Radiation Protection Dosimetry Advance Access published May 4, 2015 Radiation Protection Dosimetry (2015), pp. 1–4
doi:10.1093/rpd/ncv255
EVALUATION OF DOSE CONVERSION COEFFICIENTS FOR EXTERNAL EXPOSURE USING TAIWANESE REFERENCE MAN AND WOMAN
*Corresponding author: [email protected] Reference man has been widely used for external and internal dose evaluation of radiation protection. The parameters of the mathematical model of organs suggested by the International Commission of Radiological Protection (ICRP) are adopted from the average data of Caucasians. However, the organ masses of Asians are significantly different from the data of Caucasians, leading to potentially dosimetric errors. In this study, a total of 40 volunteers whose heights and weights corresponded to the statistical average of Taiwanese adults were recruited. Magnetic resonance imaging was performed, and T2-weighted images were acquired. The Taiwanese reference man and woman were constructed according to the measured organ masses. The dose conversion coefficients (DCFs) for anterior– posterior (AP), posterior–anterior (PA), right lateral (RLAT) and left lateral (LLAT) irradiation geometries were simulated. For the Taiwanese reference man, the average differences of the DCFs compared with the results of ICRP-74 were 7.6, 5.1 and 11.1 % for 0.1, 1 and 10 MeV photons irradiated in the AP direction. The maximum difference reached 51.7 % for the testes irradiated by 10 MeV photons. The size of the trunk, the volume and the geometric position of organs can cause a significant impact on the DCFs for external exposure of radiation. The constructed Taiwanese reference man and woman can be used in radiation protection to increase the accuracy of dose evaluation for the Taiwanese population.
INTRODUCTION Radiation protection aims to prevent deterministic effects in organs and tissues and decrease stochastic effects to an acceptable level. Therefore, it is important to assess absorbed doses by organs and tissues for radiation exposure. International Commission on Radiological Protection (ICRP) Publication 74(1) proposed dose conversion coefficients (DCFs) for external exposure, which were obtained using Monte Carlo simulation of a mathematical reference phantom. Measurable physical quantity can be converted into radiation protection quantity. However, the phantom adopted by ICRP is based on the statistical data of Caucasians. Because the average height and weight of Asians are lesser than those of Caucasians, the use of ICRP DCFs inevitably causes errors in the dose evaluation(2). Anthropomorphic phantoms can be divided into two categories: the mathematical phantom and voxel phantom. Qiu et al. (3) constructed a mathematical Chinese reference man according to the Oak Ridge National Laboratory (ORNL) reference man and calculated DCFs for external exposure. Their results differed from the data of ICRP-74 by up to 66 %. Zhu et al.(4) established elemental composition information of 18 organs and tissues and applied it to the Chinese reference man. Countries in Asia have
constructed their own mathematical reference men. The height and weight of the Korean reference man are 170.2 cm and 68.2 kg, respectively(5). Yamauchi et al.(6) constructed a Japanese reference man, whose height and weight are 170 cm and 65 kg, respectively. Voxel phantoms are constructed on the basis of individual image data, including those obtained by computed tomography (CT) and magnetic resonance imaging (MRI). Golem(7) was a 38-y-old Caucasian male phantom with a height and weight of 176 cm and 70 kg, respectively. NORMAN(8) was 176 cm in height and 73 kg in weight, having only 10 ribs. VIPman(9) was produced using colour-sliced photographs and contained only one testicle. Although voxel phantoms have better verisimilitude, they cannot represent the average physical characteristics of a particular race and therefore are not suitable for establishment of radiation protection quantities. Taiwan is an island country exhibiting significant differences from the aforementioned countries with respect to its location, climatic conditions, diet and lifestyle. Therefore, it is necessary to construct reference models representing the characteristics of Taiwanese people in order to establish DCFs for external exposure. In this study, MRI was used to assess the average organ volume and mass of Taiwanese people
# The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Washington University School of Medicine Library on November 14, 2015
S. J. Chang1,2, S. Y. Hung3,4, Y. L. Liu1, S. H. Jiang1 and J. Wu5,* 1 Institute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu, Taiwan, ROC 2 Health Physics Division, Institute of Nuclear Energy Research, Taoyuan, Taiwan, ROC 3 Department of Medical Imaging, Chi Mei Medical Center, Liouying, Tainan, Taiwan, ROC 4 Department of Medical Imaging and Radiological Science, Central Taiwan University of Science and Technology, Taichung, Taiwan, ROC 5 Department of Biomedical Imaging and Radiological Sciences, National Yang-Ming University, Taipei, Taiwan, ROC
S. J. CHANG ET AL.
to construct the mathematical Taiwanese reference man and woman. The DCFs for external radiation beams from different directions were simulated using Monte Carlo technique and compared with those of ICRP-74. MATERIALS AND METHODS
The statistical averages of the height and weight of Taiwanese adults were adopted from the report published by the Department of Health, Executive Yuan, ROC(10). The 19 age group was adopted as the standard for adult males and females. Volunteers whose heights and weights were within 3 % of the standard were recruited, that is, 152–160 cm and 55–59 kg for females and 163–173 cm and 66–71 kg for males. A total of 40 volunteers (20 males and 20 females) were available for the MRI scan, ranging from 20 to 50 y old with an average age of 37 y. MRI scans were performed using a 1.5 T Siemens MRI scanner. A fast spin echo pulse sequence was employed to acquire a series of T2-weighted images. The volunteers were asked to fast for at least 4 h and void before the scan. The T2-weighted images were reconstructed with the in-plane resolution ranging from 1.4` 1.8 mm2 (thorax) to 0.8` 0.9 mm2 (neck). The contours of 15 internal organs including the brain, eyes, thyroid, lungs, heart, liver, stomach, spleen, gall bladder, pancreas, kidneys, urinary bladder, ovaries, uterus and testes were manually delineated by three radiologists. The organ volume was calculated by multiplying the voxel size by the number of voxels in the contour. The organ mass was the product of the organ volume and the organ’s nominal density. The density information was primarily adopted from the data for the Asian reference man(2). Construction of Taiwanese reference phantoms
where Kair is the air kerma free in air, Dorgan is the absorbed dose in an organ and the unit for the DCF is Gy/Gy. The absorbed dose is evaluated by the Monte Carlo technique, which simulates energy imparted in the reference phantom from external exposure in different directions. Totally, 15 organs were simulated with photon energies ranging from 0.04 to 10 MeV. Five irradiation geometries were included: anterior–posterior Table 1. Average organ volumes and organ masses measured from 40 volunteers. Organ
Male Brain Thyroid Lungs Liver Kidneys Spleen Heart Stomach Urinary bladder Testes Pancreas Uterus Ovary
Dose conversion coefficients The equivalent dose is defined as the product of the absorbed dose of organs and tissues and the radiation weighting factor. Because the absorbed dose cannot be directly measured, the DCFs are used to convert
Mass (g)
Female Male Female
1352.7 1183.3 1393 13.9 9.1 15 3211.5 2357.7 835 1324.8 957.1 1391 315.2 221.3 331 148.9 122.1 158 659.6 527.3 679 196.2 124.3 202 105.0 102.9 109 24.6 36.4 — —
Density (g cm – 3)
— 40.3 99.2 11.2
1219 10 613 1005 232 130 543 131 107
1.03 1.05 0.26 1.05a 1.05 1.06 1.03 1.05 1.04a
— 42 103 12
1.04 1.05 1.04 1.05a
26 38 — —
a Organ density was adopted from the ORNL TM-2007 report.
Table 2. DCFs of the Taiwanese reference man/woman under 0.1, 1 and 10 MeV at the AP direction. Organ
The Taiwanese reference phantoms constructed in this study include two models: the Taiwanese reference man and woman. The body figure and organ shape were mainly designed according to the mathematical formula of the ORNL adult reference man(11). The coefficients of the formula were amended based on the statistics of the organ mass analysed in this study. The established reference male was 168.7 cm in height and 69.0 kg in weight, and the reference female was 156.2 cm and 56.6 kg.
Volume (cm3)
Brain Thyroid Lungs Liver Kidneys Spleen Heart Stomach Gall bladder Urinary bladder Testes Pancreas Uterus Ovary
Page 2 of 4
Taiwanese reference Taiwanese reference man woman 0.1
1
10
0.1
1
10
0.819 1.665 1.286 1.342 0.594 0.910 1.433 1.643 1.260 1.464 1.900 0.937 — —
0.839 1.218 1.037 0.974 0.706 0.819 0.988 1.068 0.942 0.946 1.182 0.740 — —
0.945 0.807 1.012 0.961 0.874 0.953 0.940 1.019 0.928 0.877 0.485 0.751 — —
0.823 1.856 1.101 1.457 0.671 0.918 1.494 1.440 1.307 1.094 — 1.278 1.272 1.154
0.842 1.198 0.901 1.055 0.771 0.858 1.035 0.975 0.921 0.765 — 0.940 0.997 0.974
0.931 0.435 0.928 1.053 0.945 0.931 0.973 0.885 0.952 0.683 — 1.094 1.055 1.007
Downloaded from http://rpd.oxfordjournals.org/ at Washington University School of Medicine Library on November 14, 2015
Statistics of organ mass for Taiwanese reference man and woman
the measurable exposure to the absorbed dose, as follows: Dorgan ð1Þ DCF ¼ Kair
EVALUATION OF DOSE CONVERSION COEFFICIENTS
(AP), posterior–anterior (PA), right lateral (RLAT), left lateral (LLAT) and isotropic (ISO) directions. Monte Carlo simulation
RESULTS AND DISCUSSION Table 1 indicates the measured average organ volumes, calculated average organ masses and nominal tissue
Figure 1. DCFs of the (a) liver and (b) lungs for the Taiwanese reference man/woman and ICRP-74 under the AP, PA, RLAT and LLAT directions.
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Downloaded from http://rpd.oxfordjournals.org/ at Washington University School of Medicine Library on November 14, 2015
The Monte Carlo N-Particle Transport Code 5 (MCNP 5) was used to construct the geometry of the Taiwanese reference man/woman and simulate the air kerma and absorbed dose. Regarding the simulation hardware, parallel computing techniques were used to connect eight computer servers. Each server had a quad-core Intel Xeon CPU E5606 2.13-GHz with a memory size of 3940 MB. Five million particles were tracked for each case, in order to achieve 5 % coefficient of variation (CV).
densities. Some organ densities are absent in the International Atomic Energy Agency Asian reference man report. Therefore, data from the ORNL TM-2007 report (12) were adopted for the gall bladder, urinary bladder, liver and ovaries. The volume of lungs exhibited relatively large standard deviations of 18 and 21 % for male and female, respectively. This is mainly due to the organ motion during MRI scans. The blurring effect at the edge of the lung is unavoidable, notwithstanding that the volunteers were instructed in advance to breathe in a shallow manner during the scan. Table 2 shows the DCFs of the Taiwanese reference man and woman under the AP direction. For the male phantom under 0.1 MeV, the gall bladder had the largest difference of 22.2 % in the DCF compared with the results of ICRP-74. The largest differences under 1 and 10 MeV were the urinary bladder and the testes: up to 12.7 and 51.7 %, respectively. The average differences of all organs under each of the three
S. J. CHANG ET AL.
CONCLUSION According to the statistical data from the Taiwanese population, the Taiwanese reference man and woman were constructed using MRI scans and Monte Carlo simulation was employed to calculate the DCFs under
external exposure of 40 keV to 10 MeV monoenergetic photons. The results of the present study were compared with those of the ICRP-74 and a similar trend between these three data sets was observed. However, there exist noticeable differences in some organs. In conclusion, the height and weight of reference phantoms and mass and relative geometric position of organs significantly affect the DCFs. By using the Taiwanese reference phantoms, the accuracy of radiation dose evaluation from external exposure can be improved for the Taiwanese population.
FUNDING This study was financially supported by the Ministry of Science and Technology of Taiwan (MOST 1012320-B-010-079-MY3). REFERENCES 1. ICRP. Conversion Coefficients for Use in Radiological Protection against External Radiation. ICRP 74 (1996). 2. IAEA. Compilation of Anatomical, Physiological and Metabolic Characteristics for a Reference Asian Man. IAEA-TECDOC-1005 (1998). 3. Qiu, R., Li, J., Zhang, Z., Liu, L., Bi, L. and Ren, L. Dose conversion coefficients based on the Chinese mathematical phantom and MCNP code for external phantom irradiation. Radiat. Prot. Dosim. 134, 3 –12 (2009). 4. Zhu, H. et al. Element contents in organs and tissues of Chinese adult men. Health Phys. 98, 61– 73 (2010). 5. Park, S., Lee, J. and Lee, C. Development of a Korean adult male computational phantom for internal dosimetry calculation. Radiat. Prot. Dosim. 121, 257–264 (2006). 6. Yamauchi, M., Ishikawa, M. and Hoshi, M. A stylized computational model of the head for the reference Japanese male. Med. Phys. 32, 85–92 (2005). 7. Zankl, M. and Wittmann, A. The adult male voxel model ‘Golem’ segmented from whole body CT patient data. Radiat. Environ. Biophys. 40, 153–162 (2001). 8. Ferrari, P. and Gualdrini, G. An improved MCNP version of the NORMAN voxel phantom for dosimetry studies. Phys. Med. Biol. 50, 4299– 4316 (2005). 9. Xu, X. G., Chao, T. C. and Bozkurt, A. VIP-MAN: an image-based whole-body adult male model constructed from color photographs of the Visible Human Project for multi-particle Monte Carlo calculations. Health Phys. 78, 476– 486 (2000). 10. Department of Health, Executive Yuan, ROC. Health and National Health Insurance Annual Statistics Information Service. http://nahsit.nhri.org.tw/node/14 (2009). 11. Cristy, M. and Eckerman, K. F. Specific Absorbed Fractions of Energy at Various Ages from Internal Photons Sources. ORNL Report ORNL/TM-8381 (1987). 12. Akkurt, H. and Eckerman, K. Development of PIMAL: Mathematical Phantom with Moving Arms and Legs. ORNL Report ORNL/T M-2007/14 (2007).
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Downloaded from http://rpd.oxfordjournals.org/ at Washington University School of Medicine Library on November 14, 2015
energies were 7.6, 5.1 and 11.1 %. For the female phantom, the urinary bladder had the largest differences of 33.9, 29.4 and 29.8 % in the DCFs compared with the results of ICRP-74 under the three energies. This is mainly because the Taiwanese reference woman is significantly smaller than the phantom used in ICRP-74, giving rise to a noticeable difference in the DCFs. The average differences of all organs under 0.1, 1 and 10 MeV were 9.6, 7.3 and 12.6 %, respectively. Figure 1a shows the DCFs of the liver as a function of energy in various beam directions. The curves of Taiwanese reference man/woman had similar trend compared with the curves of ICRP-74. For the Taiwanese reference man, the largest difference occurred at the low-energy range of 0.05 MeV. As the energy increased, the differences decreased below 2 %. The DCFs for the Taiwanese reference man were slightly smaller than those in ICRP-74, which is mainly because the liver volume of the male phantom is 27 % lower than that of the ICRP-74 reference, causing the position of the liver farther away from the body surface. For the Taiwanese reference woman, the DCFs were generally greater than those of ICRP-74. In particular, the difference was as large as 40 % under the LLAT direction. This is mainly due to the difference in the thickness of the trunk. The torso is described as an elliptical cylinder in the mathematic phantoms. The long axes of the elliptical cylinders of the Taiwanese female and ICRP-74 were 16.9 and 20.0 cm, leading to less photon attenuation and more absorbed dose in the liver. Figure 1b shows the DCFs of the lungs as a function of energy in various beam geometries. Compared with the data reported in ICRP-74, the difference in the AP direction for the Taiwanese male phantom was , 6.3 %. The maximum difference under the PA direction was 9.5 %. For the female phantom, the maximum differences under the AP and PA directions were 19.6 and 14.6 %, respectively. Under the high energy range, the DCFs of ICRP-74 were smaller than those of the Taiwanese reference man. The main reason is that ICRP-74 uses a hermaphrodite phantom; extra breast tissues result in photon attenuation, leading to a slightly lower absorbed dose in the lungs. Under the RLAT and LLAT directions, the DCFs were within 7 % differences among all three reference phantoms. The coefficients under RLAT were larger than those under LLAT owing to the location of the heart.
doi:10.1093/rpd/ncv255
EVALUATION OF DOSE CONVERSION COEFFICIENTS FOR EXTERNAL EXPOSURE USING TAIWANESE REFERENCE MAN AND WOMAN
*Corresponding author: [email protected] Reference man has been widely used for external and internal dose evaluation of radiation protection. The parameters of the mathematical model of organs suggested by the International Commission of Radiological Protection (ICRP) are adopted from the average data of Caucasians. However, the organ masses of Asians are significantly different from the data of Caucasians, leading to potentially dosimetric errors. In this study, a total of 40 volunteers whose heights and weights corresponded to the statistical average of Taiwanese adults were recruited. Magnetic resonance imaging was performed, and T2-weighted images were acquired. The Taiwanese reference man and woman were constructed according to the measured organ masses. The dose conversion coefficients (DCFs) for anterior– posterior (AP), posterior–anterior (PA), right lateral (RLAT) and left lateral (LLAT) irradiation geometries were simulated. For the Taiwanese reference man, the average differences of the DCFs compared with the results of ICRP-74 were 7.6, 5.1 and 11.1 % for 0.1, 1 and 10 MeV photons irradiated in the AP direction. The maximum difference reached 51.7 % for the testes irradiated by 10 MeV photons. The size of the trunk, the volume and the geometric position of organs can cause a significant impact on the DCFs for external exposure of radiation. The constructed Taiwanese reference man and woman can be used in radiation protection to increase the accuracy of dose evaluation for the Taiwanese population.
INTRODUCTION Radiation protection aims to prevent deterministic effects in organs and tissues and decrease stochastic effects to an acceptable level. Therefore, it is important to assess absorbed doses by organs and tissues for radiation exposure. International Commission on Radiological Protection (ICRP) Publication 74(1) proposed dose conversion coefficients (DCFs) for external exposure, which were obtained using Monte Carlo simulation of a mathematical reference phantom. Measurable physical quantity can be converted into radiation protection quantity. However, the phantom adopted by ICRP is based on the statistical data of Caucasians. Because the average height and weight of Asians are lesser than those of Caucasians, the use of ICRP DCFs inevitably causes errors in the dose evaluation(2). Anthropomorphic phantoms can be divided into two categories: the mathematical phantom and voxel phantom. Qiu et al. (3) constructed a mathematical Chinese reference man according to the Oak Ridge National Laboratory (ORNL) reference man and calculated DCFs for external exposure. Their results differed from the data of ICRP-74 by up to 66 %. Zhu et al.(4) established elemental composition information of 18 organs and tissues and applied it to the Chinese reference man. Countries in Asia have
constructed their own mathematical reference men. The height and weight of the Korean reference man are 170.2 cm and 68.2 kg, respectively(5). Yamauchi et al.(6) constructed a Japanese reference man, whose height and weight are 170 cm and 65 kg, respectively. Voxel phantoms are constructed on the basis of individual image data, including those obtained by computed tomography (CT) and magnetic resonance imaging (MRI). Golem(7) was a 38-y-old Caucasian male phantom with a height and weight of 176 cm and 70 kg, respectively. NORMAN(8) was 176 cm in height and 73 kg in weight, having only 10 ribs. VIPman(9) was produced using colour-sliced photographs and contained only one testicle. Although voxel phantoms have better verisimilitude, they cannot represent the average physical characteristics of a particular race and therefore are not suitable for establishment of radiation protection quantities. Taiwan is an island country exhibiting significant differences from the aforementioned countries with respect to its location, climatic conditions, diet and lifestyle. Therefore, it is necessary to construct reference models representing the characteristics of Taiwanese people in order to establish DCFs for external exposure. In this study, MRI was used to assess the average organ volume and mass of Taiwanese people
# The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Washington University School of Medicine Library on November 14, 2015
S. J. Chang1,2, S. Y. Hung3,4, Y. L. Liu1, S. H. Jiang1 and J. Wu5,* 1 Institute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu, Taiwan, ROC 2 Health Physics Division, Institute of Nuclear Energy Research, Taoyuan, Taiwan, ROC 3 Department of Medical Imaging, Chi Mei Medical Center, Liouying, Tainan, Taiwan, ROC 4 Department of Medical Imaging and Radiological Science, Central Taiwan University of Science and Technology, Taichung, Taiwan, ROC 5 Department of Biomedical Imaging and Radiological Sciences, National Yang-Ming University, Taipei, Taiwan, ROC
S. J. CHANG ET AL.
to construct the mathematical Taiwanese reference man and woman. The DCFs for external radiation beams from different directions were simulated using Monte Carlo technique and compared with those of ICRP-74. MATERIALS AND METHODS
The statistical averages of the height and weight of Taiwanese adults were adopted from the report published by the Department of Health, Executive Yuan, ROC(10). The 19 age group was adopted as the standard for adult males and females. Volunteers whose heights and weights were within 3 % of the standard were recruited, that is, 152–160 cm and 55–59 kg for females and 163–173 cm and 66–71 kg for males. A total of 40 volunteers (20 males and 20 females) were available for the MRI scan, ranging from 20 to 50 y old with an average age of 37 y. MRI scans were performed using a 1.5 T Siemens MRI scanner. A fast spin echo pulse sequence was employed to acquire a series of T2-weighted images. The volunteers were asked to fast for at least 4 h and void before the scan. The T2-weighted images were reconstructed with the in-plane resolution ranging from 1.4` 1.8 mm2 (thorax) to 0.8` 0.9 mm2 (neck). The contours of 15 internal organs including the brain, eyes, thyroid, lungs, heart, liver, stomach, spleen, gall bladder, pancreas, kidneys, urinary bladder, ovaries, uterus and testes were manually delineated by three radiologists. The organ volume was calculated by multiplying the voxel size by the number of voxels in the contour. The organ mass was the product of the organ volume and the organ’s nominal density. The density information was primarily adopted from the data for the Asian reference man(2). Construction of Taiwanese reference phantoms
where Kair is the air kerma free in air, Dorgan is the absorbed dose in an organ and the unit for the DCF is Gy/Gy. The absorbed dose is evaluated by the Monte Carlo technique, which simulates energy imparted in the reference phantom from external exposure in different directions. Totally, 15 organs were simulated with photon energies ranging from 0.04 to 10 MeV. Five irradiation geometries were included: anterior–posterior Table 1. Average organ volumes and organ masses measured from 40 volunteers. Organ
Male Brain Thyroid Lungs Liver Kidneys Spleen Heart Stomach Urinary bladder Testes Pancreas Uterus Ovary
Dose conversion coefficients The equivalent dose is defined as the product of the absorbed dose of organs and tissues and the radiation weighting factor. Because the absorbed dose cannot be directly measured, the DCFs are used to convert
Mass (g)
Female Male Female
1352.7 1183.3 1393 13.9 9.1 15 3211.5 2357.7 835 1324.8 957.1 1391 315.2 221.3 331 148.9 122.1 158 659.6 527.3 679 196.2 124.3 202 105.0 102.9 109 24.6 36.4 — —
Density (g cm – 3)
— 40.3 99.2 11.2
1219 10 613 1005 232 130 543 131 107
1.03 1.05 0.26 1.05a 1.05 1.06 1.03 1.05 1.04a
— 42 103 12
1.04 1.05 1.04 1.05a
26 38 — —
a Organ density was adopted from the ORNL TM-2007 report.
Table 2. DCFs of the Taiwanese reference man/woman under 0.1, 1 and 10 MeV at the AP direction. Organ
The Taiwanese reference phantoms constructed in this study include two models: the Taiwanese reference man and woman. The body figure and organ shape were mainly designed according to the mathematical formula of the ORNL adult reference man(11). The coefficients of the formula were amended based on the statistics of the organ mass analysed in this study. The established reference male was 168.7 cm in height and 69.0 kg in weight, and the reference female was 156.2 cm and 56.6 kg.
Volume (cm3)
Brain Thyroid Lungs Liver Kidneys Spleen Heart Stomach Gall bladder Urinary bladder Testes Pancreas Uterus Ovary
Page 2 of 4
Taiwanese reference Taiwanese reference man woman 0.1
1
10
0.1
1
10
0.819 1.665 1.286 1.342 0.594 0.910 1.433 1.643 1.260 1.464 1.900 0.937 — —
0.839 1.218 1.037 0.974 0.706 0.819 0.988 1.068 0.942 0.946 1.182 0.740 — —
0.945 0.807 1.012 0.961 0.874 0.953 0.940 1.019 0.928 0.877 0.485 0.751 — —
0.823 1.856 1.101 1.457 0.671 0.918 1.494 1.440 1.307 1.094 — 1.278 1.272 1.154
0.842 1.198 0.901 1.055 0.771 0.858 1.035 0.975 0.921 0.765 — 0.940 0.997 0.974
0.931 0.435 0.928 1.053 0.945 0.931 0.973 0.885 0.952 0.683 — 1.094 1.055 1.007
Downloaded from http://rpd.oxfordjournals.org/ at Washington University School of Medicine Library on November 14, 2015
Statistics of organ mass for Taiwanese reference man and woman
the measurable exposure to the absorbed dose, as follows: Dorgan ð1Þ DCF ¼ Kair
EVALUATION OF DOSE CONVERSION COEFFICIENTS
(AP), posterior–anterior (PA), right lateral (RLAT), left lateral (LLAT) and isotropic (ISO) directions. Monte Carlo simulation
RESULTS AND DISCUSSION Table 1 indicates the measured average organ volumes, calculated average organ masses and nominal tissue
Figure 1. DCFs of the (a) liver and (b) lungs for the Taiwanese reference man/woman and ICRP-74 under the AP, PA, RLAT and LLAT directions.
Page 3 of 4
Downloaded from http://rpd.oxfordjournals.org/ at Washington University School of Medicine Library on November 14, 2015
The Monte Carlo N-Particle Transport Code 5 (MCNP 5) was used to construct the geometry of the Taiwanese reference man/woman and simulate the air kerma and absorbed dose. Regarding the simulation hardware, parallel computing techniques were used to connect eight computer servers. Each server had a quad-core Intel Xeon CPU E5606 2.13-GHz with a memory size of 3940 MB. Five million particles were tracked for each case, in order to achieve 5 % coefficient of variation (CV).
densities. Some organ densities are absent in the International Atomic Energy Agency Asian reference man report. Therefore, data from the ORNL TM-2007 report (12) were adopted for the gall bladder, urinary bladder, liver and ovaries. The volume of lungs exhibited relatively large standard deviations of 18 and 21 % for male and female, respectively. This is mainly due to the organ motion during MRI scans. The blurring effect at the edge of the lung is unavoidable, notwithstanding that the volunteers were instructed in advance to breathe in a shallow manner during the scan. Table 2 shows the DCFs of the Taiwanese reference man and woman under the AP direction. For the male phantom under 0.1 MeV, the gall bladder had the largest difference of 22.2 % in the DCF compared with the results of ICRP-74. The largest differences under 1 and 10 MeV were the urinary bladder and the testes: up to 12.7 and 51.7 %, respectively. The average differences of all organs under each of the three
S. J. CHANG ET AL.
CONCLUSION According to the statistical data from the Taiwanese population, the Taiwanese reference man and woman were constructed using MRI scans and Monte Carlo simulation was employed to calculate the DCFs under
external exposure of 40 keV to 10 MeV monoenergetic photons. The results of the present study were compared with those of the ICRP-74 and a similar trend between these three data sets was observed. However, there exist noticeable differences in some organs. In conclusion, the height and weight of reference phantoms and mass and relative geometric position of organs significantly affect the DCFs. By using the Taiwanese reference phantoms, the accuracy of radiation dose evaluation from external exposure can be improved for the Taiwanese population.
FUNDING This study was financially supported by the Ministry of Science and Technology of Taiwan (MOST 1012320-B-010-079-MY3). REFERENCES 1. ICRP. Conversion Coefficients for Use in Radiological Protection against External Radiation. ICRP 74 (1996). 2. IAEA. Compilation of Anatomical, Physiological and Metabolic Characteristics for a Reference Asian Man. IAEA-TECDOC-1005 (1998). 3. Qiu, R., Li, J., Zhang, Z., Liu, L., Bi, L. and Ren, L. Dose conversion coefficients based on the Chinese mathematical phantom and MCNP code for external phantom irradiation. Radiat. Prot. Dosim. 134, 3 –12 (2009). 4. Zhu, H. et al. Element contents in organs and tissues of Chinese adult men. Health Phys. 98, 61– 73 (2010). 5. Park, S., Lee, J. and Lee, C. Development of a Korean adult male computational phantom for internal dosimetry calculation. Radiat. Prot. Dosim. 121, 257–264 (2006). 6. Yamauchi, M., Ishikawa, M. and Hoshi, M. A stylized computational model of the head for the reference Japanese male. Med. Phys. 32, 85–92 (2005). 7. Zankl, M. and Wittmann, A. The adult male voxel model ‘Golem’ segmented from whole body CT patient data. Radiat. Environ. Biophys. 40, 153–162 (2001). 8. Ferrari, P. and Gualdrini, G. An improved MCNP version of the NORMAN voxel phantom for dosimetry studies. Phys. Med. Biol. 50, 4299– 4316 (2005). 9. Xu, X. G., Chao, T. C. and Bozkurt, A. VIP-MAN: an image-based whole-body adult male model constructed from color photographs of the Visible Human Project for multi-particle Monte Carlo calculations. Health Phys. 78, 476– 486 (2000). 10. Department of Health, Executive Yuan, ROC. Health and National Health Insurance Annual Statistics Information Service. http://nahsit.nhri.org.tw/node/14 (2009). 11. Cristy, M. and Eckerman, K. F. Specific Absorbed Fractions of Energy at Various Ages from Internal Photons Sources. ORNL Report ORNL/TM-8381 (1987). 12. Akkurt, H. and Eckerman, K. Development of PIMAL: Mathematical Phantom with Moving Arms and Legs. ORNL Report ORNL/T M-2007/14 (2007).
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energies were 7.6, 5.1 and 11.1 %. For the female phantom, the urinary bladder had the largest differences of 33.9, 29.4 and 29.8 % in the DCFs compared with the results of ICRP-74 under the three energies. This is mainly because the Taiwanese reference woman is significantly smaller than the phantom used in ICRP-74, giving rise to a noticeable difference in the DCFs. The average differences of all organs under 0.1, 1 and 10 MeV were 9.6, 7.3 and 12.6 %, respectively. Figure 1a shows the DCFs of the liver as a function of energy in various beam directions. The curves of Taiwanese reference man/woman had similar trend compared with the curves of ICRP-74. For the Taiwanese reference man, the largest difference occurred at the low-energy range of 0.05 MeV. As the energy increased, the differences decreased below 2 %. The DCFs for the Taiwanese reference man were slightly smaller than those in ICRP-74, which is mainly because the liver volume of the male phantom is 27 % lower than that of the ICRP-74 reference, causing the position of the liver farther away from the body surface. For the Taiwanese reference woman, the DCFs were generally greater than those of ICRP-74. In particular, the difference was as large as 40 % under the LLAT direction. This is mainly due to the difference in the thickness of the trunk. The torso is described as an elliptical cylinder in the mathematic phantoms. The long axes of the elliptical cylinders of the Taiwanese female and ICRP-74 were 16.9 and 20.0 cm, leading to less photon attenuation and more absorbed dose in the liver. Figure 1b shows the DCFs of the lungs as a function of energy in various beam geometries. Compared with the data reported in ICRP-74, the difference in the AP direction for the Taiwanese male phantom was , 6.3 %. The maximum difference under the PA direction was 9.5 %. For the female phantom, the maximum differences under the AP and PA directions were 19.6 and 14.6 %, respectively. Under the high energy range, the DCFs of ICRP-74 were smaller than those of the Taiwanese reference man. The main reason is that ICRP-74 uses a hermaphrodite phantom; extra breast tissues result in photon attenuation, leading to a slightly lower absorbed dose in the lungs. Under the RLAT and LLAT directions, the DCFs were within 7 % differences among all three reference phantoms. The coefficients under RLAT were larger than those under LLAT owing to the location of the heart.
