20 Z . Mehrabankhoo et al .: Radioprotection 2024 , 59 ( 1 ), 19 – 25
Table 1 . The thickness of adipose and muscle layers in the weight percentiles more than 50 th percentile and total mass ( Karimi-Shahri , 2020 ).
Weight Percentile Adipose
Thickness of extra layers ( cm ) Muscle
65 |
0.92 |
0.22 |
83.1 |
75 |
1.6 |
0.4 |
89.1 |
85 |
2.3 |
0.5 |
95.0 |
95 |
3.0 |
0.8 |
102.8 |
Total Mass ( Kg ) an adipose tissue equivalent substitute material to slices of the 50 th physical phantom to investigate the impact of patient size on organ dose and CT scan image quality . In our previous work , the difference of the torso adipose and muscle layers ’ thickness between ORNL with VIPMAN , NORMAN05 , Mash-3 , and ICRP voxel phantoms was determined . We then added extra layers to the ORNL torso based on these differences ( Karimi-Shahri et al ., 2013 ). The Results showed that incorporating appropriate layers of muscle or adipose into different phantoms resulted in similar effective dose values ( Karimi-Shahri et al ., 2013 ). This finding provided us a clue to explore the feasibility of constructing different weight percentiles using this approach for specific situations . One of the major concerns in nuclear accidents is the presence of neutron sources , which can cause lethal biological effects ( Jamsranjav , 2019 ).
In the present study , absorbed and effective dose conversion coefficients were calculated using the revised ORNL hermaphrodite phantom ( Han et al ., 2006 ). Layers of adipose and muscle added to the existing 50 th percentile torso to create the 65 th , 75 th , 85 th , and 95 th weight percentiles . Simulations were carried out for neutrons in the range of energy 10 �9 – 20 MeV , focusing on six irradiation geometries : AP , PA , RLAT , LLAT , ROT , and ISO . According to the definitions provided by ICRP and ICRU , various radiation geometries are considered . These include the AP , PA , RLAT , and LLAT geometries . In contrast , the ROT geometry represents environmental contamination when a person moves randomly in a contaminated environment . The ISO geometry is approximated by a body suspended in a large cloud of radioactive gas ( like atmospheric contamination ) ( ICRP , 1996 ; ICRPU , 1998 ). These different orientations help in accurately assessing the radiation dose . Furthermore , organ absorbed dose and effective dose conversion coefficients of the 95 th percentile were also compared with those of VIPMAN .
2 Material and methods
The revised ORNL hermaphrodite adult phantom ( 50 th percentile ) ( Han et al ., 2006 ) with a specific weight and height ( 75.5 kg and 168.2 cm , respectively ) was used . To create weight percentiles above 50 , we focused on altering the shape of the torso while keeping all other internal organs constant . Additional layers of muscle and adipose tissue were added to the torso , respectively with adjustments made to the positions of the skin and breasts to accommodate the varying weight percentiles . The phantom ’ s torso resembles a cylinder with an elliptical cross-section , the minor and major radius underwent changes . To ensure consistency , a constant ratio between the ellipsoid minor and major radius ( r major = 2r minor ) was maintained when incorporating the additional layers . Further information on obtaining weight values for different percentiles can be found in our previous study ( Karimi-Shahri , 2020 ). Table 1 provides details on the thickness of the adipose and muscle layers , as well as the total body weight , for each weight percentile examined in this study .
The MCNPX Monte Carlo code ( Denise , 2008 ) and the revised ORNL phantom ( hermaphrodite phantom ) were applied for the simulations . The external source emits neutron with energies10 �9 , 10 �8 , 10 �7 , 10 �6 , 10 �5 , 10 �4 , 10 �3 , 0.01 , 0.1 , 0.5 , 1 , 3 , 5 , 8 , 10 , 12 , 15 , 18 and 20 MeV . The surface source was emitted neutrons in six different directions : AP , PA , RLAT , LLAT , ISO , and ROT . Neutron cross-sections were obtained from the ENDF / B-VI libraries . Molecular effects known as S ( a , b ) were taken into account for thermal neutrons . Additionally , transport of both neutrons and neutron-induced photons was considered . To calculate organ absorbed doses , the F6 tally was employed . the red bone marrow dose was computed using the F4 tally with flux to dose conversion coefficients . The radiation weighting factor was derived from equations ( 1 ) ( ICRP , 2007 ) to calculate the equivalent dose ( H T ).
! w R ¼ 2:5 þ 18:2exp �ðln ð E nÞÞ 2 E n < 1MeV
6 ! w R ¼ 5 þ 17exp �ðln ð 2E nÞÞ 2
:
1MeV E n 50MeV 6 ð1Þ
The effective dose was computed by the summation of weighted equivalent dose as :
E ¼ w breast H breast ; f emale þ X
H T ; male þ H T ; f emale w T
; ð2Þ 2
T≠breast
where , the tissue weighting factor ( w T ) was derived from ICRP103 ( ICRP , 2007 ). These data have been utilized in the calculation of the effective dose for all weight percentiles .
The effective dose of the present study was compared with the data reported by Bozkurt et al . ( 2000 ) for VIPMAN phantom . To this end , Bozkurt et al ’ s data was recalculated because the w T in that study were based on ICRP 60 ( 1991 ) and significantly differ from the updated weighting factors from ICRP 103 ( 2007 ) used in the present work . The influence of changing radiation and tissue weighting factors ( w R and w T ) on