10 mSv, the HPS position is expressed in its position statement "Radiation Risk in Perspective". This position recommends NOT attempting to quantify risks at individual exposures less than 50 mSv y^-1 or 100 mSv per lifetime. Thus, this study makes conclusions about the quantitative risk of ionizing radiation exposure below those levels at which the HPS believes it can be quantified.
2. As for the utility of performing long-term human epidemiological studies to look for the risk of low-dose radiation exposure, the HPS policy is that funds should NOT be expended to perform human epidemiological studies that have a low statistical power due to small populations or low individual doses or for which there is insufficient data to account for confounding factors. See the HPS Policy Paper "Health Physics Society Policy on Expenditure of Funds for Ionizing Radiation Health Effects Studies."
3. Due to the fact that human populations with the required characteristics (i.e., that have been exposed to low-dose radiation in the range of 50 mSv or below, that are large enough to provide an acceptable statistical study power, and that have sufficient history and data to account for likely confounding factors) do not exist, the HPS is not calling for such studies to be performed.

The HPS frequently receives questions on the Ask the Experts (ATE) feature of the HPS website from people who have received one or more diagnostic medical exams. We routinely answer this kind of ATE question with as much information taken from various standard published data sources as we can provide about typical radiation doses for the procedure.

We usually add a statement that "any theoretical risks of the radiation exposures that you received were far smaller than the direct benefits of the study." We say "theoretical" because all risk information is based on the so-called "linear no-threshold" (LNT) theory, which assumes that risks (mostly of cancer) observed in populations exposed to high doses of radiation can be extrapolated to the much lower doses that we deal with in diagnostic medical procedures, radiation-worker situations, and other circumstances.

No one knows if this extrapolation is valid, but it is used to set radiation dose limits for workers, make general evaluations about risks and benefits of medical exposures, and estimate risks in other applications. Importantly, the data upon which the LNT model is based begin at doses above 100 mSv. Many believe that the LNT approach overestimates the actual risks at such low doses and some even believe that there is evidence for a threshold for such effects.

Unfortunately an equivalent model to LNT has not been generally accepted in the scientific world for quantitative estimate of the benefits of the exposures to go along with the quantitative estimates of risk. However, Pat Zanzonico, PhD, at the Memorial Sloan-Kettering Cancer Center in New York, has prepared a paper on that subject that might be of interest to you. It can be found on the HPS website.

In a related article by Thomas C. Gerber and Raymond J. Gibbons (Weighing the Risks and Benefits of Cardiac Imaging with Ionizing Radiation, Journal of the American College of Cardiology, Cardiovascular Imaging, Vol. 3, No. 5, 2010), their review summarizes the evidence regarding both the radiation risks and the clinical benefits of cardiac imaging. Both this reference and the Zanzonico article are from recognized experts, but do not necessarily represent any official position of HPS. The HPS has provided a general position statement on risk assessment.

Howard W. Dickson, Chair
Scientific and Public Issues Committee
Health Physics Society

Download the following Portable Document File file to complete the answer to your question: Gerber_Gibbons_Article.pdf