MICRODOSIMETRY CALCULATION OF THE DOSE CONVERSION COEFFICIENT FOR RADON PROGENY. B.M.F. Lau, D. Nikezic, K.N. Yu

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1 MICRODOSIMETRY CALCULATION OF THE DOSE CONVERSION COEFFICIENT FOR RADON PROGENY B.M.F. Lau, D. Nikezic, K.N. Yu Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Kowloon, Hong Kong. (K.N. Yu). Abstract. The traditional way to assess the dose from radon progeny is through theoretical calculations of the effective dose absorbed by sensitive tissues using dosimetric lung model. However, there is considerable discrepancy of dose conversion coefficient (DCCs) for the radon progeny between the dosimetric (~15 msv/wlm) and epidemiological (~5 msv/wlm) studies. Therefore, several investigations about the reduction of the discrepancy of the DCCs for the radon progeny between the dosimetric and epidemiological studies had been carried out. One of these approaches is microdosimetry. The objective of this paper is to assess the DCC for radon progeny using the microdosimetric approach. For microdosimetric studies, the absorbed energy is calculated in the sensitive cell nuclei instead of a sensitive layer. In order to obtain the DCC for radon progeny by using microdosimetry, simulation of the absorbed dose in the sensitive cell nuclei by radon progeny needs to be performed first, which involves two programs, namely, FORMING and MICRODOSE. The first computer program, FORMING, is used to create the distribution of the cell nuclei in the airway tube. After obtaining the distribution of the cell nuclei in the airway tube, simulation of the absorbed dose in the sensitive cell nuclei by radon progeny can be started. The second computer program, MICRODOSE, is used to calculate the absorbed dose in the sensitive cell nuclei. Finally, the total DCC calculated for the target cell nuclei using microdosimetry is about 10.2 msv/wlm, which is significantly reduced by comparing to the DCCs obtained from normal dosimetric studies. 1. Introduction It is well known that tracheobronchial deposition of radon progeny in the human body can lead to lung cancers. In order to investigate the dose derived from radon progeny in the human lung, the dose conversion coefficient (DCC) for radon progeny is introduced. The DCC (in the unit of msv/wlm) is the ratio of the effective dose to the potential alpha energy exposure to radon progeny. The DCCs can be derived from different approaches. From epidemiological studies, the International Commission on Radiological Protection Publication 65 [1] recommended the DCC for radon progeny to be about 5 msv/wlm. On the other hand, the International Commission on Radiological Protection Publication 66 [2] described in details the human respiratory tract model for the dosimetric studies. Many investigations of the DCC for radon progeny with the human respiratory tract model proposed by ICRP66 [2] had been carried out in the past [3-9]. Recently, Marsh and Birchall [4] obtained a DCC of about 15 msv/wlm by using the program RADEP which was developed based on ICRP66 [2]. Obviously, there is a considerable discrepancy of the DCCs for the radon progeny between the dosimetric and epidemiological studies. The objective of this paper is to assess the DCC for radon progeny using the microdosimetric approach. The difference between dosimetric and microdosimetric approaches is the target of the lung tissue during the calculations. For dosimetric studies, the absorbed energy is calculated in the sensitive layer that contains the target cells. The abundance and distribution of cell nuclei are neglected altogether. However, according to the volume abundance of the cell nuclei and their depth distribution given by Mercer et al. [10], the cell nuclei only occupy a small volume fraction of the sensitive layer. That means only a small proportion of sensitive cell nuclei will be hit by alpha particles and most of the alpha energies will be dissipated in the insensitive tissue, but these energies are also registered in the dosimetric approach. Therefore, the DCC for radon progeny may be overestimated through the registration of these energies. In order to exclude the alpha energy dissipated in the insensitive tissue in the calculations of the DCC for radon progeny, the microdosimteric approach should be adopted. For microdosimetric studies, the absorbed energy is calculated in the sensitive cell nuclei instead of a 1

2 sensitive layer. According to some previous microdosimetric calculations of the DCCs for radon progeny [e.g. 6,11-13] the microdosimetry approach had significantly reduced the DCCs. In their studies, the shape of the sensitive cell nuclei was chosen to be a sphere. However, according to the figures showing the distribution of target cell nuclei in the bronchial and bronchiolar epithelium in ICRP66 [2] and NRC [14], the sensitive cell nuclei were ellipsoids. Therefore, in the present paper, we will adopt ellipsoid as the shape of the sensitive cell nuclei to calculate the DCCs for radon progeny using microdosimetry. 2. Methodology 2.1 Locations of Target Cell Nuclei In the present study, the lung model and the distribution of target cells proposed by ICRP66 [2] and NRC [14] have been adopted. However, they proposed the dose to be calculated in the layer containing the target cells and neglected the average abundance of target cell nuclei in the layer. In contrast, the microdosimetry approach considers the cell nuclei instead of the sensitive layer when calculating the DCC. Therefore, the average abundance of cell nuclei in the bronchial and bronchiolar wall given by Mercer et al. [10] have been employed to create the distribution of the cell nuclei in the sensitive layers. Before the simulations can be performed, the locations of the target cell nuclei in the sensitive layer need to be identified. In the bronchial (BB) region (generations 1 to 9), both basal cells and secretory cells are target cells. The distribution of target cell nuclei in the bronchial epithelium is shown in FIG. 1. According to ICRP66 [2] and NRC [14], in the BB region, the tubes have a 5 µm layer of mucous gel and a 6 µm layer of cilia and fluid; these sources were represented in the model of particle transport clearance pathways by the fast and slow surface-transport compartments, respectively. The basal cells are distributed between 35 and 50 µm below the epithelial surface and the secretory cells are distributed between 10 and 40 µm below the surface. In our study, the nuclei of both the secretory and basal cells were chosen to be an ellipsoid with a major axis of 14 µm. 14 µm FIG. 1. The distribution of the target cell nuclei in the bronchial epithelium (adopted from NRC [14]). In the bronchiolar (bb) region (generations after 9), there are no basal cells. The distribution of target cell nuclei in the bronchiolar epithelium is shown in FIG. 2. According to ICRP66 [2] and NRC [14], in the bb region, the tubes have a 2 µm layer of mucous gel and a 4 µm layer of cilia and fluid; these sources were represented in the model of particle transport clearance pathways by the fast and slow surface-transport compartments, respectively. The secretory cells are distributed between 4 and 12 µm below the epithelial surface. In our study, the nuclei of secretory cells were taken to be ellipsoids with a major axis of 5 µm. 2

3 5 µm FIG. 2. The distribution of target cell nuclei in the bronchiolar epithelium (adopted from NRC [14]). 2.2 Description of the Program FORMING In order to obtain the DCC for radon progeny by using microdosimetry, simulation of the absorbed dose in the sensitive cell nuclei from radon progeny needs to be performed first, which involves two programs, namely, FORMING and MICRODOSE. Both of them are written in the Fortran language. The first computer program, FORMING, is used to create the distribution of the cell nuclei in the airway tube wall, and consists of the following three steps. Step 1: Input of Data Six input parameters were required before running the program. The input parameters included: (1) the energies of the alpha particles, i.e., 6 and 7.69 MeV for alpha particles from 218 Po and 214 Po, respectively; (2) the axes of nuclei in ellipsoidal form; in the BB region, the major axis of both basal and secretory cell nuclei were taken as 14 µm, while in the bb region, the major axis of the secretory cell nuclei was taken as 5 µm; (3) the radii of the cylinders where the sensitive cells are located; (4) the radii of the cylinders where the centers of the cell nuclei are located and (5) the inner radius of the airway tube, which followed those given by ICRP66 [2] and the inner radii of the airway tube for the bronchial (BB) region and the bronchiolar (bb) region were 2500 µm and 500 µm, respectively; (6) the abundance of cell nuclei in the bronchial or bronchiolar wall. The average abundances of cell nuclei in the bronchial and bronchiolar wall were given by Mercer et al. [10]. According to their Tables 1 and 2, the depth distribution of nuclei in large bronchi and bronchi for basal cell nuclei were 5.8 % and 4.5 %, and for secretory cell nuclei were 1.2 % and 0.8 %. In our study, we take the average of the depth distribution of nuclei in large bronchi and bronchi to be the average volume abundance of cell nuclei in the bronchial region for both basal and secretory cell nuclei. Therefore, the average volume abundance of cell nuclei in the bronchial region for basal and secretory cell nuclei were 5.15 % and 1 %, respectively. According to Table 3 in ref. [10], the volume abundance of cell nuclei in the bronchiolar region for secretory cell nuclei was 10.3 %. Step 2: Calculation of the Numbers of Sensitive Cell Nuclei In order to know the numbers of sensitive cell nuclei in the interested layers, the volume of one nucleus and the volume of the layer containing the sensitive cell nuclei need to be calculated first. Then the product of the volume of the layer containing the sensitive cell nuclei and the volume abundance of the sensitive cell nuclei is divided by the volume of one nucleus to give the number of sensitive cell nuclei in the interested layer. 3

4 Step 3: Creating the Distribution of Cell Nuclei in the Airway Tube Wall The cell nuclei were created one by one and randomly distributed within the sensitive layer until achieving the actual numbers of the sensitive cell nuclei in the interested layer. This distribution was saved in a file to be used for the next program MICRODOSE. 2.3 Description of the Program MICRODOSE After obtaining the distribution of the cell nuclei in the airway tube, simulation of the absorbed dose in the sensitive cell nuclei from radon progeny can be started. The second computer program, MICRODOSE, is used to calculate the absorbed dose in the sensitive cell nuclei. A brief description of the program is outlined below. Step 1: Input of Data All the parameters were the same as the first program FORMING except the starting and ending radii of the source and the numbers of emitted alpha particles in different regions of the human respiratory tract model. The starting and ending radii of the source followed those from ICRP66 [2]. The numbers of emitted alpha particles N α in different regions of the human respiratory tract model were given by Nikezic & Yu [6], given as the volumetric activity (in disintegration/µm 3 ) of radon progeny per 1 WLM in different sources. One working level month (WLM) corresponds to an exposure of 1 WL during the reference-working period of one month, i.e., 170 hours per month. According to their Table 2, the numbers of emitted alpha particles with 6 MeV and 7.69 MeV were and (disintegration/µm 3 per 1WLM), respectively, for the bronchial fast region, and and (disintegration/µm 3 per 1WLM), respectively, for the bronchial slow region. The values for 6 MeV and 7.69 MeV were and (disintegration/µm 3 per 1WLM), respectively, for the bronchiolar fast region, and and (disintegration/µm 3 per 1WLM), respectively, for the bronchiolar slow region. These results were generated using the following data: breathing rate = 0.78 m 3 /h [9]; tidal volume = /breath; functional residual capacity = 3300 ml; equilibrium factor F = 0.395; unattached fraction of the potential alpha energy concentration (PAEC) f = 8 %; density of unattached particles = 1 g/cm 3 ; density of attached particles = 1.4 g/cm 3 ; shape factors equal 1 and 1.1 for unattached and attached particles, respectively [6]. Step 2: Stopping Powers of Alpha Particles in Tissue The amount of energy deposited in the basal and secretory cell nuclei depend on the stopping power of alpha particles in the tissue, so the stopping power and alpha energy as a function of traveled distance need to be determined first. The experimental information about the slowing down of alpha particles in tissue given in ICRU49 [15] have been used in these calculations. These data were fitted and converted into tables where the energy and the stopping power are given as functions of the distance traveled by the alpha particles. Step 3: Starting the Monte Carlo Simulations Monte Carlo simulations of transport of alpha particles in the airway tube are started here. Since the short-lived radon progeny were distributed in the mucous layer homogeneously, the starting point and direction for emission of alpha particles in the mucous layer are sampled randomly. Then we need to determine for each alpha particle whether it hits any cell nucleus created by the first program FORMING. If the alpha particle did not hit any cell nucleus, the above process will be repeated and a new alpha particle is chosen. If the alpha particle successfully hits a cell nucleus created in the sensitive layer, the energy absorbed in the cell nucleus will be calculated. 4

5 Step 4: Calculation of the Absorbed Dose The above steps were repeated until successful histories of 10 4 times were achieved. The absorbed energy was calculated as the difference between the incident energy and the exit energy. The specific energy is defined as the ratio between the energy imparted by radiation to the matter to the mass of that matter [16-18]. Therefore, the absorbed energy is divided by the mass of the cell nucleus to give the specific energy (with the units J/Kg or Gy) for each hit of a cell nucleus and the distribution of the specific energy is also calculated. The mean value of the specific energy is equal to the dose in the cell nuclei hit by the alpha particles (with the unit Gy). Finally, the product of the dose in cell nuclei, the fraction of hit nuclei and the number of emitted alpha particles in the sensitive layer adopted from Nikezic & Yu [6] gives the absorbed dose per WLM in the sensitive cell nuclei (with the unit Gy/WLM). 3. Results 3.1 Results of the Absorbed Dose in Sensitive Cell Nuclei In the present study, the absorbed doses per WLM in the sensitive cell nuclei were obtained from the program MICRODOSE by using microdosimetry. The basal and secretory cells in the BB region and secretory cells in the bb region were chosen to be ellipsoids with major axes of 14 µm and 5 µm, respectively. The orientations of the sensitive cell nuclei were shown in Figures 1 and 2, i.e., the major axis is parallel to the cross-section of the airway tube. The absorbed dose per WLM for the target cell nuclei with the alpha particles emitted in the BB fast and slow mucus and in the bb fast and slow mucus are shown in Table I. Table I. The absorbed dose per WLM for the target cell nuclei in BB and bb regions. Alpha-Particle Sources Target Cell Nuclei Absorbed Dose / WLM (mgy/wlm) 6 MeV Absorbed Dose / WLM (mgy/wlm) 7.69 MeV BB Fast Basal BB Slow Basal BB Fast Secretory BB Slow Secretory bb Fast Secretory bb Slow Secretory The total absorbed dose per WLM for basal cell nuclei (D BB,Basal ) and secretory cell nuclei (D BB,Secretory ) in the BB region, and for the secretory cell nuclei (D bb,secretory ) in the bb region were obtained by summing the doses in the corresponding cell nuclei in the BB and bb regions. These results are shown in the Table II. Table II. The total absorbed dose per WLM for target cell nuclei in the BB and bb regions. Total Absorbed Dose / WLM (mgy/wlm) D BB,Basal 3.75 D BB,Secretory 7.07 D bb,secretory 7.33 According to the results shown in Tables I and II, the absorbed doses for the basal cell nuclei are smaller than the corresponding values for the secretory cell nuclei in the BB region, especially for BB fast mucus with 6 MeV alpha particle energy. These results are expected because the basal cell nuclei are deeper than the secretory cell nuclei: the basal cells are distributed between 35 and 50 µm below the epithelial surface while the secretory cells are distributed between 10 and 40 µm below the surface. According to NRC [14], the range of the 6 MeV alpha particle of 218 Po in fluid or tissue is 48 µm, and that for the 7.69 MeV alpha particle of 214 Po is 71 µm. Therefore, if the alpha particles with 6 MeV 5

6 alpha particle energies are emitted in BB fast mucus, the basal cells are closed to the end of the alphaparticle range and the probability to hit a cell nucleus is very small. Due to the short range of the alpha particles, most of the alpha particles lose their energies before reaching the basal cells and this explains why the total absorbed dose in basal cell nuclei are smaller than the corresponding value for secretory cell nuclei. After obtaining the absorbed dose per WLM for the target cell nuclei in the BB and bb regions, the total DCC can be calculated. 3.2 Calculation of the DCC The total absorbed dose per WLM in the bronchial region (D BB ) is obtained by summing the dose in basal cell nuclei and secretory cell nuclei, which require the weighting factors for basal and secretory cells. In our case, the weighting factors for basal and secretory cells are both taken to be 0.5 [2]. Therefore, the total dose in the bronchial region (D BB ) is given by D BB = 0.5D BB,Basal + 0.5D BB,Secretory (mgy/wlm) (1) Since there are no basal cells in the bb region, the total absorbed dose per WLM in the bb region (D bb ) is equal to the dose in secretory cell nuclei in the bb region. Furthermore, the bronchial, bronchiolar and alveolar-interstitial regions were each assigned a weighting factor of [2]. Therefore, the total absorbed dose is given by D T-B = 0.333D BB D bb (mgy/wlm) (2) The absorbed dose is then multiplied by a radiation-weighting factor (w R ) of alpha particles of 20 to give the equivalent dose (in unit of msv). The effective dose is further obtained by multiplying the equivalent dose with the appropriate tissue-weighting factor. The tissue-weighting weighting factor is in fact the proportion of the risk resulting from irradiation of the tissue to the total risk when the whole body is uniformly irradiated. The weighting factor for lung is 0.12 [2]. Therefore, the total dose conversion coefficient is given by DCC = D T-B (msv/wlm) (3) The absorbed dose per WLM for the target cell nuclei in BB and bb regions and the total DCC are shown in Table III. Table III. The absorbed dose per WLM for the target cell nuclei in the BB and bb regions and the total DCC. Bronchial Dose, D BB 5.41 (mgy / WLM) Bronchiolar Dose, D bb 7.33 (mgy / WLM) Total absorbed dose, D T-B 4.24 (mgy / WLM) Total DCC 10.2 (msv/wlm) According to these results, the total DCC calculated for the target cell nuclei using microdosimetry is about 10.2 msv/wlm. The total DCC calculated for the layer containing the sensitive cell nuclei is about 15 msv/wlm [4]. Obviously, the total DCC has been significantly reduced by about 32 % through the microdosimetric approach. The physical explanation for the reduction is that the absorbed energy is now calculated in the sensitive cell nuclei instead of the whole sensitive layer, so that the alpha energies dissipated in the insensitive regions of the layer can be excluded in the calculations. 4. Conclusion Due to the large discrepancy of the DCC for the radon progeny between the dosimetric and epidemiological studies, the microdosimetric approach has been chosen in the present work to assess the DCC, which considers cell nuclei instead of the sensitive layer when calculating the DCC. The 6

7 total absorbed dose per WLM for basal cell nuclei and secretory cell nuclei in the bronchial (BB) region, and the secretory cell nuclei in the bronchiolar (bb) region have been calculated. After obtaining the absorbed dose per WLM for the target cell nuclei in the BB and bb regions, the total DCC can be obtained. In conclusion, the total DCC calculated for the target cell nuclei using microdosimetry is about 10.2 msv/wlm. This shows a significant reduction (about 32 %) from the total DCC calculated for the layer containing the sensitive cell nuclei, which is about 15 msv/wlm. Therefore, the absorbed dose and the corresponding DCC are significantly reduced using the microdosimetric approach. References 1. International Commission on Radiological Protection (ICRP). Protection against radon-222 at home and at work. ICRP Publication 65. Oxford: Pergamon Press (1994). 2. International Commission on Radiological Protection (ICRP). Human respiratory tract model for radiological protection. ICRP Publication 66. Oxford: Pergamon Press (1994). 3. Birchall, A. & James, A. C., Uncertainty analysis of the effective dose per unit exposure from radon progeny and implications for ICRP risk-weighting factors. Radiation Protection Dosimetry, 53: , (1994). 4. Marsh, J. W. & Birchall, A., Sensitivity analysis of the weighted equivalent lung dose per unit exposure from radon progeny. Radiation Protection Dosimetry, 87: , (2000). 5. Nikezic, D., Yu, K. N., Cheung, T. T. K., Haque, A. K. M. M., & Vucic, D., Effects of different lung morphometry models on the calculated dose conversion factor from Rn progeny. Journal of Environmental Radioactivity, 47: , (2000). 6. Nikezic, D. & Yu, K. N., Microdosimetric calculation of absorption fraction and the resulting dose conversion factor for radon progeny. Radiation and Environmental Biophysics, 40: , (2001). 7. Porstendorfer, J. & Reineking A., Radon: characteristics in air and dose conversion factors. Health Physics, 76: , (1999). 8. Porstendorfer, J., Physical parameters and dose factors of the radon and thoron decay products. Radiation Protection Dosimetry, 94: , (2001). 9. Zock, C., Portstendorfer, J., & Reineking, A., The influence of biological and aerosol parameters of inhaled short-lived radon decay products on human lung dose. Radiation Protection Dosimetry, 63: , (1996). 10. Mercer, R. R., Russell, M. L. & Crapo, J. D., Radon dosimetry based on the depth distribution of nuclei in human and rat lungs. Health Physics, 61: , (1991). 11. Hofmann, W., Menache, M. G., Crawford-Brown, D. J., Caswell, R. S. & Karam, L. R., Modeling energy deposition and cellular radiation effects in human bronchial epithelium by radon progeny alpha particles. Health Physics, 78: , (2000). 12. Hui, T. E., Poston, J. W. and Fisher, D. R., The microdosimetry of radon decay products in the respiratory tract. Radiation Protection Dosimetry, 31: , (1990). 13. Sedlak, A., Microdosimetric approach to the problem of lung cancer induced by radon progeny. Health Physics, 70: , (1996). 14. National Research Council (NRC), Comparative dosimetry of radon in mines and homes. Washington, D.C.: National Academy Press, (1991). 15. International Commission on Radiation Units and Measurements, Stopping powers and ranges for protons and alpha particles. ICRU Report 49, Maryland (1993). 16. Kellerer, A. M., Fundamentals of Microdosimetry. The dosimetry of ionizing radiation, vol 1, Edited by Kase, K. R., Bjarngard, B. E. & Attix, F. H.. Academic Press, , (1985). 17. Rossi, H. & Zaider, M., Microdosimetry and its applications. Berlin: Springer (1996). 18. Nikezic, D. & Yu, K. N., Distribution of specific energy in sensitive layers of human respiratory tract. Radiation Research, 157:92-98, (2002). 7