24 Z . Mehrabankhoo et al .: Radioprotection 2024 , 59 ( 1 ), 19 – 25
4 Discussion
In external dosimetry , the effective dose depends on the distance between the organ boundaries and the irradiated body surface . Thus , the thickness of the outer layers of the body can be favored over the distance between internal organs . Results indicated that the effective dose was decreased by increasing the weight percentile . AP and RLAT show the highest and lowest effective dose for the investigated neutron energy range at all weight percentiles among the six irradiation geometries . It was expected , whereas radiosensitive organs such as breasts are directly exposed to radiation in AP geometry .
The comparison between the 95 th percentile effective dose conversion coefficients and VIPMAN data shows varying degrees of agreement depending on the irradiation geometry . There is a good agreement between results for AP , ISO , and ROT geometries . The VIPMAN ’ s back is thinner than the 95 th percentile , resulting in higher absorbed dose in abdominal organs when VIPMAN is irradiated from PA geometry . Thus , a relative difference of about 47 % of data falls within the range of 15 – 25 %. In lateral geometries ( RLAT and LLAT ), there are larger discrepancies due to thicker external layers on the sides of the 95 th percentile phantom acting as a shield against neutrons . Generally , the relative difference between the 95 th percentile effective dose and VIPMAN was less than 15 % for about 57 % of the data and only 33 % of data fell within the range of 25 – 50 % across all neutron energies and irradiation geometries used in this study .
The observed differences in absorbed dose values between VIPMAN and the 95 th percentile can be attributed to various factors . Differences in organ topology between individuals can affect the distribution of absorbed dose . Variation in organ depth or the average distance between organs and the radiation source can also contribute to variations in absorbed dose . Also , the choice of Monte Carlo code used for calculations can introduce differences in the calculated absorbed dose values . Despite these discrepancies , a majority of the data ( more than 58 %) showed a relative difference below 15 % between VIPMAN and the 95 th percentile . However , it is important to note that in some cases ( 23 % of the data ), the difference exceeded 25 % in all studied irradiation geometries and energies .
The similarities between the data from Bozkurt et al . and McHale et al . in the AP geometry suggest that the differences in MCNP versions may have a smaller impact in this particular irradiation geometry . However , large discrepancies are observed in the ISO and ROT geometries , indicating that the choice of MCNP version can significantly affect the calculated absorbed dose values . Interestingly , if the trend of changes is examined for the stomach in Figures 3c and 4c ( green symbols ), as well as for the liver in Figures 3d and 4d , it is observed that the trend is completely same . This indicates that a significant part of the differences in the absorbed dose data compared to VIPMAN data may originate from differences in the two code versions ( MCNP4B versus MCNPX2.6 ). When versions change can indeed be accompanied by changes in the data cross sections or sampling from different data files . In MCNP6.2 , which was used by
McHale et al ., neutron interaction cross sections are obtained from the endf71x library based on ENDF / B-VII . 1 nuclear data , whereas in MCNP4B , the data is get from the endf60 library and ENDF / B-VI data files . These finding highlight the importance of using consistent Monte Carlo code versions when comparing absorbed dose data between studies . It is essential to consider these discrepancies when interpreting and comparing absorbed dose values obtained from different studies , as they can affect the accuracy and reliability of the results .
5 Conclusion
For radiation protection aims , the use of a reference phantom is usually recommended by ICRP . As the results of this study show , there is a difference between the data of the 50 th and other weight percentiles . These differences reach about 30 % between the 50 th and 95 th weight percentiles . Therefore , in critical situations where medical interventions are needed and considering that these interventions depended on the level of radiation received , it is suggested that an individual-specific phantom be used instead of a reference phantom . However , constructing a phantom for each individual can be time-consuming and may not be feasible in critical times . In this study , a solution has been examined in these conditions and an attempt to answer the question : “ Are data extracted from phantoms with a weight percentile heavier than the 50 th percentile , obtained by adding only layers of muscle and adipose to the torso , reliable in critical situations ”? To this end , effective and absorbed dose conversion coefficients were calculated for 19 monoenergetic neutrons spanning 10 �9 to 20 MeV for six irradiation geometries by MCNPX2.6 Monte Carlo code . The 95 th weight percentile results were compared to VIPMAN . Results showed that about 57 % and 58 % of the effective dose and organ ’ s absorbed dose data , respectively have a relative difference of less than 15 %. Furthermore , the most organs have a good agreement in ISO and ROT irradiation geometries , which are closer to the actual conditions of radiation exposure in accidents compared to other irradiation geometries . A detailed analysis identified that the difference of data cross section used in different versions of MCNP code can be the main factor causing the differences . According to the results , it can be concluded that this method is very reliable when especially the whole body is exposed to radiation especially in the energies above 1 MeV . It should be noted that the value of w T reported by ICRP 103 are based on population of both sexes and all ages , Therefore , it should be investigated the dependency of tissue weight factors on weight percentiles in future .
Conflict of interest
The authors declare that they have no conflicts of interest in relation to this article .
Funding This research did not receive any specific funding .