Analyses of local dose distributions in the lungs for the determination of risk apportionment factors

Effects of spatial variation in dose delivery: what can we learn from radon-related lung cancer studies?

Exposure to radon progeny results in heterogeneous dose distributions in many different spatial scales. The aim of this review is to provide an overview on the state of the art in epidemiology, clinical observations, cell biology, dosimetry, and modelling related to radon exposure and its association with lung cancer, along with priorities for future research. Particular attention is paid on the effects of spatial variation in dose delivery within the organs, a factor not considered in radiation protection. It is concluded that a multidisciplinary approach is required to improve risk assessment and mechanistic understanding of carcinogenesis related to radon exposure. To achieve these goals, important steps would be to clarify whether radon can cause other diseases than lung cancer, and to investigate radon-related health risks in children or persons at young ages. Also, a better understanding of the combined effects of radon and smoking is needed, which can be achieved by integrating epidemiological, clinical, pathological, and molecular oncology data to obtain a radon-associated signature. While in vitro models derived from primary human bronchial epithelial cells can help to identify new and corroborate existing biomarkers, they also allow to study the effects of heterogeneous dose distributions including the effects of locally high doses. These novel approaches can provide valuable input and validation data for mathematical models for risk assessment. These models can be applied to quantitatively translate the knowledge obtained from radon exposure to other exposures resulting in heterogeneous dose distributions within an organ to support radiation protection in general.

EFFECTIVE DOSE COEFFICIENTS FOR RADON AND PROGENY: A REVIEW OF ICRP AND UNSCEAR VALUES

The International Commission on Radiological Protection (ICRP) publishes guidance on protection against radon exposure in homes and workplaces. ICRP Publication 137 recommends a dose coefficient of 3 mSv per mJ h m-3 (~10 mSv WLM-1) to be used in most circumstances of radon exposure, for workers in buildings and in underground mines. Recently, United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) reviewed radon epidemiology and dosimetry and concluded that its established dose coefficient of 1.6 mSv per mJ h m-3 (5.7 mSv WLM-1) should be retained for use in its comparisons of radiation exposures from different sources in a population. This paper explains and compares the reviews of the scientific evidence from UNSCEAR and ICRP. It is shown that the UNSCEAR and ICRP reviews are consistent and support the use of the ICRP reference dose coefficients for radiation protection purposes. It is concluded that the ICRP dose coefficient should be used to calculate doses to workers.

Establishment of detailed respiratory tract model and Monte Carlo simulation of radon progeny caused dose

As radon is one of the most important natural radiation sources, its radiation hazard has always been a concern. α and β particles emitted by short-lived radioactive radon progeny nuclides could result in a high local dose and induce radiation damage to the respiratory tract. A detailed respiratory tract model needs to be built and dose distribution in the respiratory tract should be studied to reflect the characteristics of energy deposition caused by radon and its progeny. Therefore, in the present work, a dosimetric study was conducted on the respiratory tract and non-uniform dose distribution in the bronchial region was studied. First, a detailed voxel respiratory tract model was established based on the anatomic bronchial parameters of an adult Chinese male. The dimensional parameters of the tracheo-bronchial tree of an adult male adopted in ICRP Publication 66 (ICRP 1994 Human Respiratory Tract Model for Radiological Protection ICRP Publication 66 (Oxford: Pergamon)), featured by consecutive 16 generations of bronchi structures to express the irregular structure of the respiratory tract and the radiosensitive tissues in the bronchial region, were also built for dosimetric study. Then the deposition and clearance models recommended by ICRP were used to analyse the regional deposition and transfer in the respiratory tract, and a fluid dynamic simulation was used to obtain 3D distribution of radon progeny aerosol particles in the bronchial region. The result showed that the highest deposition fraction density occurs at the first and second generations of bronchi. Furthermore, the detailed voxel respiratory tract model along with the Monte Carlo method were used to obtain dose distribution in the BB region. It was found that the dose distribution in the respiratory tract is very non-uniform and the maximum voxel dose is about 30 times higher than the average voxel dose. The dose conversion factor (DCF) for lung in the home environment derived with the dosimetry method in the present work is 9.86 mSv·WLM^-1. Sensitivity analysis was performed for the parameters involved in the DCF calculation and it was found that the unattached fraction and breathing rate influence the DCF the most.