RERF Report No. 8-15

Paper published in The American Journal of Medicine §“Radiation Effects on Cognitive Function among Atomic Bomb Survivors Exposedat or after Adolescence”
Michiko Yamada, Reid D. Landes, Yasuyo Mimori, Yoshito Nagano, Hideo Sasaki

Am J Med 2016 (June); 129(6):586-91(doi: 10.1016/j.amjmed.2015.09.002)

Study Findings
We analyzed longitudinal cognitive decline data spanning two decades from atomic bomb
survivors exposed in or after adolescence. Radiation exposure was not significantly associatedwith late-life pre-dementia cognition level or age-associated cognitive decline.
Explanation
Cognitive function was evaluated in medical examinations performed every other year
from 1992 through 2011. The effects of radiation exposure on longitudinal pre-dementia
cognitive decline were estimated after adjustment for age, sex, education level, and occurrenceof dementia during follow-up.
1. Study purpose
The objective was to investigate what effects radiation exposure in or after adolescence hason late-life cognitive function by estimating longitudinal cognitive decline in the participants
who did and in those who did not develop dementia during follow-up.
2. Study methods
To observe longitudinal pre-dementia cognitive decline, cognitive function was assessed in
medical examinations of 1,844 participants in the Hiroshima Adult Health Study (AHS) who
were exposed to atomic bomb radiation at or after the age of 13 and, in 1992, were between 60
and 80 years of age and free of dementia. The Cognitive Abilities Screening Instrument (CASI),
a standardized survey of cognitive ability, was used to evaluate cognitive function. Of that total,
313 participants developed dementia during follow-up from 1992 through 2011. A
mixed-effects* model was used to analyze changes over time in pre-dementia cognitive scores.
*Mixed-effects model: This statistical model is useful for analyzing repeated measurements
over time. It factors in both fixed and random effects (hence its name).
3. Study results(1) Cognition level was significantly lower in participants who were older, had a low level of1

RERF Report No. 8-15

education, or developed dementia but was unrelated to radiation dose or sex.
(2) Cognitive decline accelerated with age, dropping particularly rapidly in participants who had
developed dementia. Tendencies in age-associated cognitive decline were unrelated to radiation oreducation.
(3) Radiation dose did not significantly affect cognition level or tendencies in age-related cognitive
decline in the participants who did and in those who did not develop dementia.
Study SignificanceInterest is growing in the effects of radiation exposure on cognitive function as morepatients survive malignancies. Research on survivors of cancer radiation therapy has shown
cognitive decline many years after exposure in younger patients exposed to high doses. But theeffects of high radiation therapy doses for cancer treatment are not easily distinguished from theeffects of the underlying cancer and chemotherapy. The effects of moderate and lower radiation
doses or in patients undergoing radiation therapy at relatively older ages, moreover, have notbeen sufficiently studied. In this study, we evaluated cognitive function in elderly atomic bomb
survivors exposed once to a moderate or lower dose (≤4 Gy) in or after adolescence. We did not
find that radiation exposure significantly affected cognition. A similar cognitive study has been
carried out since 2011 of AHS participants who were 12 years or younger at the time of theA-bombings. Upon completion of the four-year cross-sectional study, those data will beanalyzed.

The Radiation Effects Research Foundation has studied A-bomb survivors and their offspring in
Hiroshima and Nagasaki for more than 60 years. RERF’s research achievements are considered theprincipal scientific basis for radiation risk assessment by the United Nations Scientific Committee on theEffects of Atomic Radiation (UNSCEAR) and for recommendations regarding radiation protection
standards by the International Commission on Radiological Protection (ICRP). RERF expresses its
profound gratitude to the A-bomb survivors and survivors’ offspring for their cooperation in our studies.

§The American Journal of Medicine provides a medium for the prompt publication of originalarticles on internal medicine from around the world. The Journal places special emphasis on articles
that provide guidance to clinicians around the world, report cutting-edge science related to internal
medicine, educate readers about relevant and practical clinical outcomes in internal medicine, andsummarize or editorialize the current state of the literature. (Impact factor in 2014: 5.003)

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