DoseCV: Dose coefficient calculation from MCNP simulations with MRCP phantoms.

A new series of organ equivalent dose conversion coefficients for whole body external photon exposure is presented for a standardized couple of human voxel models, called Rex and Regina. Irradiations from broad parallel beams in antero-posterior, postero-anterior, left- and right-side lateral directions as well as from a 360° rotational source have been performed numerically by the Monte Carlo transport code EGSnrc. Dose conversion coefficients from an isotropically distributed source were computed, too. The voxel models Rex and Regina originating from real patient CT data comply in body and organ dimensions with the currently valid reference values given by the International Commission on Radiological Protection (ICRP) for the average Caucasian man and woman, respectively. While the equivalent dose conversion coefficients of many organs are in quite good agreement with the reference values of ICRP Publication 74, for some organs and certain geometries the discrepancies amount to 30% or more. Differences between the sexes are of the same order with mostly higher dose conversion coefficients in the smaller female model. However, much smaller deviations from the ICRP values are observed for the resulting effective dose conversion coefficients. With the still valid definition for the effective dose (ICRP Publication 60), the greatest change appears in lateral exposures with a decrease in the new models of at most 9%. However, when the modified definition of the effective dose as suggested by an ICRP draft is applied, the largest deviation from the current reference values is obtained in postero-anterior geometry with a reduction of the effective dose conversion coefficient by at most 12%.

When accidents by industrial radiography sources occur, it is necessary to accurately and quickly estimate radiation doses for the effective treatment of those individuals with acute radiation syndrome (ARS). In the present study, a comprehensive set of absorbed dose coefficients (DCs) was obtained by performing Monte Carlo simulations using computational human phantoms of different body sizes. These DCs provide an “initial and rapid” dose estimation for individuals accidentally exposed to industrial radiography sources. The adult mesh-type ICRP reference computational phantoms (MRCPs) and the adult 10th and 90th percentile computational phantoms, constructed by deforming the MRCPs, were implemented in the Geant4 Monte Carlo code. We subsequently simulated the most commonly used industrial radiography sources (i.e., 192Ir and 60Co) placed in 72 different locations near the human body. It was found that body size significantly influences the DCs, especially when the source is closer than 1 m to the human body, which is a case frequently encountered during industrial radiography accidents. Acknowledging the significance of these results, the ICRP is planning to include the full set of the calculated DCs from this study in a forthcoming ICRP Publication, which is being prepared by the ICRP Committee 2 Task Group 103 “Mesh-type Reference Computational Phantoms.”

In a previous study, posture-dependent dose coefficients (DCs) for photon external exposures were calculated using the adult male and female mesh-type reference computational phantoms (MRCPs) of the International Commission on Radiological Protection (ICRP) that had been transformed into five non-standing postures (i.e. walking, sitting, bending, kneeling, and squatting). As an extension, the present study was conducted to establish another DC dataset for external exposures to neutrons by performing Monte Carlo radiation transport simulations with the adult male and female MRCPs in the five non-standing postures. The resulting dataset included the DCs for absorbed doses (i.e., organ/tissue-averaged absorbed doses) delivered to 29 individual organs/tissues, and for effective doses for neutron energies ranging from 10-9 to 104MeV in six irradiation geometries: antero-posterior (AP), posteroanterior (PA), left-lateral (LLAT), right-lateral (RLAT), rotational (ROT), and isotropic (ISO) geometries. The comparison of DCs for the non-standing MRCPs with those of the standing MRCPs showed significant differences. In the lateral irradiation geometries, for example, the standing MRCPs overestimate the breast DCs of the squatting MRCPs by up to a factor of 4 due to the different arm positions but underestimate the gonad DCs by up to about 17 times due to the different leg positions. The impact of different postures on effective doses was generally less than that on organ doses but still significant; for example, the standing MRCPs overestimate the effective doses of the bending MRCPs only by 20% in the AP geometry at neutron energies less than 50MeV, but underestimate those of the kneeling MRCPs by up to 40% in the lateral geometries at energies less than 0.1MeV.

In computed tomography (CT), organ dose, effective dose, and risk index can be estimated from volume-weighted CT dose index (CTDI(vol)) or dose-length product (DLP) using conversion coefficients. Studies have investigated how these coefficients vary across scanner models, scan parameters, and patient size. However, their variability across CT protocols has not been systematically studied. Furthermore, earlier studies of the effect of patient size have not included obese individuals, which currently represent more than one-third of U.S. adults. The purpose of this study was to assess the effects of protocol and obesity on dose and risk conversion coefficients in adult body CT. Whole-body computational phantoms were created from clinical CT images of six adult patients (three males, three females), representing normal-weight patients and patients of three obesity classes. Body CT protocols at our institution were selected and categorized into ten examination categories based on anatomical region examined. A validated Monte Carlo program was used to estimate organ dose. Organ dose estimates were normalized by CTDI(vol) and size-specific dose estimate (SSDE) to obtain organ dose conversion coefficients (denoted as h and h(ss) factors, respectively). Assuming each phantom to be 20, 40, and 60 years old, effective dose and risk index were calculated and normalized by DLP to obtain effective dose and risk index conversion coefficients (denoted as k and q factors, respectively). Coefficient of variation was used to quantify the variability of each conversion coefficient across examination categories. The effect of obesity was assessed by comparing each obese phantom with the normal-weight phantom of the same gender. For a given organ, the variability of h factor across examination categories that encompassed the entire organ volume was generally within 15%. However, k factor varied more across examination categories (15%-27%). For all three ages, the variability of q factor was small for male (<10%), but large for female phantoms (21%-43%). Relative to the normal-weight phantoms, the reduction in h factor (an average across fully encompassed organs) was 17%-42%, 17%-40%, and 51%-63% for obese-class-I, obese-class-II, and obese-class-III phantoms, respectively. h(ss) factor was not independent of patient diameter and generally decreased with increasing obesity. Relative to the normal-weight phantoms, the reduction in k factor was 12%-40%, 14%-46%, and 44%-59% for obese-class-I, obese-class-II, and obese-class-III phantoms, respectively. The respective reduction in q factor was 11%-36%, 17%-42%, and 48%-59% at 20 years of age and similar at other ages. In adult body CT, dose to an organ fully encompassed by the primary radiation beam can be estimated from CTDI(vol) using a protocol-independent conversion coefficient. However, fully encompassed organs only account for 50% ± 19% of k factor and 46% ± 24% of q factor. Dose received by partially encompassed organs is also substantial. To estimate effective dose and risk index from DLP, it is necessary to use conversion coefficients specific to the anatomical region examined. Obesity has a significant effect on dose and risk conversion coefficients, which cannot be predicted using body diameter alone. SSDE-normalized organ dose is not independent of diameter. SSDE itself generally overestimates organ dose for obese patients.

The purpose of this study was to determine a new conversion factor (kSSDE-LP) that would allow more accurate calculation of the effective dose that is minimally influenced by patient body size, using the Dose Management Software (DMS), and to verify its accuracy in estimating patient-specific effective dose compared to existing methods. A simple regression equation was obtained using the product of the size-specific dose estimate (SSDE) and scan length on the horizontal axis and the effective dose on the vertical axis, and the slope was taken as the effective dose conversion coefficient, kSSDE-LP. Similarly, the slope obtained from another simple regression equation, using the dose-length product (DLP) on the horizontal axis and effective dose on the vertical axis, was defined as kDLP. The effective dose conversion factors and coefficients of determination (R2) were compared for males, females, and both sexes. The DLP, SSDE, scan length, and effective dose, which are dose indices necessary for determining effective dose conversion coefficients, were obtained from the DMS. The kSSDE-LP values in males, females, and both sexes were 0.012 (R2 = 0.997), 0.014 (R2 = 0.996), and 0.013 (R2 = 0.993), respectively. Using the SSDE, a dose index that takes into account information about the patient's physique, we calculated the kSSDE-LP, which reflects the current tissue weighting coefficients and mathematical voxel phantom and does not easily deviate from the regression equation even in the high-weight group.

PAEDIATRIC PHANTOMS FOR THE NEXT RECOMMENDATIONS.

Dose coefficients of mesh-type ICRP reference computational phantoms for external exposures of neutrons, protons, and helium ions

Conversion coefficients have been calculated for fluence-to-absorbed dose, fluence-to-effective dose and fluence-to-gray equivalent for isotropic exposure of an adult male and an adult female to (56)Fe(26+) in the energy range of 10 MeV to 1 TeV (0.01-1000 GeV). The coefficients were calculated using Monte Carlo transport code MCNPX 2.7.A and BodyBuilder 1.3 anthropomorphic phantoms modified to allow calculation of effective dose using tissues and tissue weighting factors from either the 1990 or 2007 recommendations of the International Commission on Radiological Protection (ICRP) and gray equivalent to selected tissues as recommended by the National Council on Radiation Protection and Measurements. Calculations using ICRP 2007 recommendations result in fluence-to-effective dose conversion coefficients that are almost identical at most energies to those calculated using ICRP 1990 recommendations.

Recently, the International Commission on Radiological Protection (ICRP) developed new mesh-type reference computational phantoms (MRCPs) that provide high deformability compared with the current voxel-type reference computational phantoms of ICRP Publication 110. Taking advantage of this deformability, in the present study, the MRCPs were deformed to five non-standing postures (i.e. walking, sitting, bending, kneeling, and squatting) by developing and using a systematic posture-change method based on the as-rigid-as-possible (ARAP) shape-deformation algorithm and motion-capture technology. The non-standing MRCPs were then implemented in the Geant4 Monte Carlo code to calculate a comprehensive dataset of dose coefficients (DCs) for photon external exposures. These include the dose coefficients for 29 individual organs/tissues and the dose coefficients for effective doses from 0.01 MeV to 10 GeV in the antero-posterior (AP), postero-anterior (PA), left-lateral (LLAT), right-lateral (RLAT), rotational (ROT), and isotropic (ISO) geometries. To investigate the dosimetric impact of posture, the DCs of the non-standing MRCPs were compared with those of the original MRCPs (in the standing posture). The results showed that organ/tissue doses are significantly influenced by posture, with arm position mostly influencing dose to organs/tissues in the torso region and leg position influencing dose in the pelvic region. For most cases, the gonads showed notably large differences, ranging from a few tens of percentage points to several orders of magnitude, depending on posture and irradiation geometry. The effective doses showed much smaller differences than the organ/tissue doses, but they were nonetheless significant: for example, the kneeling MRCPs in the AP geometry showed lower values at energies <10 MeV by up to 30% and greater values at higher energies by up to 40%. The presented results indicate that not only different irradiation geometries, but also different postures might be necessary in DC calculations for reliable dose estimates for radiological protection purposes.

A calculational method is presented to evaluate equivalent doses of tissues or organs and effective dose for high energy radiations, where the radiation weighting factor given in ICRP 60 was replaced by an average quality factor estimated from the Q-L relationship in ICRP 60. The equivalent doses of tissues or organs and fluence to effective dose conversion coefficients are calculated for photons up to 10GeV with a Monte Carlo code EGS4 in which a mathematical model of an anthropomorphic phantom is included. The equivalent doses for 24 tissue and organs have been calculated for AP and PA incidence of aligned and expanded photons from I MeV to 10 GeV. The effective doses were obtained by summing the equivalent doses with tissue weighting factors from ICRP 60. It is also demonstrated that the kerma approximation overestimates significantly the effective dose for photons above 10 MeV.

Fluence-to-dose conversion coefficients based on the posture modification of Adult Male (AM) and Adult Female (AF) reference phantoms of ICRP 110

Protection and operational quantities as defined by the International Commission on Radiological Protection (ICRP) and the International Commission on Radiation Units and Measurements (ICRU) are the two sets of quantities recommended for use in radiological protection for external radiation. Since the ’80s, the protection quantities have evolved from the concept of dose equivalent to effective dose equivalent to effective dose, and the associated conversion coefficients have undergone changes. In this work, the influence of three different versions of ICRP photon dose conversion coefficients in the synchrotron radiation shielding calculations of an experimental enclosure has been examined. The versions are effective dose equivalent (ICRP Publication 51), effective dose (ICRP Publication 74), and effective dose (ICRP Publication 116) conversion coefficients. The sources of the synchrotron radiation white beam into the enclosure were a bending magnet, an undulator and a wiggler. The ranges of photons energy from these sources were 10–200 keV for the bending magnet and undulator, and 10–500 keV for the wiggler. The design criterion aimed a radiation leakage less than 0.5 µSv h−1 from the enclosure. As expected, larger conversion coefficients in ICRP Publication 51 lead to higher calculated dose rates. However, the percentage differences among the calculated dose rates get smaller once shielding is added, and the choice of conversion coefficients set did not affect the final shielding decision.

Purpose: To compare organ dose, effective dose, and dose conversion coefficients in adult CT estimated using three types of reference phantoms: (1) hermaphrodite mathematical phantom employed by the imPACT CT dose calculator, (2) reference male and female extended cardiac-torso (XCAT) phantoms, and (3) ICRP 110 reference male and female phantoms. Methods: Representative CT examinations were selected from the database of clinical adult CT protocols in use at our institution. Organ dose, effective dose, and dose conversion coefficients (the k factor) were estimated for these examinations for a clinical CT system (LightSpeed VCT, GE Healthcare) using three methods, each employing a different type of reference phantoms. The first method employed commercial dosimetry software imPACT using Monte Carlo methods for a reference mathematical hermaphrodite phantom. The second and third methods employed a Monte Carlo program previously developed and validated in our laboratory. In the second method, the male and female extended cardiac-torso (XCAT) phantoms created from the Visible Human data were used. In the third method, the ICRP 110 male and female phantoms based on ICRP 89 tomographic anatomic data were used. Results: The k factor varied very little between phantoms, with average 0.019 mSv/mGy-cm (ranging from 0.009 to 0.025) for imPACT phantom and 0.018 mSv/mGy-cm (ranging from 0.013 to 0.022) for XCAT male phantom. Distributed organs tended to have similar dose values among different phantoms for the same protocol. The calculated percent difference between imPACT and XCAT male phantom was 1.4% for bone marrow and 9.3% for bone surface. Percent differences between phantoms were generally inversely proportional with scan length ranging from 21.1% for chest-abdomen-pelvis protocol to −94% for liver to kidneys protocol. Conclusions: This work suggests that gender averaged dose from anthropomorphic phantoms should be used as reference to adult patient CT dose.

In a recent study, a comprehensive library composed of 212 phantoms with different body sizes was established by deforming the adult male and female mesh-type reference computational phantoms (MRCPs) of ICRP Publication 145 and the next-generation ICRP reference phantoms over the current voxel-type reference phantoms of ICRP Publication 110. In this study, as an application of the MRCP-based phantom library, we investigated dosimetric impacts due to the different body sizes for neutron external exposures. A comprehensive dataset of organ/tissue dose coefficients (DCs) for idealized external neutron beams with four phantoms for each sex representatively selected from the phantom library were produced by performing Monte Carlo simulations using the Geant4 code. The body size-dependent DCs produced in this study were systematically analyzed, observing that the variation of the body weights overall played a more important role in organ/tissue dose calculations than the variation of the body heights. We also observed that the reference body-size DCs based on the MRCPs indeed significantly under- or overestimated the DCs produced using the phantoms, especially for those much heavier (male: 175 cm and 140 kg; female: 165 cm and 140 kg) than the reference body sizes (male: 176 cm and 73 kg; female: 163 cm and 60 kg) by up to 1.6 or 3.3 times, respectively. We believe that the use of the body size-dependent DCs, together with the reference body-size DCs, should be beneficial for more reliable organ/tissue dose estimates of individuals considering their body sizes rather than the most common conventional approach, i.e., the sole use of the reference body size DCs.

This study introduces a refined approach for more accurately estimating radiation doses to alimentary tract organs in nuclear medicine, by utilizing the ICRP pediatric and adult mesh-type reference computational phantoms (MRCPs) that improved the anatomical representation of these organs. Our initial step involved compiling a comprehensive dataset of electron Specific Absorbed Fractions (SAFs) for all source-target pairs of alimentary tract organs in both adult and pediatric phantoms, calculating SAFs for all cases in the present study only except those computed in the previous study for certain pediatric phantom cases. Subsequently, we determined S values for 1,252 radionuclides, facilitating dosimetry applications. The consistency of target and source masses for alimentary tract organs in the MRCPs with the reference values in ICRP Publication 89 led to noticeable differences in SAF, S values, and consequently, absorbed dose coefficients when compared to the stylized models in ICRP Publication 100. Notably, the S value ratios (MRCP/stylized) for selected radionuclides—11C, 18F, 68Ga, and 131I—ranged from 0.41 to 7.60. Particularly for therapeutic 131I-iodide in thyroid cancer, the use of MRCPs resulted in up to 1.49 times higher absorbed dose coefficients for the colon than those derived from stylized models, while the stomach dose coefficients decreased by a factor of 0.72. The application of our findings promises enhanced, more realistic dosimetry for alimentary tract organs, especially beneficial for radiopharmaceuticals likely to accumulate within these organs.