Comparison of internal doses calculated using the specific absorbed fractions of the average adult Japanese male phantom with those of the reference computational phantom-adult male of ICRP publication 110

Organ Dose Reconstruction Applicable for a Japanese Nuclear Worker Cohort: J-EPISODE

ABSTRACT

An evaluation of cancer risk based on organ-absorbed dose is underway for the Japanese Epidemiological Study on Low-Dose Radiation Effects (J-EPISODE), which has analyzed health effects in association with radiation exposure evaluated with the personal dose equivalent Hp(10). Although the concept of effective dose and its operational definition of Hp(10) are widely used for radiological protection purposes, effective dose is not recommended for epidemiological evaluation. Organ-absorbed dose was instead adopted for the IARC 15-Country Collaborative study (15-Country study), the International Nuclear Workers Study (INWORKS), the Mayak worker study, and the Life Span Study (LSS) of atomic bomb survivors. The reconstruction method in J-EPISODE followed in principle the approach adopted in the 15-Country Study. As part of the approach of J-EPISODE, a conversion factor from photon dosimeter reading to air kerma was developed using dosimeter response data, which were measured by the experiment using an anthropomorphic phantom, and it was confirmed that the 15-Country study's assumption of photon energy and geometry distribution in a work environment applied to Japanese nuclear workers. This article focuses on a method for reconstructing the conversion factor from photon dosimeter reading to organ-absorbed photon dose for a Japanese nuclear worker cohort. The model for estimating the conversion factor was defined under the assumption of a lognormal distribution from three concerned bias factors: (1) a dosimeter reading per air kerma, i.e., dosimeter response; (2) an organ-absorbed dose per air kerma; and (3) a factor relating to the differences in dose concepts and calibration practices between the roentgen dosimeter era and the present. Dosimeter response data were cited from the companion paper. Data on organ-absorbed photon dose per air kerma were estimated using a voxel phantom with the average Japanese adult male height and weight. The bias factor for the recorded dose in the roentgen era was defined, considering the backscatter radiation from the human body. The estimated values of organ-absorbed photon dose per air kerma were almost the same as those in ICRP Publication 116, revealing that the effect of differences in body size was almost negligible. The conversion factors from dosimeter reading to organ-absorbed dose were estimated by period (the roentgen era or from then), nuclear facility type (nuclear power plant or other), dosimeter type, and tissue or organ. The estimated conversion factors ranged from 0.7 to 0.9 (Gy Sv-1). The estimated cumulative organ-absorbed photon dose for the participants of J-EPISODE demonstrated that organ-absorbed dose values were approximately 0.8 times the recorded doses if neglecting dose-unit differences. J-EPISODE reconstructed an organ-absorbed dose conversion factor and will evaluate the risk of cancer mortality and morbidity using the organ-absorbed dose in the future.

ORGAN ABSORBED DOSE ESTIMATION REFLECTING SPECIFIC ORGAN MASSES WITH SIMPLE SCALING OF REFERENCE DOSES USING THE ORGAN MASSES

Estimating organ absorbed doses in consideration of person-specific parameters is important for radiation protection in diagnostic nuclear medicine. This study proposes a straightforward method for estimating the organ dose that reflects a specific organ mass by scaling the reference organ dose using the inverse ratio of the specific organ mass to the reference organ mass. For the administration of radiopharmaceuticals labelled by 99mTc or 123I, the organ doses for the liver, spleen, red marrow and thyroid obtained by the method were compared with those generated by a Monte Carlo simulation. The discrepancies were less than 14% for the liver, spleen and thyroid. Conversely, in some cases, the red marrow discrepancies were greater than 30% due to the wide distribution of red marrow in the trunk and head regions. This study confirms that the method of scaling organ doses can be effective for estimating mass-specific doses for solid organs.

Calculations of S values and effective dose for the radioiodine carrier and surrounding individuals based on Chinese hybrid reference phantoms using the Monte Carlo technique

The S values for the thyroid as the radioiodine source organ to other target organs were investigated using Chinese hybrid reference phantoms and the Monte Carlo code MCNP5. Two radioiodine isotopes (125)I and (131)I uniformly distributed in the thyroid were investigated separately. We compared our S values for (131)I in Chinese phantoms with previous studies using other types of phantoms: Oak Ridge National Laboratory (ORNL) stylized phantoms, International Commission on Radiological Protection (ICRP) voxel phantoms, and University of Florida (UF) phantoms. Our results are much closer to the UF phantoms. For each specific target organ, the S value for (131)I is larger than for (125)I in both male and female phantoms. In addition, the S values and effective dose to surrounding face-to-face exposed individuals, including different genders and ages (10- and 15-year-old juniors, and adults) from an adult male radioiodine carrier were also investigated. The target organ S values and effective dose for surrounding individuals obey the inverse square law with the distance between source and target phantoms. The obtained effective dose data in Chinese phantoms are comparable to the results in a previous study using the UF phantoms. The data generated in this study can serve as the reference to make recommendations for radiation protection of the Chinese patients or nuclear workers.