In this issue of The American Journal of Medicine, Yamada et al found no effects on the decline in cognitive function among atomic bomb survivors exposed at or after adolescence. In recent years, clinicians have been increasingly bombarded with information about the harmful effects of radiation exposure, so it is novel to receive some apparent good news about radiation.
Despite the widespread use of radiation in medical imaging, most professionals have a limited understanding of the significance of radiation exposure and its effects. Because radiation effects are dependent on dose, a basic understanding of radiation dose is required. Although dose seems like a simple concept, it is complicated by the fact that there are many different doses (eg, skin dose, organ dose, effective dose) and there are 2 different dose systems (the US is one of the few countries that still does not routinely use SI units). In the US, roentgens, rads, and rems are routinely used in clinical practice instead of the SI units of coulomb/kg, gray, and sievert. Many journals, including this one, have adopted SI units.
Average radiation doses from natural background radiation in the US are about 3 mSv per year. The cumulative 70-year dose from natural background radiation in the US is 210 mSv. The radiation dose (effective dose—see below) from a simple computed tomography (CT) scan is typically around 10 mSv (3-4 times the annual dose from natural background radiation).
Effective dose is probably the most commonly used dose estimate in medicine. The effective dose is calculated by summing up organ doses after they are weighted by a factor that is related to the radiation sensitivity of that organ. Effective doses are popular because they provide a way to compare the risk of very different radiation exposures in medicine (for example, the risk of a chest CT compared with an abdominal CT or a nuclear medicine study; Table 1, Table 2). The numbers in these tables provide reasonable dose estimates for the average patient who receives an examination at an institution that participates in dose reporting; however, these numbers can vary greatly.
Radiation effects are determined by the radiation dose. There are 2 very different types of radiation effects (stochastic and deterministic). Stochastic effects are rare chance events such as cancer induction. Radiation is a relatively weak carcinogen. The second type of radiation effect is a deterministic effect. Deterministic effects are very different from stochastic effects. Deterministic effects include effects such as skin reddening as a result of radiation therapy. Deterministic effects have very different characteristics than stochastic effects. For deterministic effects, there is a threshold (they do not occur at low doses); many cells need to be affected and the severity of the effect varies with the dose (the higher the dose, the worse the effect). Dementia would likely be a deterministic effect. Dementia would not occur unless many cells were affected.
One of the earliest studies of the atomic bomb survivors was assessing cognition impairment in children who were exposed in utero.6 There was a significantly increased occurrence of severe mental retardation and a significant decrease in IQ related to dose. These findings were greatest when the exposure occurred at 8-15 weeks of gestation and were not significant when the exposure occurred prior to 8 weeks of gestation or after 25 weeks of gestation. As would be expected with a deterministic effect, there appears to be a threshold between 200 and 400 mSv (the equivalent of about 20-40 CTs of the brain). The threshold is likely higher in adolescence and adults where there is not the same rapid growth of the central nervous system as there is in the developing fetus. Loss of cognitive function has also been reported after radiation therapy involving the brain,7 especially in children, but these radiation doses are thousands of times greater than doses for diagnostic studies.
Given this background, the results of the Yamada et al paper are not surprising because their subjects were exposed at or after adolescence. Studies of the atomic bomb survivor have been very valuable in furthering our understanding of radiation effects because of the size of the cohort and the length and rigor of the study. In addition, the cohort is representative of the general population (men, women, children), and the dose is very uniform, so information has been provided about the radiation sensitivity of all organs. The paper by Yamada et al adds to our extensive knowledge of radiation effects.
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-Henry D. Royal, MD
This article originally appeared in the June 2016 issue of The American Journal of Medicine.
