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At least 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

A GPU-accelerated Monte Carlo dose calculation platform and its application toward validating an MRI-guided radiation therapy beam model

Purpose: The clinical commissioning of IMRT subject to a magnetic field is challenging. The purpose of this work is to develop a GPU-accelerated Monte Carlo dose calculation platform based on PENELOPE and then use the platform to validate a vendor-provided MRIdian head model toward quality assurance of clinical IMRT treatment plans subject to a 0.35 T magnetic field. Methods: PENELOPE was first translated from FORTRAN to C++ and the result was confirmed to produce equivalent results to the original code. The C++ code was then adapted to CUDA in a workflow optimized for GPU architecture. The original code was expanded to include voxelized transport with Woodcock tracking, faster electron/positron propagation in a magnetic field, and several features that make gPENELOPE highly user-friendly. Moreover, the vendor-provided MRIdian head model was incorporated into the code in an effort to apply gPENELOPE as both an accurate and rapid dose validation system. A set of experimental measurements were performed on the MRIdian system to examine the accuracy of both the head model and gPENELOPE. Ultimately, gPENELOPE was applied toward independent validation of patient doses calculated by MRIdian’s KMC. Results: An acceleration factor of 152 was achieved in comparison to the original single-thread FORTRAN implementation with the original accuracy being preserved. For 16 treatment plans including stomach (4), lung (2), liver (3), adrenal gland (2), pancreas (2), spleen(1), mediastinum (1), and breast (1), the MRIdian dose calculation engine agrees with gPENELOPE with a mean gamma passing rate of 99.1% ± 0.6% (2%/2 mm). Conclusions: A Monte Carlo simulation platform was developed based on a GPU- accelerated version of PENELOPE. This platform was used to validate that both the vendor-provided head model and fast Monte Carlo engine used by the MRIdian system are accurate in modeling radiation transport in a patient using 2%/2 mm gamma criteria. Future applications of this platform will include dose validation and accumulation, IMRT optimization, and dosimetry system modeling for next generation MR-IGRT systems.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2012

EchoSeed Model 6733 Iodine-125 brachytherapy source: Improved dosimetric characterization using the MCNP5 Monte Carlo code

This study primarily aimed to obtain the dosimetric characteristics of the Model 6733 {sup 125}I seed (EchoSeed) with improved precision and accuracy using a more up-to-date Monte-Carlo code and data (MCNP5) compared to previously published results, including an uncertainty analysis. Its secondary aim was to compare the results obtained using the MCNP5, MCNP4c2, and PTRAN codes for simulation of this low-energy photon-emitting source. The EchoSeed geometry and chemical compositions together with a published {sup 125}I spectrum were used to perform dosimetric characterization of this source as per the updated AAPM TG-43 protocol. These simulations were performed in liquid water material in order to obtain the clinically applicable dosimetric parameters for this source model. Dose rate constants in liquid water, derived from MCNP4c2 and MCNP5 simulations, were found to be 0.993 cGyh{sup -1} U{sup -1} ({+-}1.73%) and 0.965 cGyh{sup -1} U{sup -1} ({+-}1.68%), respectively. Overall, the MCNP5 derived radial dose and 2D anisotropy functions results were generally closer to the measured data (within {+-}4%) than MCNP4c and the published data for PTRAN code (Version 7.43), while the opposite was seen for dose rate constant. The generally improved MCNP5 Monte Carlo simulation may be attributed to a more recent and accurate cross-section library. However, some of the data points in the results obtained from the above-mentioned Monte Carlo codes showed no statistically significant differences. Derived dosimetric characteristics in liquid water are provided for clinical applications of this source model.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2011

Integration of Three-Dimensional Rotational Angiography in Radiosurgical Treatment Planning of Cerebral Arteriovenous Malformations

Purpose: Accuracy in delineating the target volume is a major issue for successful stereotactic radiosurgery for arteriovenous malformations. The aim of the present study was to describe a method to integrate three-dimensional (3D) rotational angiography ( (3DRA)) into CyberKnife treatment planning and to investigate its potential advantages compared with computed tomography angiography (CTA) and magnetic resonance angiography. Methods and Materials: A total of 20 patients with a diagnosis of cerebral arteriovenous malformation were included in the present study. All patients underwent multislice computed tomography and 3D-volumetric CTA, (3DRA), and 3D magnetic resonance angiography. The contouring of the target and critical volumes was done separately using CTA and thereafter directly using (3DRA). The composite, conjoint, and disjoint volumes were measured. Results: The use of CTA or (3DRA) resulted in significant differences in the target and critical volumes. The target volume averaged 3.49 {+-} 3.01 mL measured using CTA and 3.26 {+-} 2.93 mL measured using (3DRA), for a difference of 8% (p < .05). The conjoint and disjoint volume analysis showed an 88% volume overlap. The qualitative evaluation showed that the excess volume obtained using CTA was mostly tissue surrounding the nidus and venous structures. The mean contoured venous volume was 0.67 mL measured using CTA and 0.88 mL (range, 0.1-2.7) measured using (3DRA) (p < .05). Conclusions: (3DRA) is a volumetric angiographic study that can be integrated into computer-based treatment planning. Although whether (3DRA) provides superior accuracy has not yet been proved, its high spatial resolution is attractive and offers a superior 3D view. This allows a better 3D understanding of the target volume and distribution of the radiation doses within the volume. Additional technical efforts to improve the temporal resolution and the development of software tools aimed at improving the performance of 3D contouring are warranted.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2015

SU-E-I-31: Differences Observed in Radiation Doses Across 2 Similar CT Scanners From Adult Brain-Neck CT Angiography

Purpose: The aim of this study is to evaluate the difference in radiation doses from adult Brain-Neck CT angiography (CTA) between two CT scanners. Methods: We collected CT dose index data (CTDIvol, DLP) from adult Brain-Neck CTA performed with two CT scanners (Sensation 64 (S64) and Definition AS (AS), Siemens Healthcare) performed at two of our facilities from Jan 1st to Dec 31th, 2014. X-ray dose management software (Radmetrics, Bayer Healthcare) was used to mine these data. All exams were performed with Tube Current Modulation (Care Dose 4D), tube voltage of 120 kVp, quality reference mAs of 300, beam collimation of 64*0.6 mm. The rotation time was set to 0.5 sec for S64 and 1.0 sec for AS. We also scanned an anthropomorphic skull and chest phantom under routine Brain-Neck CTA protocol with the two scanners and extracted the tube current values from the raw projection data. Results: The mean CTDIvol and DLP in Brain-Neck CTA was 72 mGy and 2554 mGy*cm for AS, which was substantially larger than the mean values of 46 mGy and 1699 mGy*cm for S64. The maximum tube current was 583 mA for most cases on the S64 while the maximum was 666 mA for AS even though the rotation time set for AS was 1.0 sec. Measurements obtained with the anthropomorphic phantom showed that the tube current reached 583 mA at the shoulder region for S64 while it reached to 666 mA for AS. Conclusion: The results of this study showed that substantially different CT doses can Result from Brain-Neck CTA protocols even when similar scanners and similar settings are used. Though both scanners have a similar maximum mA rating, differences in mA were observed through the shoulders, resulting in substantially different CTDIvol values.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

Quality indexes based on water measurements for low and medium energy x-ray beams: A theoretical study with PENELOPE

Purpose : To study the use of quality indexes based on ratios of absorbed doses in water at two different depths to characterize x-ray beams of low and medium energies. Methods : A total of 55 x-ray beam spectra were generated with the codes XCOMP5R and SPEKCALC and used as input of a series of Monte Carlo simulations performed with PENELOPE, in which the percentage depth doses in water and thek{sub Q,Q{sub 0}} factors, defined in the TRS-398 protocol, were determined for each beam. Some of these calculations were performed by simulating the ionization chamber PTW 30010. Results : The authors found that the relation betweenk{sub Q,Q{sub 0}} and the ratios of absorbed doses at two depths is almost linear. A set of ratios statistically compatible with that showing the best fit has been determined. Conclusions : The results of this study point out which of these ratios of absorbed doses in water could be used to better characterize x-ray beams of low and medium energies.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2015

SU-E-T-142: Automatic Linac Log File: Analysis and Reporting

Purpose: End to end QA for IMRT/VMAT is time consuming. Automated linac log file analysis and recalculation of daily recorded fluence, and hence dose, distribution bring this closer. Methods: Matlab (R2014b, Mathworks) software was written to read in and analyse IMRT/VMAT trajectory log files (TrueBeam 1.5, Varian Medical Systems) overnight, and are archived on a backed-up network drive (figure). A summary report (PDF) is sent by email to the duty linac physicist. A structured summary report (PDF) for each patient is automatically updated for embedding into the R&V system (Mosaiq 2.5, Elekta AG). The report contains cross-referenced hyperlinks to ease navigation between treatment fractions. Gamma analysis can be performed on planned (DICOM RTPlan) and treated (trajectory log) fluence distributions. Trajectory log files can be converted into RTPlan files for dose distribution calculation (Eclipse, AAA10.0.28, VMS). Results: All leaf positions are within +/−0.10mm: 57% within +/−0.01mm; 89% within 0.05mm. Mean leaf position deviation is 0.02mm. Gantry angle variations lie in the range −0.1 to 0.3 degrees, mean 0.04 degrees. Fluence verification shows excellent agreement between planned and treated fluence. Agreement between planned and treated dose distribution, the derived from log files, is very good. Conclusion: Automated log file analysis is a valuable tool for the busy physicist, enabling potential treated fluence distribution errors to be quickly identified. In the near future we will correlate trajectory log analysis with routine IMRT/VMAT QA analysis. This has the potential to reduce, but not eliminate, the QA workload.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2018

Hepatic Arterial Embolization Using Cone Beam CT with Tumor Feeding Vessel Detection Software: Impact on Hepatocellular Carcinoma Response

PurposeDedicated tumor feeding vessel detection software (TFVDS) using cone beam CT has shown a higher sensitivity to detect tumor feeding vessels during hepatic arterial embolization (HAE) of hepatocellular carcinoma (HCC) compared to 2D imaging. Our primary hypothesis was that HCC tumors treated with HAE guided by a TFVDS would show more complete response (CR) than when treated with 2D imaging alone. Secondary analysis of the impact on X-ray exposure was performed.Materials and MethodsNineteen males and 8 females (median age: 69 year, 46–85) with 44 tumors (median size: 38 mm, 6–100) treated with selective HAE between January 2013 and December 2014 were included. Exclusion criteria were: extra-hepatic supply, >4 tumors, tumor size >10 cm, and adjunctive local therapy. Baseline patient and procedure characteristics were reviewed. Differences in CR per modified Response Evaluation Criteria in Solid Tumors were assessed by univariate and multivariate analyses for tumor size, number, location, particles size, and use of TFVDS.ResultsMedian imaging follow-up was 20.1 months (2–33). Use of TFVDS (13 patients, 19 tumors) was the only factor predictive of CR (OR = 3.85 [CI95%: 1.09, 13.67], p = 0.04) on univariate analysis but not on multivariate analysis (OR = 3.26 [0.87, 12.23], p = 0.08). A higher rate of CR was observed for HAE using TFVDS guidance versus 2D imaging alone (68.4%, 13–19, vs. 36%, 9–25, p = 0.03). Median dose area product was lower when TFVDS was used (149.7 Gy.cm{sup 2}, 38–365, vs. 227.8 Gy.cm{sup 2}, 85.3–468.6, p = 0.05).ConclusionsHCC embolized with TFVDS may result in improved local tumor response without increasing the dose exposure.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2012

Dosimetric Verification Using Monte Carlo Calculations for Tissue Heterogeneity-Corrected Conformal Treatment Plans Following RTOG 0813 Dosimetric Criteria for Lung Cancer Stereotactic Body Radiotherapy

Purpose: The recently activated Radiation Therapy Oncology Group (RTOG) studies of stereotactic body radiation therapy (SBRT) for non-small-cell lung cancer (NSCLC) require tissue density heterogeneity correction, where the high and intermediate dose compliance criteria were established based on superposition algorithm dose calculations. The study was aimed at comparing superposition algorithm dose calculations with Monte Carlo (MC) dose calculations for SBRT for NSCLC and to evaluate whether compliance criteria need to be adjusted for MC dose calculations. Methods and Materials: Fifteen RTOG 0236 study sets were used. The planning tumor volumes (PTV) ranged from 10.7 to 117.1 cm{sup 3}. SBRT conformal treatment plans were generated using XiO (CMS Inc.) treatment planning software with superposition algorithm to meet the dosimetric high and intermediate compliance criteria recommended by the RTOG 0813 protocol. Plans were recalculated using the MC algorithm of a Monaco (CMS, Inc.) treatment planning system. Tissue density heterogeneity correction was applied in both calculations. Results: Overall, the dosimetric quantities of the MC calculations have larger magnitudes than those of the superposition calculations. On average, R{sub 100%} (ratio of prescription isodose volume to PTV), R{sub 50%} (ratio of 50% prescription isodose volume to PTV), D{sub 2cm} (maximal dose 2 cm from PTV in any direction as a percentage of prescription dose), and V{sub 20} (percentage of lung receiving dose equal to or larger than 20 Gy) increased by 9%, 12%, 7%, and 18%, respectively. In the superposition plans, 3 cases did not meet criteria for R{sub 50%} or D{sub 2cm}. In the MC-recalculated plans, 8 cases did not meet criteria for R{sub 100%}, R{sub 50%}, or D{sub 2cm}. After reoptimization with MC calculations, 5 cases did not meet the criteria for R{sub 50%} or D{sub 2cm}. Conclusions: Results indicate that the dosimetric criteria, e.g., the criteria for R{sub 50%} recommended by RTOG 0813 protocol, may need to be adjusted when the MC dose calculation algorithm is used.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2015

SU-E-T-616: Plan Quality Assessment of Both Treatment Planning System Dose and Measurement-Based 3D Reconstructed Dose in the Patient

Purpose: Systematic radiotherapy plan quality assessment promotes quality improvement. Software tools can perform this analysis by applying site-specific structure dose metrics. The next step is to similarly evaluate the quality of the dose delivery. This study defines metrics for acceptable doses to targets and normal organs for a particular treatment site and scores each plan accordingly. The input can be the TPS or the measurement-based 3D patient dose. From this analysis, one can determine whether the delivered dose distribution to the patient receives a score which is comparable to the TPS plan score, otherwise replanning may be indicated. Methods: Eleven neuroblastoma patient plans were exported from Eclipse to the Quality Reports program. A scoring algorithm defined a score for each normal and target structure based on dose-volume parameters. Each plan was scored by this algorithm and the percentage of total possible points was obtained. Each plan also underwent IMRT QA measurements with a Mapcheck2 or ArcCheck. These measurements were input into the 3DVH program to compute the patient 3D dose distribution which was analyzed using the same scoring algorithm as the TPS plan. Results: The mean quality score for the TPS plans was 75.37% (std dev=14.15%) compared to 71.95% (std dev=13.45%) for the 3DVH dose distribution. For 3/11 plans, the 3DVH-based quality score was higher than the TPS score, by between 0.5 to 8.4 percentage points. Eight/11 plans scores decreased based on IMRT QA measurements by 1.2 to 18.6 points. Conclusion: Software was used to determine the degree to which the plan quality score differed between the TPS and measurement-based dose. Although the delivery score was generally in good agreement with the planned dose score, there were some that improved while there was one plan whose delivered dose quality was significantly less than planned. This methodology helps evaluate both planned and delivered dose quality. Sun Nuclear Corporation has provded a license for the software described.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2016

SU-G-BRB-06: Commissioning and Evaluation of EPID-Based in Vivo Dosimetry Software Using a Tissue-Maximum Ratio Approach

Purpose: To commission and evaluate an in vivo EPID-based transit dosimetry software (EPIgray, DOSIsoft, Cachan, France) using simple fields and TG119-based IMRT treatment plans. Methods: EPIgray was commissioned on a Truebeam based on finite tissue-maximum ratio (fTMR) measurements with solid water blocks of thicknesses between 0 and 37 cm. Field sizes varied from 2×2 to 20×20 cm{sup 2}. Subsequently, treatment plans of single and opposed beams with field sizes between 4×4 and 15×15 cm{sup 2} as well as IMRT plans were measured to evaluate the dose reconstruction accuracy. Single field dose predictions were made for anterior-posterior and lateral beams. IMRT plans were created based on TG119 recommendations. The reconstructed dose was compared to the planned dose for selected points at isocenter and away from isocenter. Results: For single square fields, the dose in EPIgray was reconstructed within 3% accuracy at isocenter relative to the planned dose. Similarly, the relative deviation of the total dose was accurately reconstructed within 3% for all IMRT plans with points placed inside a high dose region near the isocenter. Predictions became less accurate than 5% when the evaluation point was outside the majority of IMRT beam segments. Additionally, points 5 cm or more away from the isocenter or within an avoidance structure were predicted less reliably. Conclusion: EPIgray formalism accuracy is adequate for an efficient error detection system. It provides immediate intra-fractional feedback on the delivery of treatment plans without affecting the treatment beam. Besides the EPID, no additional hardware is required, which makes it accessible to all clinics. The software evaluates point dose measurements to verify treatment plan delivery and patient positioning within 5% accuracy, depending on the placement of evaluation points. EPIgray is not intended to replace patient-specific quality assurance but should be utilized as an additional layer of safety for continuous patient treatment verification. This research is supported by DOSIsoft.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-E-T-203: Comparison of a Commercial MRI-Linear Accelerator Based Monte Carlo Dose Calculation Algorithm and Geant4

Purpose: An MRI-linear accelerator is currently being developed by the vendor Elekta™. The treatment planning system that will be used to model dose for this unit uses a Monte Carlo dose calculation algorithm, GPUMCD, that allows for the application of a magnetic field. We tested this radiation transport code against an independent Monte-Carlo toolkit Geant4 (v.4.10.01) both with and without the magnetic field applied. Methods: The setup comprised a 6 MeV mono-energetic photon beam emerging from a point source impinging on a homogeneous water phantom at 100 cm SSD. The comparisons were drawn from the percentage depth doses (PDD) for three different field sizes (1.5 x 1.5 cm{sup 2}, 5 x 5 cm{sup 2}, 10 x 10 cm{sup 2}) and dose profiles at various depths. A 1.5 T magnetic field was applied perpendicular to the direction of the beam. The transport thresholds were kept the same for both codes. Results: All of the normalized PDDs and profiles agreed within ± 1 %. In the presence of the magnetic field, PDDs rise more quickly reducing the depth of maximum dose. Near the beam exit point in the phantom a hot spot is created due to the electron return effect. This effect is more pronounced for the larger field sizes. Profiles selected parallel to the external field show no effect, however, the ones selected perpendicular to the direction of the applied magnetic field are shifted towards the direction of the Lorentz force applied by the magnetic field on the secondary electrons. It is observed that these profiles are not symmetric which indicates a lateral build up of the dose. Conclusion: There is a good general agreement between the PDDs/profiles calculated by both algorithms thus far. We are proceeding towards clinically relevant comparisons in a heterogeneous phantom for polyenergetic beams. Funding for this work has been provided by Elekta.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

SU-F-T-657: In-Room Neutron Dose From High Energy Photon Beams

Purpose: To estimate neutron dose inside the treatment room from photodisintegration events in high energy photon beams using Monte Carlo simulations and experimental measurements. Methods: The Monte Carlo code MCNP6 was used for the simulations. An Eberline ESP-1 Smart Portable Neutron Detector was used to measure neutron dose. A water phantom was centered at isocenter on the treatment couch, and the detector was placed near the phantom. A Varian 2100EX linear accelerator delivered an 18MV open field photon beam to the phantom at 400MU/min, and a camera captured the detector readings. The experimental setup was modeled in the Monte Carlo simulation. The source was modeled for two extreme cases: a) hemispherical photon source emitting from the target and b) cone source with an angle of the primary collimator cone. The model includes the target, primary collimator, flattening filter, secondary collimators, water phantom, detector and concrete walls. Energy deposition tallies were measured for neutrons in the detector and for photons at the center of the phantom. Results: For an 18MV beam with an open 10cm by 10cm field and the gantry at 180°, the Monte Carlo simulations predict the neutron dose in the detector to be 0.11% of the photon dose in the water phantom for case a) and 0.01% for case b). The measured neutron dose is 0.04% of the photon dose. Considering the range of neutron dose predicted by Monte Carlo simulations, the calculated results are in good agreement with measurements. Conclusion: We calculated in-room neutron dose by using Monte Carlo techniques, and the predicted neutron dose is confirmed by experimental measurements. If we remodel the source as an electron beam hitting the target for a more accurate representation of the bremsstrahlung fluence, it is feasible that the Monte Carlo simulations can be used to help in shielding designs.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-167: Evaluation of Mobius Dose Calculation Engine Using Out of the Box Preconfigured Beam Data

Purpose: Determine the dose calculation accuracy of a preconfigured Mobius server for use in secondary checks of a treatment planning system. Methods: 10 plans were created for irradiation on two of the IROC (formerly RPC) accreditation phantoms: 4 for the head and neck phantom and 6 for the lung phantom. The plans each were created using one of four different photon energies (6FFF, 10 FFF, 6X, and 15X) and were varied in treatment type including VMAT, step and shoot IMRT, dynamic MLC IMRT (DMLC), and conformal RT (CRT). The TLDs in the phantoms were contoured, and each plan was sent for calculation to Mobius software (Mobius Medical Systems, Houston, TX) with a default configuration. Each plan was then irradiated on the planned phantom 3 times to create an average reading across 56 TLDs. These readings were then compared against the corresponding Mobius calculation at each TLD location. Results: The mean difference (MD) normalized to the plan prescription dose between each TLD and Mobius calculation for all measurements was 0.5 ± 3.3%, with a maximum difference of 8.4%. The MD was 0.6 ± 3.8%, − 2.0 ± 1.9%, 1.7 ± 3.7%, and 1.9 ± 1.2% across the 6FFF, 10FFF, 6X and 15X energies respectively. The MD was −1.2 ± 2.3% for lung plans and 1.8 ± 3.5% for head/neck plans. Across treatment types, the MD ranged from − 1.8 ± 1.7% for CRT to 4.3 ± 2.4 % for DMLC. Conclusion: Out of the box and preconfigured, Mobius provides accurate dose calculations with respect to beam energy, treatment type, and treatment site.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2011

Changes in dose with segmentation of breast tissues in Monte Carlo calculations for low-energy brachytherapy

Purpose: To investigate the use of various breast tissue segmentation models in Monte Carlo dose calculations for low-energy brachytherapy. Methods: The EGSnrc user-code BrachyDose is used to perform Monte Carlo simulations of a breast brachytherapy treatment using TheraSeed Pd-103 seeds with various breast tissue segmentation models. Models used include a phantom where voxels are randomly assigned to be gland or adipose (randomly segmented), a phantom where a single tissue of averaged gland and adipose is present (averaged tissue), and a realistically segmented phantom created from previously published numerical phantoms. Radiation transport in averaged tissue while scoring in gland along with other combinations is investigated. The inclusion of calcifications in the breast is also studied in averaged tissue and randomly segmented phantoms. Results: In randomly segmented and averaged tissue phantoms, the photon energy fluence is approximately the same; however, differences occur in the dose volume histograms (DVHs) as a result of scoring in the different tissues (gland and adipose versus averaged tissue), whose mass energy absorption coefficients differ by 30%. A realistically segmented phantom is shown to significantly change the photon energy fluence compared to that in averaged tissue or randomly segmented phantoms. Despite this, resulting DVHs for the entire treatment volume agree reasonably because fluence differences are compensated by dose scoring differences. DVHs for the dose to only the gland voxels in a realistically segmented phantom do not agree with those for dose to gland in an averaged tissue phantom. Calcifications affect photon energy fluence to such a degree that the differences in fluence are not compensated for (as they are in the no calcification case) by dose scoring in averaged tissue phantoms. Conclusions: For low-energy brachytherapy, if photon transport and dose scoring both occur in an averaged tissue, the resulting DVH for the entire treatment volume is reasonably accurate because inaccuracies in photon energy fluence are compensated for by inaccuracies in localized dose scoring. If dose to fibroglandular tissue in the breast is of interest, then the inaccurate photon energy fluence calculated in an averaged tissue phantom will result in inaccurate DVHs and average doses for those tissues. Including calcifications necessitates the use of proper tissue segmentation.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2016

SU-F-T-658: Out-Of-Field Dose Comparison for TrueBeam Low Energy Beams for Extended Distances: Measurement Vs Monte Carlo Simulation

Purpose: Patient dose far from the treatment field is comprised of scatter from within the patient, and treatment head leakage. We quantify the treatment head leakage for TrueBeam linear accelerator for 6X and 6X-FFF beams by comparing measurements to Monte Carlo simulations for a variety of jaw sizes and collimator rotations. This work is conceptually similar to that of Kry et al. (Medical Physics 2006; 33: 4405), who considered a Clinac linear accelerator. Methods: Measurements were made using an EXRADIN A101 ion chamber positioned in the patient plane, at distances up to 100 cm from isocenter. Simulations were done using VirtuaLinac, the GEANT4-based Monte Carlo model of the TrueBeam treatment head, and an in-house (U. Virginia) GEANT4-based code. In-house code modelled an ion chamber with build-up, based on a CT scan of the chamber. VirtuaLinac included a detailed model of the treatment head shielding, and was run on the Amazon Web Services cloud to generate spherical phase space files surrounding the treatment head. These phase space files were imported into the in-house code. Results: Initial comparisons between measurements and simulation showed an excess of dose in the in-plane direction, away from the gantry, in the simulations. This was traced to an incomplete model of the shielding—specifically, the component holding the primary collimator was smaller in the model than in the TrueBeam. Modifications were made to VirtuaLinac to more closely match the engineering drawings. In the in-plane direction, the lowest out of field dose was away from gantry (negative abscissa values) at around 60 cm from isocenter, for fields smaller than 10×10 cm2. Out of field dose decreased with decreasing jaw size. Flattening-filter free beam produced out-of-field doses as low as 65% of those with flattened beam. Conclusion: Doses determined from simulation and measurement were in close agreement. Funding support from the Jefferson Trust Foundation.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2010

Dose calculations using MARS for Bremsstrahlung beam stops and collimators in APS beamline stations.

The Monte Carlo radiation transport code MARS is used to model the generation of gas bremsstrahlung (GB) radiation from 7-GeV electrons which scatter from residual gas atoms in undulator straight sections within the Advanced Photon Source (APS) storage ring. Additionally, MARS is employed to model the interactions of the GB radiation with components along the x-ray beamlines and then determine the expected radiation dose-rates that result. In this manner, MARS can be used to assess the adequacy of existing shielding or the specifications for new shielding when required. The GB radiation generated in the 'thin-target' of an ID straight section will consist only of photons in a 1/E-distribution up to the full energy of the stored electron beam. Using this analytical model, the predicted GB power for a typical APS 15.38-m insertion device (ID) straight section is 4.59 x 10{sup -7} W/nTorr/mA, assuming a background gas composed of air (Z{sub eff} = 7.31) at room temperature (293K). The total GB power provides a useful benchmark for comparisons between analytical and numerical approaches. We find good agreement between MARS and analytical estimates for total GB power. The extended straight section 'target' creates a radial profile of GB, which is highly peaked centered on the electron beam. The GB distribution reflects the size of the electron beam that creates the radiation. Optimizing the performance of MARS in terms of CPU time per incident trajectory requires the use of a relatively short, high-density gas target (air); in this report, the target density is {rho}L = 2.89 x 10{sup -2} g/cm{sup 2} over a length of 24 cm. MARS results are compared with the contact dose levels reported in TB-20, which used EGS4 for radiation transport simulations. Maximum dose-rates in 1 cc of tissue phantom form the initial basis for comparison. MARS and EGS4 results are approximately the same for maximum 1-cc dose-rates and attenuation in the photon-dominated regions; for thicker targets, however, the dose-rate no longer depends only on photon attenuation, as photoneutrons (PNs) begin to dominate. The GB radiation-induced photoneutron measurements from four different metals (Fe, Cu, W, and Pb) are compared with MARS predictions. The simulated dose-rates for beamline 6-ID are approximately 3-5 times larger than the measured values, whereas those for beamline 11-ID are much closer. Given the uncertainty in local values of pressure and Z, the degree of agreement between MARS and the PN measurements is good. MARS simulations of GB-induced radiation in and around the FOE show the importance of using actual pressure and gas composition (Z{sub eff}) to obtain accurate PN dose. For a beam current of 300 mA, extrapolating pressure data measured in previously published studies predicts an average background gas pressure of 27 nTorr. An average atomic number of Z{sub eff} = 4.0 is obtained from the same studies. In addition, models of copper masks presently in use at the APS are included. Simulations show that inclusion of exit masks make significant differences in both the radiation spatial distribution within the FOE, as well as the peak intensity. Two studies have been conducted with MARS to assess shielding requirements. First, dose levels in contact with the outside wall of the FOE are examined when GB radiation strikes Pb or W beam stops of varying transverse size within the FOE. Four separate phantom regions are utilized to measure the dose, two at beam elevation and two at the horizontal beam position. The first two phantoms are used for scoring FOE dose along the outside and back walls, horizontally; the second two collect dose on the roof and vertically on the back wall. In all cases, the beam stop depth is maintained at 30 cm. Inclusion of front end (FE) exit masks typically cause a 1-2 order-of-magnitude increase in the dose-rates relative to the case with no masks. Masks place secondary bremsstrahlung sources inside the FOE, and therefore they must be shielded appropriately. The MARS model does not fully account for all shielding present in the hutches; localized shielding is employed in individual hutches. Typically, a collimator, placed downstream of the FE exit masks, mitigates the possible increase in dose. Regarding beam stop transverse size, a modest reduction in dose on the back wall is noted as the stop dimension (square cross section) is increased from 12 cm to 24 cm. In the second study, the thickness of Pb required to shield against the GB extremal ray is determined. In this study, we are interested in finding the thickness of material necessary to add at the edge of a stop to adequately block GB radiation; therefore, we look at the case of no masks in order to have a well-defined GB beam edge. Simulations show the separation between the extremal ray and the edge of the shielding should be 2R{sub m}, where R{sub m} is the Moliere radius.

43 PARTICLE ACCELERATORS↗
DOE OSTI2014

Design of an Yb-169 source optimized for gold nanoparticle-aided radiation therapy

Purpose: To find an optimum design of a new high-dose rate ytterbium (Yb)-169 brachytherapy source that would maximize the dose enhancement during gold nanoparticle-aided radiation therapy (GNRT), while meeting practical constraints for manufacturing a clinically relevant brachytherapy source. Methods: Four different Yb-169 source designs were considered in this investigation. The first three source models had a single encapsulation made of one of the following materials: aluminum, titanium, and stainless steel. The last source model adopted a dual encapsulation design with an inner aluminum capsule surrounding the Yb-core and an outer titanium capsule. Monte Carlo (MC) simulations using the Monte Carlo N-Particle code version 5 (MCNP5) were conducted initially to investigate the spectral changes caused by these four source designs and the associated variations in macroscopic dose enhancement across the tumor loaded with gold nanoparticles (GNPs) at 0.7% by weight. Subsequent MC simulations were performed using the EGSnrc and NOREC codes to determine the secondary electron spectra and microscopic dose enhancement as a result of irradiating the GNP-loaded tumor with the MCNP-calculated source spectra. Results: Effects of the source filter design were apparent in the current MC results. The intensity-weighted average energy of the Yb-169 source varied from 108.9 to 122.9 keV, as the source encapsulation material changed from aluminum to stainless steel. Accordingly, the macroscopic dose enhancement calculated at 1 cm away from the source changed from 51.0% to 45.3%. The sources encapsulated by titanium and aluminum/titanium combination showed similar levels of dose enhancement, 49.3% at 1 cm, and average energies of 113.0 and 112.3 keV, respectively. While the secondary electron spectra due to the investigated source designs appeared to look similar in general, some differences were noted especially in the low energy region (<50 keV) of the spectra suggesting the dependence of the photoelectron yield on the atomic number of source filter material, consistent with the macroscopic dose enhancement results. A similar trend was also shown in the so-called microscopic dose enhancement factor, for example, resulting in the maximum values of 138 and 119 for the titanium- and the stainless steel-encapsulated Yb-169 sources, respectively. Conclusions: The current results consistently show that the dose enhancement achievable from the Yb-169 source is closely related with the atomic number (Z) of source encapsulation material. While the observed range of improvement in the dose enhancement may be considered moderate after factoring all uncertainties in the MC results, the current study provides a reasonable support for the encapsulation of the Yb-core with lower-Z materials than stainless steel, for GNRT applications. Overall, the titanium capsule design can be favored over the aluminum or dual aluminum/titanium capsule designs, due to its superior structural integrity and improved safety during manufacturing and clinical use.

07 ISOTOPE AND RADIATION SOURCES↗