RERF Report No. 3-17

Paper published in Radiation Research§“Cytogenetic reconstruction of gamma-ray doses delivered to atomic bomb survivors:
Dealing with wide distributions of photon energies and contributions fromhematopoietic stem/progenitor cells”
Nori Nakamura, Yuko Hirai, Yoshiaki Kodama, Kanya Hamasaki, Harry M. Cullings,
Kismet A. Cordova and Akio Awa
Radiat Res 2017 (October); 187(4):412-8(doi: 10.1667/RR14832.1)
Study Findings
Chromosome aberration frequencies in blood lymphocytes are one marker used to biologicallyestimate radiation doses received by individuals. However, biodosimetry based on chromosome
aberration frequencies has not been used in the case of A-bomb survivors for two reasons: one is thatchromosome aberration frequency differs by gamma ray energy, and A-bomb gamma rays have arange of energies. The other reason is that survivors were exposed so long ago that some lymphocytes
examined were hematopoietic stem/progenitor cells at the time of exposure of which theirradiosensitivities are not well understood.
To overcome these hurdles, we compared tooth enamel radiation doses estimated using electron
spin resonance (ESR) with doses in the same survivors estimated from chromosome aberration
frequencies. (We assumed that all cells had been lymphocytes at the time of exposure and that theA-bomb gamma-ray spectrum was equivalent to that of 60Co gamma rays.) This comparison, which
factored in shielding differences between the teeth and bone marrow, showed close agreement between
these two sets of biologically estimated doses.
Based on these study results, we concluded that unadjusted 60Co gamma-ray equivalent doses
estimated from chromosome aberration frequencies may be an accurate indication of A-bomb
gamma-ray doses.
Explanation
Chromosome aberration frequencies in blood lymphocytes can be used in one method to estimate
radiation doses received by individuals after exposure. Blood lymphocytes are irradiated in vitro with60
Co gamma rays to obtain a dose-response curve between radiation dose and chromosome aberration
frequency. The chromosome aberration frequencies are then determined to estimate exposure doses.
This procedure, however, has not been successfully used to estimate radiation doses received byA-bomb survivors for two reasons. One is that the wide distribution of energies released from theA-bombs makes model experiments impossible. Yet model experiments are needed to account for thefact that a given dose yields different chromosome aberration frequencies depending on thegamma-ray energy.
The second is that cells that were immature and differentiating at the time of exposure could haveeventually entered into the lymphocyte pool after considerable time had passed. Immature cells could
be more radiosensitive than blood lymphocytes but cannot be differentiated from old lymphocytes.
When doses are estimated, all lymphocytes examined must therefore be assumed to have been mature
lymphocytes at the time of exposure.
To overcome these hurdles, we estimated doses in about 100 A-bomb survivors from tooth1

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enamel, which is less dependent on photon energies, using ESR and from chromosome aberration
frequencies. The latter estimates assume that all cells exposed to radiation were blood lymphocytes(differentiated mature cells) and that the effect of A-bomb gamma rays was the same as that ofmonoenergetic 60Co gamma rays.
Study Results
Radiation doses estimated from tooth enamel using ESR were slightly larger than 60Co
gamma-ray equivalent doses estimated from chromosome aberration frequencies. This difference isattributable to the larger shielding effect of bone on the bone marrow, where many lymphocytes reside,than the shielding effect of skin on teeth. We therefore used individual shielding data to estimate
kerma doses before estimating organ doses. (Kerma dose refers to the dose present in the space theindividual occupied at the time of exposure.) The two sets of dose estimates agreed closely.
Based on these study results, we concluded that unadjusted 60Co gamma-ray equivalent doses
estimated from chromosome aberration frequencies may be an accurate indication of A-bomb
gamma-ray doses.
Study SignificanceOur findings support the idea that radiation doses estimated from chromosome aberration
frequencies are directly comparable to those physically estimated based on distance from thehypocenters and shielding of A-bomb survivors (DS02R1 doses). This understanding paves the way
for use of chromosome aberration frequency data to determine whether physically estimated doses
contain any systematic biases.
The Radiation Effects Research Foundation has studied A-bomb survivors and their offspring in Hiroshima
and Nagasaki for around 70 years. RERF’s research achievements are considered the principal scientific basis forradiation risk assessment by the United Nations Scientific Committee on the Effects 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.
§Radiation Research, which is an official monthly journal of the Radiation Research Society, publishes
original peer-reviewed papers and review articles on radiation effects and related issues in the fields ofphysics, chemistry, biology, and medicine. (Impact factor in 2016/2017: 2.539)

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