EGS4 Monte Carlo Code Research Articles

Technical Note: Implications of using EGSnrc instead of EGS4 for extracting electron stopping powers from measured energy spectra.

NRC Report PIRS-0626 (https://doi.org/10.4224/40000364) describes how measured electron energy deposition spectra can be used to determine the electronic stopping power. The stopping power is obtained by comparing measured spectra with spectra calculated using Monte Carlo techniques. The stopping powers reported in PIRS-0626 were obtained using the EGS4 Monte Carlo code. Since then, the EGSnrc code has been released which has more accurate electron transport algorithms. We calculate the effect on the measured stopping powers of using EGSnrc instead of EGS4. The EGS4 spectra calculated in PIRS-0626 were based on primary electron histories. We first show that those spectra, calculated in 1997, are consistent with current EGS4 spectra calculated using histories. EGSnrc spectra are also calculated using histories and these high-precision spectra are compared to extract any energy difference. The energy differences between the spectra are used to estimate the effect on the measured electronic stopping powers. The energy differences depend on the absorber material, the absorber thickness and the beam energy. The improved electron elastic scattering cross section of EGSnrc accounts for only part of the difference between the two codes. The effect on the extracted stopping power is largest for the lowest electron energies and can be as large as 0.9%. The calculated spectra show differences for lower energies, with the EGSnrc spectra having a larger proportion of low-energy electrons. The differences introduced by using EGSnrc instead of EGS4 can affect the estimated stopping power by almost 1% in the worst case but generally the effect is much smaller. We report corrections that can be applied to all the stopping power data in PIRS-0626. An experiment to measure the average energy to create an ion pair in air, , using aluminum detectors will provide an interesting test of the aluminum stopping power data as reported in PIRS-0626 and revised by this work.

Monte Carlo Evaluation of Gamma Knife Dose Profile in Real Brain Phantom

Introduction The Gamma Knife system is designed solely for non-invasive treatment of brain disorders, and it benefits from stereotactic surgical techniques. Dose calculations required in the system are performed by GammaPlan code; in this code, brain tissue is considered uniform. In the present study, we evaluated the effect of Gamma Knife system on the obtained dose through simulating a real human brain phantom. Materials and Methods In this study, a Monte Carlo simulation code (MCNPX2.7) was employed to simulate Gamma Knife system. Brain tissue equivalent Snyder phantom and combinations were considered according to International Commission on Radiological Units (ICRU)-44 report. Results To ensure accuracy of the simulations, patient’s head was modeled by a spherical water phantom. At this point, the dosimetry parameters were compared with those obtained by the Monte Carlo code EGS4 and good consistency was observed (less than 7% difference). At the next stage, the above dosimetry parameters were compared with those obtained experimentally by polystyrene phantom and EDR2 dosimetry film and improved consistency was detected (less than 0.5% difference). Finally, the Snyder phantom, as the human brain, was simulated. The Full Width at Half Maximum (FWHM) and penumbra decreased by 4.7% and 18%, respectively. Moreover, an isocenter dose reduction of 30-40%, compared to the water phantom, was noted. Conclusion The calculation of the real phantom showed that water and polystyrene could function similarly, while evaluating dosimetry parameters in the Gamma Knife system; thus, water and polystyrene are not appropriate phantom matters for this purpose.

Computer simulation of a backscattered X-ray fluorescence system.

An EGSnrc user code is developed to simulate a backscattered geometry in vivo x-ray fluorescence system for the measurement of platinum concentration in head and neck tumours. The user code is fundamentally based on a previous study which used the EGS4 Monte Carlo code. The new user code, which we have developed in this study, has new improvements which made it able to simulate the process of photon transportation through the different components of the modelled x-ray fluorescence system. The simulation process included modelling of the photon source, collimators, phantoms and detector. Simulation results were compared and evaluated against x-ray fluorescence data obtained experimentally from an existing system developed by the Swansea In vivo Analysis and Cancer Research Group. In addition, simulation results of this study were also compared with our previous study in which the EGS4 user code was used. Comparison between results has shown that the new EGSnrc user code was able to reproduce the spectral shape obtained using the experimental x-ray fluorescence system. The area under the Compton peak differs by 2.5% between the experimental measurement and the EGSnrc simulation. Similarly, the area under the two Pt Kα peaks differs by 2.3% and 2.2%.

Kilo-Voltage Imaging Doses to Organs-at-Risk in CBCT-guided Breast Radiation Therapy

Recently kilo-voltage cone beam computed tomography (kVCBCT) scans have been widely used in the external beam radiation therapy of breast cancer patients to improve daily setup accuracy so that high dose of tumoricidal radiation can be delivered accurately to the partial volume, as well as to the whole breast. Although early studies investigated the kV imaging doses from kVCBCT scans to critical structures in the pelvis and head regions, the knowledge of dose depositions to the various organs-at-risk (OARs) in thoracic kVCBCT scans of the patient chest is largely missing. Therefore, the goal of this study is to systematically investigate the kV imaging doses to the OARs during CBCT-guided radiation therapy of breast cancer patients. Twenty breast cancer patients in supine orientation were retrospectively assorted into two groups by the lesion site (left vs. right breast). Critical structures such as the heart, the lungs, the normal breast and the thyroid were delineated on the Eclipse treatment planning system based on the planning CT images, which together were subsequently converted into patient CT phantoms for Monte Carlo simulations with an in-house DICOM utility tool. A benchmarked EGS4 Monte Carlo code was used to calculate the 3D dose distributions in the patients scanned by the kVCBCT with a half-fan pelvis protocol pre-defined by the manufacturer. The correlation between the imaging doses received by the OARs and the patient size as well as the lesion site was studied. Furthermore, a similar study was carried out on an adult female anthropomorphic phantom as defined in ICRP report 110 with cortical bone and red bone marrow delineated in the ribs, scapulae, clavicles, thoracic and cervical spines. The mean doses deposited to the heart, the lungs, the normal breast and the thyroid were found to be 3.1, 3.2, 3.3, and 1.5 cGy per CBCT scan, respectively. There were no significant differences observed between the dose depositions to the left and the right lungs, due primarily to the 364° arc rotation defined for the half-fan pelvis scan. A strong correlation was observed between the patient size and the imaging doses, with lower organ doses in the patients of larger sizes. The surface dose for the skin of the breast could reach 7-8 cGy per scan due to proximity to the kV source. The ICRP phantom study indicated that the kVCBCT-contributed mean doses to the cortical bones and the red bone marrows in the thoracic region were 6.1 and 4.0 cGy, respectively for each scan. Compared to the negligible doses (< 0.2% of therapeutic dose except for ipsilateral lung) from the mega-voltage photon beams in breast radiation therapy, each kVCBCT scan contributed 4-10 times more doses to the various OARs on average. Caution should be exercised to avoid multiple kVCBCT acquisitions for each fraction in CBCT-guided breast radiation therapy.

SU‐E‐T‐365: IMRT Monte Carlo Dose Calculations for a Novalis TX LINAC Equipped with HD‐120 MLC

Purpose: To investigate the accuracy of a Monte Carlo dose calculation engine for IMRT using a Novalis Tx LINAC with the HD‐120 MLC. Methods: PDD and beam profiles ranging from 1×1 to 20×20 cm2 were calculated and compared against measured data in water to commission the LINAC built on the BEAMNRC platform. An in‐house, MATLAB‐based software was used to extract the parameters needed for the MC simulation from the DICOM‐RT plan and create input files for the MCSim egs4 Monte Carlo code. MCSim egs4 software was used to simulate the IMRT treatment plans in a cylindrical solid‐water phantom and ultimately used to compute the dose. In‐house software using a MATLAB platform was utilized to convert the 3D dose file obtained from the Monte Carlo simulation to a universal file which can be read on the RIT Software platform. Within RIT, the MC and treatment planning system (TPS) dose calculations could be compared. Results: 5 IMRT plan simulations were performed and good agreement results were achieved for TPS and MC dose comparison. Gamma analysis with a 3% dose and 3 mm DTA criteria shows a mean gamma index value of 92% for the cases studied. Conclusions: Based on our preliminary results, the Monte Carlo based Novalis Tx linear accelerator and the MCSim simulations of the IMRT treatments on a cylindrical phantom demonstrate good accuracy with regards to the treatment planning dose calculation.

Personalized Assessment of kV Cone Beam Computed Tomography Doses in Image-guided Radiotherapy of Pediatric Cancer Patients

To develop a quantitative method for the estimation of kV cone beam computed tomography (kVCBCT) doses in pediatric patients undergoing image-guided radiotherapy. Forty-two children were retrospectively analyzed in subgroups of different scanned regions: one group in the head-and-neck and the other group in the pelvis. Critical structures in planning CT images were delineated on an Eclipse treatment planning system before being converted into CT phantoms for Monte Carlo simulations. A benchmarked EGS4 Monte Carlo code was used to calculate three-dimensional dose distributions of kVCBCT scans with full-fan high-quality head or half-fan pelvis protocols predefined by the manufacturer. Based on planning CT images and structures exported in DICOM RT format, occipital-frontal circumferences (OFC) were calculated for head-and-neck patients using DICOMan software. Similarly, hip circumferences (HIP) were acquired for the pelvic group. Correlations between mean organ doses and age, weight, OFC, and HIP values were analyzed with SigmaPlot software suite, where regression performances were analyzed with relative dose differences (RDD) and coefficients of determination (R(2)). kVCBCT-contributed mean doses to all critical structures decreased monotonically with studied parameters, with a steeper decrease in the pelvis than in the head. Empirical functions have been developed for a dose estimation of the major organs at risk in the head and pelvis, respectively. If evaluated with physical parameters other than age, a mean RDD of up to 7.9% was observed for all the structures in our population of 42 patients. kVCBCT doses are highly correlated with patient size. According to this study, weight can be used as a primary index for dose assessment in both head and pelvis scans, while OFC and HIP may serve as secondary indices for dose estimation in corresponding regions. With the proposed empirical functions, it is possible to perform an individualized quantitative dose assessment of kVCBCT scans.

TH-E-BRC-06: Optimizing KVCBCT Scan Protocol for Individual Patient Undergoing IGRT

Purpose: To develop an automatic tool for optimal mAs and kVp settings in kVCBCT based on patient CT anatomy and user-defined priorities on normal tissue sparing and image quality. Methods: An EGS4 Monte Carlo code was employed to calculate 3D dose distributions in patients scanned with kVCBCT at both half-fan and full-fan modes. Absorbed doses to various organs were analyzed. The relationship between absorbed doses, mAs and kVp was studied with phantom measurements and fit with empirical functions. An optimizer tool based on conjugated gradient searching algorithm in multi-dimensions was developed to generate the optimal settings of mAs and kVp based on 3D dose depositions to various organs of individual patient, with consideration of user-defined priorities on normal tissue sparing and image quality. The effectiveness of the optimizer was tested on Catphan and various patient CT anatomies. Results: Our phantom study indicates that the dose contributions from OBI CBCT half- fan and full-fan modes depend almost linearly on mAs while exponentially on kVp. Empirical functions can be used to reproduce the kVCBCT doses accurately at any allowed mAs and kVp settings. In general, our optimizer recommends much less mAs and kVp than default settings without compromising kVCBCT image quality. Compared to default modes, the optimized modes deposit much less doses, with reduction of 61–66% to the adult and 45–68% to the pediatric patients, respectively. Conclusions: A novel kVCBCT scan protocol optimizer has been developed to help clinicians choose appropriate mAs and kVp settings for individual patient while maintaining a balance between normal tissue sparing and image quality of kVCBCT scans. With optimal settings, the dose depositions to the patients could reduce by 45–68% on average in comparison to the default without compromising image quality. The developed optimizer could potentially improve radiation safety in medical imaging of both adult and pediatric patients.

TU-C-BRB-02: Personalized Assessment of KV-CBCT Doses in Image Guided Radiotherapy of Pediatric Cancer Patients

Purpose: To develop an empirical method for estimation of doses to critical structures of children undergoing kV‐CBCT imaging for image‐guidedradiotherapy. Methods: Forty pediatric patients were retrospectively assorted into four groups by gender and scanned sites with approval of IRB. Critical structures in CTimages were delineated on Eclipse treatment planning system and were subsequently packaged into patient CT phantoms for Monte Carlo simulations. A benchmarked EGS4 Monte Carlo code was used to calculate 3D dose distributions produced by High‐quality head and Pelvic protocols that were pre‐defined by manufacturer. The ratios of irradiated volume over longitudinal length (V/L) and the equivalent sphere diameters (ESDs) were calculated by Eclipse. In accordance with the scanned region, either occipital‐frontal circumferences (OFCs) or hip circumferences (HIPs) were calculated by DICOMan software. Regression was performed with SigmaPlot software suite where average standard errors of estimate (SEEs) were compared to evaluate the accuracy of regression and reliability of various parameters. Results: For both sexes, mean doses of all critical structures were observed to decrease linearly with the increase of all physical parameters including age, weight, V/L, ESD, OFC, and HIP. As suggested by smaller standard errors of estimates, OFC and weight are recommended for head dose estimation while ESD and weight are more appropriate for pelvic/abdominal dose regression. Unique empirical functions for the estimation of doses to critical organs for male or female patients are presented which make a personalized dose assessment readily feasible. Conclusions: Using Monte Carlo simulations in a population of 40 selected children, kV‐CBCT doses were found highly correlated with body sizes of children. Considering uneven body development levels among children of same age, OFC and weight are recommended primarily for dose estimation in head, while weight and ESD are preferred for pelvic dose assessment. This work was partially supported by National Basic Research Subject (973) of China (No.2011CB707701), China Scholarship Council (No. 2010601107), and Joint Research Foundation of Beijing Education Committee, China (No. JD100010607)

Testicular Doses in Image-Guided Radiotherapy of Prostate Cancer

To investigate testicular doses contributed by kilovoltage cone-beam computed tomography (kVCBCT) during image-guided radiotherapy (IGRT) of prostate cancer. An EGS4 Monte Carlo code was used to calculate three-dimensional dose distributions from kVCBCT on 3 prostate cancer patients. Absorbed doses to various organs were compared between intensity-modulated radiotherapy (IMRT) treatments and kVCBCT scans. The impact of CBCT scanning mode, kilovoltage peak energy (kVp), and CBCT field span on dose deposition to testes and other organs was investigated. In comparison with one 10-MV IMRT treatment, a 125-kV half-fan CBCT scan delivered 3.4, 3.8, 4.1, and 5.7 cGy to the prostate, rectum, bladder, and femoral heads, respectively, accounting for 1.7%, 3.2%, 3.2%, and 8.4% of megavoltage photon dose contributions. However, the testes received 2.9 cGy from the same CBCT scan, a threefold increase as compared with 0.7 cGy received during IMRT. With the same kVp, full-fan mode deposited much less dose to organs than half-fan mode, ranging from 9% less for prostate to 69% less for testes, except for rectum, where full-fan mode delivered 34% more dose. As photon beam energy increased from 60 to 125 kV, kVCBCT-contributed doses increased exponentially for all organs, irrespective of scanning mode. Reducing CBCT field span from 30 to 10 cm in the superior-inferior direction cut testicular doses from 5.7 to 0.2 cGy in half-fan mode and from 1.5 to 0.1 cGy in full-fan mode. Compared with IMRT, kVCBCT-contributed doses to the prostate, rectum, bladder, and femoral heads are clinically insignificant, whereas dose to the testes is threefold more. Full-fan CBCT usually deposits much less dose to organs (except for rectum) than half-fan mode in prostate patients. Kilovoltage CBCT-contributed doses increase exponentially with photon beam energy. Reducing CBCT field significantly cuts doses to testes and other organs.

Kilovoltage Imaging Doses in the Radiotherapy of Pediatric Cancer Patients

To investigate doses induced by kilovoltage cone-beam computed tomography (kVCBCT) to pediatric cancer patients undergoing radiotherapy, as well as strategies for dose reduction. An EGS4 Monte Carlo code was used to calculate three-dimensional dose deposition due to kVCBCT on 4 pediatric cancer patients. Absorbed doses to various organs were analyzed for both half-fan and full-fan modes. Clinical conditions, such as distance from organ at risk (OAR) to CBCT field border, kV peak energy, and testicular shielding, were studied. The mean doses induced by one CBCT scan operated at 125 kV in half-fan mode to testes, liver, kidneys, femoral heads, spinal cord, brain, eyes, lens, and optical nerves were 2.9, 4.7, 7.7, 10.5, 8.8, 7.6, 7.7, 7.8, and 7.2 cGy, respectively. Increasing the distances from OARs to CBCT field border greatly reduced the doses to OARs, ranging from 33% reduction for spinal cord to 2300% reduction for testes. As photon beam energy increased from 60 to 125 kV, the dose increase due to kVCBCT ranged from 170% for lens to 460% for brain and spinal cord. A testicular shielding made of 1-cm cerrobend could reduce CBCT doses down to 31%, 51%, 68%, and 82%, respectively, for 60, 80, 100, and 125 kV when the testes lay within the CBCT field. Generally speaking, kVCBCT deposits much larger doses to critical structures in children than in adults, usually by a factor of 2 to 3. Increasing the distances from OARs to CBCT field border greatly reduces doses to OARs. Depending on OARs, kVCBCT-induced doses increase linearly or exponentially with photon beam energy. Testicular shielding works more efficiently at lower kV energies. On the basis of our study, it is essential to choose an appropriate scanning protocol when kVCBCT is applied to pediatric cancer patients routinely.

Influence of voxel size on specific absorbed fractions and S-values in a mouse voxel phantom

Photon and electron specific absorbed fractions (SAFs) and S-values have been evaluated using mouse voxel phantoms. In voxel phantoms, it is important to choose the voxel size carefully since it affects the accuracy of results. In this study, two mouse voxel phantoms were constructed, with cubic voxels, one with 0.1-mm sides and the other with 0.4-mm sides. The sources were considered to be distributed uniformly in the main organs and the radiation transport was simulated using the Monte Carlo code EGS4. It was found that the effect of voxel size on SAFs for self-irradiation was not high (<10 %) for electrons and photons. However, it was appreciable for cross-irradiation especially for electrons. The effect of voxel size was investigated on S-values for some beta emitters such as (131)I, (153)Sm, (188)Re and (90)Y.

SU-DD-A4-02: Investigation of KV Imaging Doses in the Radiotherapy of Pediatric Cancer Patients

Purpose: To investigate kVCBCT‐induced doses to pediatric cancer patients undergoing radiotherapy and strategies for optimal dose reduction. Method and Materials: Four pediatric cases were selected with patient CTs and contours exported from Eclipse TPS via DICOM and converted into EGS4 patient phantoms. An EGS4 Monte Carlo code was employed to calculate dose distributions on four patients scanned by kVCBCT. 3D dose distributions and DVHs were analyzed. In addition, distance from organ‐at‐risk (OAR) to CBCT field border, kilo‐voltage peak energy and testicular shielding have been studied for strategies to reduce doses to OARs. Results: Due to circular gantry rotations of CBCT and smaller dimensions in pediatric patients compared to adults, kVCBCT‐induced doses were quite symmetrically deposited around patient anatomy, with much higher doses to bony structures due to enhanced photoelectric effect. DVH analyses indicated mean doses induced by kVCBCT ranged from 2.9 cGy to testes to 10.5 cGy to femur heads. Increasing distances from OARs to CBCT field border would greatly cut doses to OARs, ranging from 50% reduction for spinal cord to 2300% reduction for testes. Reducing beam energy from 125 to 60 kV generally increased doses to OARs, but further reduction would yield more doses to lens and testes while less doses to other OARs. The dose reduction via a testicular shielding made of lead would be more efficient in lower energy when testes lied within CBCT field. Conclusions: kVCBCT deposits considerably large doses to critical structures in children. Increasing the distances from OARs to CBCT field border and use of a testicular shielding are effective ways to minimize the doses to testes and other critical structures. It is essential to evaluate the doses induced by kilo‐voltage beams as a part of treatment planning and choose an appropriate scanning protocol when kVCBCT is applied frequently, especially in young patients.

SU‐GG‐T‐426: Feasibility of an Independent Dose Calculation for VMAT with Monte Carlo

Purpose: To investigate the feasibility of a Monte Carlo based independent dose calculation for VMAT in a Novalis Tx linear accelerator equipped with high definition MLC. Method and Materials: Percent depth dose curves were calculated and compared against measured ones in water to commissioning the linear accelerator built on BEAMNRC. From the DICOM containing the information of the VMAT plan all the parameters needed for the simulation were extracted using a MATLAB in‐house software, which with this data create the files for the Monte Carlo simulation and is exported to a LINUX environment where the simulation is done in MCSim egs4 Monte Carlo code. MCSim egs4 software was used to simulate the VMAT treatment plans, in which a cylindrical phantom (15 cm radius and 18 cm depth) is used to compute the dose. In‐house software using a MATLAB platform was utilized to convert the 3D dose file obtained from the Monte Carlo simulation to a file which can be read using commercially‐available software for dose visualization (RIT software), in which can be compared with the treatment plan exported form the TPS. Results: Several simulations in the MCSim egs4 platform were performed and good results have been observed. The accuracy of the calculation is still low but the main purpose of this preliminary work is just for demonstrating the feasibility of the VMAT calculation in Monte Carlo software. Conclusion: Based on the results, the Monte Carlo based NovalisTx linear accelerator and the MCSim simulations of treatment on a cylindrical phantom it was demonstrate the feasibility of VMAT independent dose calculation in Monte Carlo.

Computation of gamma-ray exposure buildup factors up to 10 mfp using generalized feed-forward neural network

This paper presents an approach based on generalized feed-forward neural network (GFFNN) to compute exposure buildup factors ( B D ) for point isotropic sources in infinite homogeneous media at energies varying from 0.03 MeV to 15 MeV and up to depths of 10 mean free paths (mfp). The results obtained by using the proposed model have been compared with the ANSI standard data, the calculations by use of EGS4 Monte Carlo code and Invariant Embedding (IE) Method for water, iron, lead and concrete. The comparisons have shown that the GFFNN model improved B D estimation with respect to the other methods, particularly for lead and concrete.

TU‐E‐304A‐04: Methods of Assessing the Dose to the Red Bone Marrow for Diagnostic and Therapeutic Procedures

Purpose: To demonstrate a more accurate method of assessing whole body red bone marrow dose in clinical applications. Method and Materials: Assessment of methods of red bone marrow (RBM) dose calculations is performed using a model constructed from the CT data of the Visible Human Project (VHP) and the EGS4 Monte Carlo code. Calculation of local marrow dose is achieved using the CT Number Method, which describes the fraction of marrow in a given CT voxel. Calculation of whole‐body average RBM dose is performed using two methods: one that assumes an homogeneous mixture of red and yellow bone marrow (HM Method), and one that uses a cellularity‐factor weighted marrow mixture in each bone (CW Method). Distribution of RBM mass in the skeleton calculated using both methods are compared to the color photograph data of the VHP used to construct the VIP‐Man anatomical model. Red bone marrow doses are calculated for broad beam exposures over a large energy range, and for several clinical applications including various CT scan protocols and electron whole‐body irradiation treatments. Results: The average difference in red bone marrow mass in each bone segment compared to the quantity present in VIP‐Man is 45% smaller when calculated with the CW method than the HM method. For clinical CT protocols, the RBM dose shows significant differences, up to 33% for head scans and 25% for abdominal scans. For the electron whole‐body irradiation (WBI) treatments, the RBM dose differs by 39%. Conclusion: Two common methods for calculating whole body average red bone marrow dose have been compared to the observed distribution standard available in the Visible Human Project data. Significant differences in dose are observed between the two methods, suggesting the importance of using appropriate cellularity factors in calculating red bone marrow dose.

Differences among Monte Carlo codes in the calculations of voxel values for radionuclide targeted therapy and analysis of their impact on absorbed dose evaluations

Several updated Monte Carlo (MC) codes are available to perform calculations of voxel S values for radionuclide targeted therapy. The aim of this work is to analyze the differences in the calculations obtained by different MC codes and their impact on absorbed dose evaluations performed by voxel dosimetry. Voxel S values for monoenergetic sources (electrons and photons) and different radionuclides (90Y, 131I, and 188Re) were calculated. Simulations were performed in soft tissue. Three general-purpose MC codes were employed for simulating radiation transport: MCNP4C, EGSnrc, and GEANT4. The data published by the MIRD Committee in Pamphlet No. 17, obtained with the EGS4 MC code, were also included in the comparisons. The impact of the differences (in terms of voxel S values) among the MC codes was also studied by convolution calculations of the absorbed dose in a volume of interest. For uniform activity distribution of a given radionuclide, dose calculations were performed on spherical and elliptical volumes, varying the mass from 1 to 500 g. For simulations with monochromatic sources, differences for self-irradiation voxel S values were mostly confined within 10% for both photons and electrons, but with electron energy less than 500 keV, the voxel S values referred to the first neighbor voxels showed large differences (up to 130%, with respect to EGSnrc) among the updated MC codes. For radionuclide simulations, noticeable differences arose in voxel S values, especially in the bremsstrahlung tails, or when a high contribution from electrons with energy of less than 500 keV is involved. In particular, for 90Y the updated codes showed a remarkable divergence in the bremsstrahlung region (up to about 90% in terms of voxel S values) with respect to the EGS4 code. Further, variations were observed up to about 30%, for small source-target voxel distances, when low-energy electrons cover an important part of the emission spectrum of the radionuclide (in our case, for 131I). For 90Y and 188Re, the differences among the various codes have a negligible impact (within few percents) on convolution calculations of the absorbed dose; thus either one of the MC programs is suitable to produce voxel S values for radionuclide targeted therapy dosimetry. However, if a low-energy beta-emitting radionuclide is considered, these differences can affect also dose depositions at small source-target voxel distances, leading to more conspicuous variations (about 9% for 1311) when calculating the absorbed dose in the volume of interest.

Effect of transverse magnetic fields on dose distribution and RBE of photon beams: comparing PENELOPE and EGS4 Monte Carlo codes

The application of a strong transverse magnetic field to a volume undergoing irradiation by a photon beam can produce localized regions of dose enhancement and dose reduction. This study uses the PENELOPE Monte Carlo code to investigate the effect of a slice of uniform transverse magnetic field on a photon beam using different magnetic field strengths and photon beam energies. The maximum and minimum dose yields obtained in the regions of dose enhancement and dose reduction are compared to those obtained with the EGS4 Monte Carlo code in a study by Li et al (2001), who investigated the effect of a slice of uniform transverse magnetic field (1 to 20 Tesla) applied to high-energy photon beams. PENELOPE simulations yielded maximum dose enhancements and dose reductions as much as 111% and 77%, respectively, where most results were within 6% of the EGS4 result. Further PENELOPE simulations were performed with the Sheikh-Bagheri and Rogers (2002) input spectra for 6, 10 and 15 MV photon beams, yielding results within 4% of those obtained with the Mohan et al (1985) spectra. Small discrepancies between a few of the EGS4 and PENELOPE results prompted an investigation into the influence of the PENELOPE elastic scattering parameters C1 and C2 and low-energy electron and photon transport cut-offs. Repeating the simulations with smaller scoring bins improved the resolution of the regions of dose enhancement and dose reduction, especially near the magnetic field boundaries where the dose deposition can abruptly increase or decrease. This study also investigates the effect of a magnetic field on the low-energy electron spectrum that may correspond to a change in the radiobiological effectiveness (RBE). Simulations show that the increase in dose is achieved predominantly through the lower energy electron population.

SU-GG-T-41: Dosimetric Impacts On Tissue Homogeneity Corrections in Electronic Brachytherapy

Purpose: With increasing interests in utilizing electronic brachytherapy in accelerated partial breast irradiation, the lack of tissue inhomogeneity correction in the dose calculation draws clinical attentions. In this study, a Monte Carlo based dose evaluation was assessed in a human anatomy phantom to demonstrate these dosimetricimpacts.Method and Materials: The dosimetriccharacteristics of Xoft S700 Axxent™ X‐ray source (Xoft, Fremont,CA) were evaluated using EGS4 Monte Carlo code to verify the calculation method and source parameter accuracy. A virtual female human phantom utilizing all clinical parameters was used in the simulation. A balloon (diameter = 5 cm) was inserted in the right breast of the phantom. A 1 cm expanded volume from the balloon surface was designed to define as planning target volume (PTV). The prescribed dose was 3.4 Gy to 5 selected points (anterior, posterior, right lateral, superior and inferior) of the PTV. Dose,dose volume histograms of the PTV and normal tissues, isodose distributions in axial, sagittal and coronal planes crossing the center of the balloon were evaluated. Results: Using measured 50 kVp source spectrum provided by Xoft Inc, the simulated radial dose function agrees well with the published data within 3%. The simulation shows clear dose uniformity with 90.6% of the prescription dose enclosing PTV. The two dimensional sagittal dose distributions show high skin and rib doses with reasonably acceptable doses to all other surrounding normal tissues. When accounted for the tissue heterogeneity, changes in the isodose distribution were observed. Conclusion: This has been our first EGS4 Monte Carlo simulation study for the Xoft electronic brachytherapy system. Preliminary results demonstrate the dosimetricimpact especially on the surrounding normal tissues, when tissue inhomogeneity consideration is taken into account.

A boundary-representation method for designing whole-body radiation dosimetry models: pregnant females at the ends of three gestational periods—RPI-P3, -P6 and -P9

Fetuses are extremely radiosensitive and the protection of pregnant females against ionizing radiation is of particular interest in many health and medical physics applications. Existing models of pregnant females relied on simplified anatomical shapes or partial-body images of low resolutions. This paper reviews two general types of solid geometry modeling: constructive solid geometry (CSG) and boundary representation (BREP). It presents in detail a project to adopt the BREP modeling approach to systematically design whole-body radiation dosimetry models: a pregnant female and her fetus at the ends of three gestational periods of 3, 6 and 9 months. Based on previously published CT images of a 7-month pregnant female, the VIP-Man model and mesh organ models, this new set of pregnant female models was constructed using 3D surface modeling technologies instead of voxels. The organ masses were adjusted to agree with the reference data provided by the International Commission on Radiological Protection (ICRP) and previously published papers within 0.5%. The models were then voxelized for the purpose of performing dose calculations in identically implemented EGS4 and MCNPX Monte Carlo codes. The agreements of the fetal doses obtained from these two codes for this set of models were found to be within 2% for the majority of the external photon irradiation geometries of AP, PA, LAT, ROT and ISO at various energies. It is concluded that the so-called RPI-P3, RPI-P6 and RPI-P9 models have been reliably defined for Monte Carlo calculations. The paper also discusses the needs for future research and the possibility for the BREP method to become a major tool in the anatomical modeling for radiation dosimetry.For more information on this article, see medicalphysicsweb.org

Characteristics of a 85Kr beta-particle source applied in Series 1 reference irradiations of TMDIS-1 direct ion storage dosemeters

Characteristics necessary to specify an ISO 6980 Series 1 reference radiation field were determined for a commercially available 85Kr beta-particle source, using a BEAM EGS4 Monte Carlo code. The characteristics include residual maximum beta energy, E(res), and the uniformity of the dose rate over the calibration area. The E(res) and the uniformity were also determined experimentally, using an extrapolation ionization chamber (EC) and a 0.2 cm3 parallel plate ionization chamber, respectively. The depth-dose curve measured with the EC gave a value 0.62 MeV for the E(res). Series 2 90Sr + 90Y and Series 1(85) Kr beta-particle sources calibrated for H(p)(0.07) at the secondary standard dosimetry laboratory (SSDL) of STUK were used to determine the energy and angular responses of DIS-1 direct ion storage dosemeters. The averaged zero angle H(p)(0.07) responses to the 90Sr + 90Y and 85Kr reference radiations were 135 and 80%, respectively. The responses were normalized to 100%, H(p)(0.07) response to 137Cs photon radiation.