Reconstruction of local fallout composition and gamma-ray exposure in a village contaminated by the first USSR nuclear test in the Semipalatinsk nuclear test site in Kazakhstan

Accounting for shared and unshared dosimetric uncertainties in the dose response for ultrasound-detected thyroid nodules after exposure to radioactive fallout

Dosimetic uncertainties, particularly those that are shared among subgroups of a study population, can bias, distort or reduce the slope or significance of a dose response. Exposure estimates in studies of health risks from environmental radiation exposures are generally highly uncertain and thus, susceptible to these methodological limitations. An analysis was published in 2008 concerning radiation-related thyroid nodule prevalence in a study population of 2,994 villagers under the age of 21 years old between August 1949 and September 1962 and who lived downwind from the Semipalatinsk Nuclear Test Site in Kazakhstan. This dose-response analysis identified a statistically significant association between thyroid nodule prevalence and reconstructed doses of fallout-related internal and external radiation to the thyroid gland; however, the effects of dosimetric uncertainty were not evaluated since the doses were simple point "best estimates". In this work, we revised the 2008 study by a comprehensive treatment of dosimetric uncertainties. Our present analysis improves upon the previous study, specifically by accounting for shared and unshared uncertainties in dose estimation and risk analysis, and differs from the 2008 analysis in the following ways: 1. The study population size was reduced from 2,994 to 2,376 subjects, removing 618 persons with uncertain residence histories; 2. Simulation of multiple population dose sets (vectors) was performed using a two-dimensional Monte Carlo dose estimation method; and 3. A Bayesian model averaging approach was employed for evaluating the dose response, explicitly accounting for large and complex uncertainty in dose estimation. The results were compared against conventional regression techniques. The Bayesian approach utilizes 5,000 independent realizations of population dose vectors, each of which corresponds to a set of conditional individual median internal and external doses for the 2,376 subjects. These 5,000 population dose vectors reflect uncertainties in dosimetric parameters, partly shared and partly independent, among individual members of the study population. Risk estimates for thyroid nodules from internal irradiation were higher than those published in 2008, which results, to the best of our knowledge, from explicitly accounting for dose uncertainty. In contrast to earlier findings, the use of Bayesian methods led to the conclusion that the biological effectiveness for internal and external dose was similar. Estimates of excess relative risk per unit dose (ERR/Gy) for males (177 thyroid nodule cases) were almost 30 times those for females (571 cases) and were similar to those reported for thyroid cancers related to childhood exposures to external and internal sources in other studies. For confirmed cases of papillary thyroid cancers (3 in males, 18 in females), the ERR/Gy was also comparable to risk estimates from other studies, but not significantly different from zero. These findings represent the first reported dose response for a radiation epidemiologic study considering all known sources of shared and unshared errors in dose estimation and using a Bayesian model averaging (BMA) method for analysis of the dose response.

Gamma-ray thermoluminescence measurements: a record of fallout deposition in Hiroshima?

In certain Hiroshima neighborhoods, radiation measurements using thermoluminescence dosimetry (TLD) exceed what can be explained by the initial gamma-ray doses and uncertainties from the Dosimetry System 2002 (DS02). This problem was not previously recognized as being isolated to certain parts of that city. The ratio between TLD measurements and DS02 dose calculations for gamma rays appear to grow larger than unity up to more than three with increasing ground range, but closer examination shows the excess TLD dose (0.1, 0.2, or possibly up to 0.8 Gray) is correlated with certain neighborhoods and could be due to radioactive fallout. At Nagasaki, the TLD measurements do not show this same excess, probably because there were no TLD measurements taken more than 800 m downwind (eastward) from the Nagasaki hypocenter, so that any small excess TLD dose was masked by larger initial gamma-ray doses of 25-80 Gray in the few downwind samples. The DS02 Report had noted many measurements lower than the DS02 calculation for several Nagasaki TLD samples, independent of ground range. This was explained as being the result of previously unaccounted urban shielding which was observed from Nagasaki pre-bomb aerial photos. However, the Hiroshima excess TLD dose issue was not resolved. If the excess TLD doses at Hiroshima are an indication of fallout, it may be possible to use additional TLD studies to make better estimates of the locations and radiation doses to survivors from the fallout after the bombings at both cities.

Estimation of beta-ray skin dose from exposure to fission fallout from the Hiroshima atomic bomb

Beta-ray skin dose due to the fission fallout from the Hiroshima atomic bomb is potentially related to the epilation in the black rain area. The absorbed dose to the skin from beta-rays emitted by fission fallout has been estimated for an initial ¹³⁷Cs deposition of 1 kBq m⁻² on the ground at 0.5 h after the explosion. The estimated skin dose takes into account both external exposure from fission fallout radionuclides uniformly distributed in 1 mm of soil on the surface of the ground and from a 26 μm thickness of contaminated soil on the skin, using the Monte Carlo radiation transport code MCNP-4C. The cumulative skin dose for 1 month after the explosion is taken as the representative value. The estimated skin dose for an initial ¹³⁷Cs deposition of 1 kBq m⁻² was determined to be about 500 mSv.