Answer to Question #11154 Submitted to "Ask the Experts"

I heard some people are more sensitive to radiation than others. How can I know whether I am more sensitive to radiation or not?

Will the immune system decline after I had a computed tomography (CT) head scan if I am more sensitive to radiation?

I also want to know how people calculate the cancer risk probability and whether different authorities calculate different probabilities.

Just as some people are more sensitive to various drugs and some to certain chemical toxic agents than others, the same is true for sensitivity to radiation. To my knowledge, at the levels of radiation exposure that you are concerned with (typically about 0.02 sievert (Sv) effective dose for a CT head scan) there is no likelihood that any natural elevation in sensitivity to radiation that you might have would produce any negative effect. The CT head scan that you had would not produce any effects on your immune system. At considerably higher doses we could be concerned, but we do not presently have any way to identify which individuals may be inherently more sensitive than others.

If you were concerned about much higher doses, greater than perhaps 1 Sv, there would be some identifiable sensitivity factors, independent of any genetic predispositions, that might be significant. Any deficiencies in one's blood-forming system (e.g., anemia, leukemia) may result in increased sensitivity because of possible additional impacts of the radiation on the blood-forming system, which is one of the body's critical and relatively sensitive systems. Individuals who suffer from gastrointestinal (GI) tract disorders/diseases may also sustain greater effects from high doses of ionizing radiation because of possible effects to sensitive stem cells in the GI tract and subsequent damage to the lining of the GI tract. Any illness or disease that reduces an individual's ability to respond effectively to insults from radiation or other toxic agents may enhance sensitivity to radiation.

The cancer risk numbers that we have adopted are based on results of epidemiological studies of populations who have received large doses of radiation, most notably the survivors of the Hiroshima and Nagasaki bombings. These doses were delivered over very short intervals of time and were much higher doses than the dose that you accrued from your medical scan. For radiation protection purposes, our cancer risk values have been developed by plotting the cancer responses vs. dose for the high-dose individuals and then extrapolating the curve (or line) backwards to zero dose with the assumption that the cancer risk per unit dose is the same at very low doses as it was at the high doses. There is no direct evidence that doses at the level of concern to you are capable of inducing cancer but, for protection purposes, we assume that the extrapolated risks are legitimate. This is an area of ongoing controversy.

Two major and most authoritative groups that have done reviews of available data and generated results for cancer risk estimates are the Committee on the Biological Effects of Ionizing Radiations, Board on Radiation Effects Research, Division on Earth and Life Studies, National Research Council of the National Academies, and the United Nations Scientific Committee on the Effects of Atomic Radiation. Both of these groups have done extensive investigations and have reached pretty much the same conclusions regarding biological risks from ionizing radiation exposure. The most recent report of the first group (BEIR VII 2006) showed a cancer incidence risk factor of 0.08 per man-Sv for males and about 0.13 per woman-Sv for females. Using an approximate overall value of 0.1 cancers per person-Sv we would predict, for example, an excess of 10 cancers in a population of 100 people exposed each to 1 Sv (i.e., 100 people x 1 Sv x 0.1 cancers/person-Sv).

I hope this is helpful to you.

George Chabot, PhD

Reference

National Academies/National Research Council. BEIR VII Phase 2: Health risks from exposure to low levels of ionizing radiation. Washington, DC: National Academies Press; 2006.