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Title: Understanding the discrepancy between dose conversion coefficients for radon progeny from dosimetric and epidemiological approaches through microdosimetry
Other Titles: Li yong wei ji liang xue jie shi ji liang xue yu liu xing bing xue zhi dong zi ti ji liang bian huan xi shu zhi cha yi
Authors: Lau, Bonnie Mei Fung (劉美鳳)
Department: Dept. of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2006
Publisher: City University of Hong Kong
Subjects: Microdosimetry
Radiation dosimetry
Radon -- Measurement
Notes: CityU Call Number: RA1247.R33 L38 2006
Includes bibliographical references (leaves 162-173)
Thesis (Ph.D.)--City University of Hong Kong, 2006
xii, 174, A16 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: It is well known that tracheobronchial deposition of radon progeny in the human body can lead to lung cancers. The dose conversion coefficient (DCC) is used to assess the risk due to inhaled radon progeny in the human lung. This can be calculated through dosimetric modelling (which is called the dosimetric approach). In following the dosimetric approach, ICRP focused on the layers containing the target cell, i.e., the basal and secretory cells. However, the DCC thus obtained (15 mSv/WLM) has a considerable discrepancy from the epidemiologically derived DCC (4 mSv/WLM). Therefore, several investigations about the reduction of the discrepancy of the DCC for the radon progeny between the dosimetric and epidemiological studies had been carried out. One of these approaches is microdosimetry. The objective of this study is to assess the DCC for radon progeny using the microdosimetric approach and microdosimetry with cell killing to explain this discrepancy. The difference between dosimetric and microdosimetric approaches is the target of the lung tissue during the calculations. For microdosimetric studies, the absorbed energy is calculated in the sensitive cell nuclei instead of a sensitive layer. The present work consists of three parts. The first part is to reproduce the DCC value following the ICRP66 dosimetric model. Here, the DCC value had been successfully reproduced as 15 mSv/WLM, and the intermediate results of the equilibrium activities of radon progeny in different regions of the T-B region obtained from this approach were used in the microdosimetric approach. The second part is to investigate the effects of the geometry of basal and secretory cells on the microdosimetric parameters and distribution, as well as on the absorbed dose received by the nuclei of these cells and the DCC value by using microdosimetric approach. The absorbed energy of alpha particles emitted by radon progeny in the human respiratory tract were calculated in basal and secretory cell nuclei, assuming conical and elliptical forms for these cells. Furthermore, the absorbed energy was also calculated in the spherical forms of the basal and secretory cell nuclei. The distribution of specific absorbed energy is presented for different geometry of the cell nuclei. It is shown that the DCC for radon progeny is reduced for about 2 mSv/WLM when calculating the DCC in the elliptical and conical cell nuclei when compared to that in the sensitive layer. In the last part of the present study, the microdosimetric approach was further developed. Killing of target cells was also taken into account through the effect specific track length model (Crawford-Brown and Hofmann 1991, 2001; Hofmann et al. 2000). To focus on the relevant part of the absorbed dose in the cell nuclei, the absorbed dose which causes cell killing is discarded in the final calculations of the DCC. Following this approach, the calculated DCC has become 3.4 mSv/WLM which is very close to the epidemiologically derived value. Other useful information obtained in the course of obtaining the final DCC are also presented, including the cell death rates, the specific energy distributions and hit frequencies in the bronchial (BB) and bronchiolar (bb) regions.
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