Monte Carlo Calculated Dose Distributions Research Articles

Monte Carlo investigation of dose distribution of uniformly and non-uniformly loaded standard and notched eye plaques.

To investigate the effect of using non-uniform loading and notched plaques on dose distribution for eye plaques. Using EGSnrc Monte Carlo (MC) simulations, we investigate eye plaque dose distributions in water and in an anatomically representative eye phantom. Simulations were performed in accordance with TG43 formalism and compared against full MC simulations which account for inter-seed and inhomogeneity effects. For standard plaque configurations, uniformly and non-uniformly loaded plaque dose distributions in water showed virtually no difference between each other. For standard plaque, the MC calculated dose distribution in planes parallel to the plaque is narrower than the TG43 calculation due to attenuation at the periphery of the plaque by the modulay. MC calculated the dose behind the plaque is fully attenuated. Similar results were found for the notched plaque, with asymmetric attenuation along the plane of the notch. Cumulative dose volume histograms showed significant reductions in the calculated MC doses for both tumor and eye structures, compared to TG43 calculations. The effect was most pronounced for the notch plaque where the MC dose to the optic nerve was greatly attenuated by the modulay surrounding the optic nerve compared to the TG43. Thus, a reduction of optic nerve D95% from 14 to 0.2Gy was observed, when comparing the TG43 calculation to the MC result. The tumor D95% reduced from 89.2 to 79.95Gy for TG43 and MC calculations, respectively. TG43 calculations overestimate the absolute dose and the lateral dose distribution of both standard and notched eye plaques, leading to the dose overestimation for the target and organs at risk. The dose matching along the central axis for the non-uniformly loaded plaques to that of uniformly loaded ones was found to be sufficient for providing comparable coverage and can be clinically used in eye-cancer-busy centers.

P-232 - EGSnrc Monte Carlo calculated dose distribution around brachytherapy sources

P-232 - EGSnrc Monte Carlo calculated dose distribution around brachytherapy sources

Development of an extended Macro Monte Carlo method for efficient and accurate dose calculation in magnetic fields.

Progress in the field of magnetic resonance (MR)-guided radiotherapy has triggered the need for fast and accurate dose calculation in presence of magnetic fields. The aim of this work is to satisfy this need by extending the macro Monte Carlo (MMC) method to enable dose calculation for photon, electron, and proton beams in a magnetic field. The MMC method is based on the transport of particles in macroscopic steps through an absorber by sampling the relevant physical quantities from a precalculated database containing probability distribution functions. To enable MMC particle transport in a magnetic field, a transformation accounting for the Lorentz force is applied for each macro step by rotating the sampled position and direction around the magnetic field vector. The transformed position and direction distributions on local geometries are validated against full MC for electron and proton pencil beams. To enable photon dose calculation, an in-house MC algorithm is used for photon transport and interaction. Emerging secondary charged particles are passed to MMC for transport and energy deposition. The extended MMC dose calculation accuracy and efficiency is assessed by comparison with EGSnrc (photon and electron beams) and Geant4 (proton beam) calculated dose distributions of different energies and homogeneous magnetic fields for broad beams impinging on water phantoms with bone and lung inhomogeneities. The geometric transformation on the local geometries is able to reproduce the results of full MC for all investigated settings (difference in mean value and standard deviation <1%). Macro Monte Carlo calculated dose distributions in a homogeneous magnetic field are in agreement with EGSnrc and Geant4, respectively, with gamma passing rates >99.6% (global 2%, 2mm and 10% threshold criteria) for all situations. MMC achieves a substantial efficiency gain of up to a factor of 21 (photon beam), 66 (electron beam), and 356 (proton beam) compared to EGSnrc or Geant4. Efficient and accurate dose calculation in magnetic fields was successfully enabled by utilizing the developed extended MMC transport method for photon, electron, and proton beams.

Comparison of Measured and Monte Carlo Calculated Dose Distributions from Indigenously Developed 6 MV Flattening Filter Free Medical Linear Accelerator.

Purpose:Monte Carlo simulation was carried out for a 6 MV flattening filter-free (FFF) indigenously developed linear accelerator (linac) using the BEAMnrc user-code of the EGSnrc code system. The model was benchmarked against the measurements. A Gaussian distributed electron beam of kinetic energy 6.2 MeV with full-width half maximum of 1 mm was used in this study.Methods:The simulation of indigenously developed linac unit has been carried out by using the Monte Carlo-based BEAMnrc user-code of the EGSnrc code system. Using the simulated model, depth and lateral dose profiles were studied using the DOSXYZnrc user-code. The calculated dose data were compared against the measurements using an RFA dosimertic system made by PTW, Germany (water tank MP3-M and 0.125 cm3 ion chamber).Results:The BEAMDP code was used to analyze photon fluence spectra, mean energy distribution, and electron contamination fluence spectra. Percentage depth dose (PDD) and beam profiles (along both X and Y directions) were calculated for the field sizes 5 cm × 5 cm - 25 cm × 25 cm. The dose difference between the calculated and measured PDD and profile values were under 1%, except for the penumbra region where the maximum deviation was found to be around 3%.Conclusions:A Monte Carlo model of indigenous FFF linac (6 MV) has been developed and benchmarked against the measured data.

A general mechanistic model enables predictions of the biological effectiveness of different qualities of radiation

Predicting the responses of biological systems to ionising radiation is extremely challenging, particularly when comparing X-rays and heavy charged particles, due to the uncertainty in their Relative Biological Effectiveness (RBE). Here we assess the power of a novel mechanistic model of DNA damage repair to predict the sensitivity of cells to X-ray, proton or carbon ion exposures in vitro against over 800 published experiments. By specifying the phenotypic characteristics of cells, the model was able to effectively stratify X-ray radiosensitivity (R2 = 0.74) without the use of any cell-specific fitting parameters. This model was extended to charged particle exposures by integrating Monte Carlo calculated dose distributions, and successfully fit to cellular proton radiosensitivity using a single dose-related parameter (R2 = 0.66). Using these parameters, the model was also shown to be predictive of carbon ion RBE (R2 = 0.77). This model can effectively predict cellular sensitivity to a range of radiations, and has the potential to support developments of personalised radiotherapy independent of radiation type.

SU‐F‐T‐28: Evaluation of BEBIG HDR Co‐60 After‐Loading System for Skin Cancer Treatment Using Conical Surface Applicator

Purpose:To evaluate the possibility of utilizing the BEBIG HDR 60Co remote after‐loading system for malignant skin surface treatment using Monte Carlo (MC) simulation technique.Methods:First TG‐43 parameters of BEBIG‐Co‐60 and Nucletron Ir‐192‐mHDR‐V2 brachytherapy sources were simulated using MCNP6 code to benchmark the sources against the literature. Second a conical tungsten‐alloy with 3‐cm diameter of Planning‐Target‐Volume (PTV) at surface for use with a single stepping HDR source is designed. The HDR source is modeled parallel to treatment plane at the center of the conical applicator with a source surface distance (SSD) of 1.5‐cm and a removable plastic end‐cap with a 1‐mm thickness. Third, MC calculated dose distributions from HDR Co‐60 for conical surface applicator were compared with the simulated data using HDR Ir‐192 source. The initial calculations were made with the same conical surface applicator (standard‐applicator) dimensions as the ones used with the Ir‐192 system. Fourth, the applicator wall‐thickness for the Co‐60 system was increased (doubled) to diminish leakage dose to levels received when using the Ir‐192 system. With this geometry, percentage depth dose (PDD), and relative 2D‐dose profiles in transverse/coronal planes were normalized at 3‐mm prescription‐depth evaluated along the central axis.Results:PDD for Ir‐192 and Co‐60 were similar with standard and thick‐walled applicator. 2D‐relative dose distribution of Co‐60, inside the standard‐conical‐applicator, generated higher penumbra (7.6%). For thick‐walled applicator, it created smaller penumbra (&lt;4%) compared to Ir‐192 source in the standard‐conicalapplicator. Dose leakage outside of thick‐walled applicator with Co‐60 source was approximately equal (≤3%) with standard applicator using Ir‐192 source.Conclusion:Skin cancer treatment with equal quality can be performed with Co‐60 source and thick‐walled conical applicators instead of Ir‐192 with standard applicators. These conical surface applicator must be used with a protective plastic end‐cap to eliminate electron contamination and over‐dosage of the skin.

Monte Carlo evaluation of tissue heterogeneities corrections in the treatment of head and neck cancer patients using stereotactic radiotherapy.

The purpose of this study was to generate Monte Carlo computed dose distributions with the X‐ray voxel Monte Carlo (XVMC) algorithm in the treatment of head and neck cancer patients using stereotactic radiotherapy (SRT) and compare to heterogeneity corrected pencil‐beam (PB‐hete) algorithm. This study includes 10 head and neck cancer patients who underwent SRT re‐irradiation using heterogeneity corrected pencil‐beam (PB‐hete) algorithm for dose calculation. Prescription dose was 24‐40 Gy in 3‐5 fractions (treated 3‐5 fractions per week) with at least 95% of the PTV volume receiving 100% of the prescription dose. A stereotactic head and neck localization box was attached to the base of the thermoplastic mask fixation for target localization. The gross tumor volume (GTV) and organs‐at‐risk (OARs) were contoured on the 3D CT images. The planning target volume (PTV) was generated from the GTV with 0 to 5 mm uniform expansion; PTV ranged from 10.2 to 64.3 cc (average=35.0±17.5 cc). OARs were contoured on the 3D planning CT and consisted of spinal cord, brainstem, optic structures, parotids, and skin. In the BrainLab treatment planning system (TPS), clinically optimal SRT plans were generated using hybrid planning technique (combination of 3D conformal noncoplanar arcs and nonopposing static beams) for the Novalis‐Tx linear accelerator consisting of high‐definition multileaf collimators (HD‐MLCs: 2.5 mm leaf width at isocenter) and 6 MV‐SRS (1000 MU/min) beam. For the purposes of this study, treatment plans were recomputed using XVMC algorithm utilizing identical beam geometry, multileaf positions, and monitor units and compared to the corresponding clinical PB‐hete plans. The Monte Carlo calculated dose distributions show small decreases (<1.5%) in calculated dose for D99,Dmean, and Dmax of the PTV coverage between the two algorithms. However, the average target volume encompassed by the prescribed percent dose (Vp) was about 2.5% less with XVMC vs. PB‐hete and ranged between ‐0.1 and 7.8%. The averages for D100 and D10 of the GTV were lower by about 2% and ranged between ‐0.8 and 3.1%. For the spinal cord, both the maximal dose difference and the dose to 0.35 cc of the structure were higher by an average of 4.2% (ranged 1.2 to −13.6%) and 1.4% (ranged 7.5 to −11.3%), respectively, with XVMC calculation. For the brainstem, the maximal dose differences and the dose to 0.5 cc of the structure were, on average, higher by 2.4% (ranged 6.4 to −8.0%) and 3.6% (ranged 6.4 to −9.0%), respectively. For the parotids, both the mean dose and the dose to 20 cc of parotids were higher by an average of 3% (ranged ‐0.2 to −5.9%) and 4% (ranged ‐0.2 to ‐8%), respectively, with XVMC calculation. For the optic apparatus, results from both algorithms were similar. However, the mean dose to skin was 3% higher (ranged 0 to ‐6%), on average, with XVMC compared to PB‐hete, although the maximum dose to skin was 2% lower (ranged −5% to 15.5%). The results from our XVMC dose calculations for head and neck SRT patients indicate small to moderate underdosing of the tumor volume when compared to PB‐hete calculation. However, Vp was up to 7.8% less for the lower‐neck patient with XVMC. Critical structures, such as spinal cord, brainstem, or parotids, could potentially receive higher doses when using XVMC algorithm. Given the proximity to critical structures and the smaller volumes treated with SRT in the region of the head and neck, the differences between XVMC and PB‐hete calculation methods may be of clinical interest.PACS number(s): 87.55.K‐

Evaluation of BEBIG HDR (60)Co system for non-invasive image-guided breast brachytherapy.

PurposeHDR 60Co system has recently been developed and utilized for brachytherapy in many countries outside of the U.S. as an alternative to 192Ir. In addition, the AccuBoost® technique has been demonstrated to be a successful non-invasive image-guided breast brachytherapy treatment option. The goal of this project is to evaluate the possibility of utilizing the BEBIG HDR 60Co system for AccuBoost treatment. These evaluations are performed with Monte Carlo (MC) simulation technique.Material and methodsIn this project, the MC calculated dose distributions from HDR 60Co for various breast sizes have been compared with the simulated data using an HDR 192Ir source. These calculations were performed using the MCNP5 code. The initial calculations were made with the same applicator dimensions as the ones used with the HDR 192Ir system (referred here after as standard applicator). The activity of the 60Co source was selected such that the dose at the center of the breast would be the same as the values from the 192Ir source. Then, the applicator wall-thickness for the HDR 60Co system was increased to diminish skin dose to levels received when using the HDR 192Ir system. With this geometry, dose values to the chest wall and the skin were evaluated. Finally, the impact of a conical attenuator with the modified applicator for the HDR 60Co system was analyzed.ResultsThese investigations demonstrated that loading the 60Co sources inside the thick-walled applicators created similar dose distributions to those of the 192Ir source in the standard applicators. However, dose to the chest wall and breast skin with 60Co source was reduced using the thick-walled applicators relative to the standard applicators. The applicators with conical attenuator reduced the skin dose for both source types.ConclusionsThe AccuBoost treatment can be performed with the 60Co source and thick-wall applicators instead of 192Ir with standard applicators.

SU-E-T-85: Comparison of Treatment Plans Calculated Using Ray Tracing and Monte Carlo Algorithms for Lung Cancer Patients Having Undergone Radiotherapy with Cyberknife

Purpose: The latest publications indicate that the Ray Tracing algorithm significantly overestimates the dose delivered as compared to the Monte Carlo (MC) algorithm. The purpose of this study is to quantify this overestimation and to identify significant correlations between the RT and MC calculated dose distributions. Methods: Preliminary results are based on 50 preexisting RT algorithm dose optimization and calculation treatment plans prepared on the Multiplan treatment planning system (Accuray Inc., Sunnyvale, CA). The analysis will be expanded to include 100 plans. These plans are recalculated using the MC algorithm, with high resolution and 1% uncertainty. The geometry and number of beams for a given plan, as well as the number of monitor units, is constant for the calculations for both algorithms and normalized differences are compared. Results: MC calculated doses were significantly smaller than RT doses. The D95 of the PTV was 27% lower for the MC calculation. The GTV and PTV mean coverage were 13 and 39% less for MC calculation. The first parameter of conformality, as defined as the ratio of the Prescription Isodose Volume to the PTV Volume was on average 1.18 for RT and 0.62 for MC. Maximum doses delivered to OARs was reduced in the MC plans. The doses for 1000 and 1500 cc of total lung minus PTV, respectively were reduced by 39% and 53% for the MC plans. The correlation of the ratio of air in PTV to the PTV with the difference in PTV coverage had a coefficient of −0.54. Conclusion: The preliminary results confirm that the RT algorithm significantly overestimates the dosages delivered confirming previous analyses. Finally, subdividing the data into different size regimes increased the correlation for the smaller size PTVs indicating the MC algorithm improvement verses the RT algorithm is dependent upon the size of the PTV.

An optically stimulated luminescence dosimeter for measuring patient exposure from imaging guidance procedures

There is a growing interest in patient exposure resulting from an x-ray imaging procedure used in image-guided radiation therapy. This study explores a feasibility to use a commercially available optically stimulated luminescence (OSL) dosimeter, nanoDot, for estimating imaging radiation exposure to patients. The kilovoltage x-ray sources used for kV-cone-beam CT (CBCT) imaging acquisition procedures were from a Varian on-board imager (OBI) image system. An ionization chamber was used to determine the energy response of nanoDot dosimeters. The chamber calibration factors for x-ray beam quality specified by half-value layer were obtained from an Accredited Dosimetry Calibration Laboratory. The Monte Carlo calculated dose distributions were used to validate the dose distributions measured by using the nanoDot dosimeters in phantom and in vivo. The range of the energy correction factors for the nanoDot as a function of photon energy and bow-tie filters was found to be 0.88–1.13 for different kVp and bow-tie filters. Measurement uncertainties of nanoDot were approximately 2–4% after applying the energy correction factors. The tests of nanoDot placed on a RANDO phantom and on patient's skin showed consistent results. The nanoDot is suitable dosimeter for in vivo dosimetry due to its small size and manageable energy dependence. The dosimeter placed on a patient's skin has potential to serve as an experimental method to monitor and to estimate patient exposure resulting from a kilovoltage x-ray imaging procedure. Due to its large variation in energy response, nanoDot is not suitable to measure radiation doses resulting from mixed beams of megavoltage therapeutic and kilovoltage imaging radiations.

SU‐E‐T‐584: Evaluation of Collapsed Cone Dose Calculation Algorithm for HDR Brachytherapy Using Monte Carlo

Purpose: In clinical routine, dose calculation for HDR brachytherapy plans is usually based on the AAPM‐TG43 formalism. Because the patient's anatomy is not taken into account, the resulting dose calculation may be inaccurate. Monte Carlo (MC) methods are able to calculate dose distributions accurately, but due to long computation times they are mainly used for benchmarking purposes. In this work, a test version of the collapsed cone (CC) algorithm from Nucletron was evaluated using MC. Methods: The Nucletron brachytherapy source micro Selectron‐HDR v2 has been implemented in the MC environment. CC and MC calculated dose distributions for a single source placed in a water phantom are validated against published AAPM‐TG43 data. Furthermore, dose distributions for clinical breast cancer patient treatment plans are calculated with CC and MC. Primary and scatter dose components are separated and compared individually. Results: The local differences between the MC calculated dose distributions in the water phantom and the AAPM‐TG43 data are less than 1%, while the differences between MC and CC for distances &gt; 2 cm from the source are up to 2% of the reference dose (dose at 1 cm from the source in its equatorial plane). These differences are assigned to the first and residual scatter dose components. For the clinical breast cancer patients, CC underestimates the MC dose up to 2% in the high‐dose region (&gt; 100% of the prescribed dose). This deviation is correlated with the scan‐length of the patient CT data set. If the CT data set extends the PTV by an appropriate amount, the underestimation falls below 1%. Conclusion: The CC algorithm is an accurate dose calculation algorithm and improves the dose calculation accuracy in brachytherapy compared to AAPM‐TG43 based algorithms. This work was supported by Nucletron. This work was supported by Nucletron.

Patient-Specific 3D Pretreatment and Potential 3D Online Dose Verification of Monte Carlo–Calculated IMRT Prostate Treatment Plans

Fast and reliable comprehensive quality assurance tools are required to validate the safety and accuracy of complex intensity-modulated radiotherapy (IMRT) plans for prostate treatment. In this study, we evaluated the performance of the COMPASS system for both off-line and potential online procedures for the verification of IMRT treatment plans. COMPASS has a dedicated beam model and dose engine, it can reconstruct three-dimensional dose distributions on the patient anatomy based on measured fluences using either the MatriXX two-dimensional (2D) array (offline) or a 2D transmission detector (T2D) (online). For benchmarking the COMPASS dose calculation, various dose-volume indices were compared against Monte Carlo-calculated dose distributions for five prostate patient treatment plans. Gamma index evaluation and absolute point dose measurements were also performed in an inhomogeneous pelvis phantom using extended dose range films and ion chamber for five additional treatment plans. MatriXX-based dose reconstruction showed excellent agreement with the ion chamber (<0.5%, except for one treatment plan, which showed 1.5%), film (∼100% pixels passing gamma criteria 3%/3 mm) and mean dose-volume indices (<2%). The T2D based dose reconstruction showed good agreement as well with ion chamber (<2%), film (∼99% pixels passing gamma criteria 3%/3 mm), and mean dose-volume indices (<5.5%). The COMPASS system qualifies for routine prostate IMRT pretreatment verification with the MatriXX detector and has the potential for on-line verification of treatment delivery using T2D.

TU‐C‐BRA‐04: A New Approach to Model‐Based Dose Calculations for Kilovoltage Beams

Purpose: The additional radiation dose delivered to radiotherapy patients from repeated image guidance procedures is a growing concern. This study presents a new approach to model‐based dose calculation that overcomes the deficiencies of currently available model‐based algorithms in the kilovoltage regime. Method and Materials: Monte Carlo techniques were used to calculate the dose to patients resulting from kV‐CBCT scans for multiple scan sites including the head‐and‐neck, chest, and pelvis. Both dose‐to‐medium and dose‐to‐water were calculated using realistic simulated kV‐CBCT beams to find a correlation between the effective thickness of bone (dEB) traversed by an x‐ray beam to reach a voxel and the correction factor needed to correct the model‐based dose calculation. A program was written to facilitate the calculation of dEB in which the incident beam profiles and isocenter relative to a scanned patient were taken into account. Results: A strong correlation between dose correction factor and dEB was observed. A unique correction factor calibration curve as a function of dEB was derived and used to predict patient dose. Compared to gold standard Monte Carlo calculated dose distributions, the resulting mean dose error for each patient was less than 3% for bone and 2% for soft‐tissue. This was in contrast to model‐based calculations, which resulted in mean errors of up to −103% for bone and 8% for soft‐tissue. Conclusion: The derived correction factor as a function of effective bone thickness is capable of accurately calculating the dose to bone and soft‐tissue by making use of patient CT data and the incident kV‐CBCT beam information. The accuracy of this new approach is patient and scan site independent because it takes into account the spectral changes of the x‐ray beam. With the addition of a commissioned kV‐CBCT beam this new approach can facilitate inclusion of imaging dose in radiotherapy treatment planning.

TH‐C‐BRB‐03: On the Suitability of Using NanoDot for Measuring Dose Distributions Resulting from KV‐CBCT Acquisitions

Purpose: There is a growing need to determine radiation exposure and dose distributions resulting from an image‐guidance procedure. Although Monte Carlo simulations are capable to calculate the dose distributions experimental measurements remain the gold standards to validate the accuracy of calculations. This study investigated the suitability of nanoDot dosimeters for measuring accumulative dose at several locations simultaneously to determine 3D dose distributions resulting from a kilovoltage cone‐beam CT (kV‐CBCT) scan. Method and Materials: The nanoDot dosimeters read with a microStar reader by Landauer were used in the study. The system can be calibrated to measure dose from kilovoltage to megavoltage energy beams. The manufacturer provides a set of calibration dosimeters for 80 kVp x‐rays. The accuracy of the nanoDot dosimeters for absorbed dose measurements for kV‐CBCT scans was validated by using an ionization chamber in which the air‐kerma calibration factors were traceable to National Standards Laboratory. The relative dose distributions measured with nanoDot were compared with Monte Carlo calculated dose distributions in a PLASTIC WATER® LR phantom. Results: Seven nanoDot dosimeters were placed at different locations to determine dose distributions resulting from each kV‐CBCT scan protocol. There are five different kV‐CBCT scan protocols in the Varian OBI 1.4 system including Head Low‐dose Thorax Pelvis and Pelvis Spot Light with two different Bow‐tie filters. Both absolute and relative dose distributions were investigated. The dose values measured from 35 points generally agree with that of Monte Carlo calculations within 2–4%. Conclusion: The nanoDot dosimeters have been shown to be very suitable detectors for measuring 3D dose distributions resulting from kV‐CBCT scans due to its small size the ease of use good reproducibility and manageable photon energy dependence.Conflict of Interest: The microStar reader used in the research was provided by Landauer Inc. Glenwood IL

SU-GG-T-238: In-Silico Verification of Rapid Arc Using Swiss Monte Carlo Plan

Purpose: Nowadays, patient specific quality assurance (QA) for RapidArc treatments is performed by pre‐treatment verification based on measured dose distribution within phantoms. This procedure is well‐established in clinical practice for IMRT and IMAT verification. However, the procedure is in principle a mixture of machine and patient specific QA and lacks of the fact that linac treatment time is needed. We believe that instead of performing the patient specific QA on the machine the QA could be realized using computers, which is referred to as in‐silico verification of RapidArd treatments. The aim of this work is to use the Monte Carlo(MC) method as an in‐silico verification of RapidARc treatments. Method and Materials: We extended and modified our in‐house developed Swiss Monte Carlo Plan (SMCP) such that dose distributions for RapidArc treatments on the Novalis TX can be calculated. Several methods were developed in order to calculate dose distributions within reasonable computing time. The validation of this approach is performed by comparing MC calculated dose distributions with measured dose distributions for open as well as RapidArc treatment fields. The usefulness of this verification approach is shown by comparing MC calculated dose distributions within the patient with corresponding dose distributions using Eclipse (Varian Medical Systems). Results: The measured and SMCP calculated dose distributions are in good agreement for open as well as for RapidArc treatment fields. In general, the agreement between SMCP and AAA calculated dose distributions is also good. However, due to the presence of inhomogeneities in the patient anatomy, the AAA typically overestimates the mean PTV dose and underestimates the dose for organs‐at‐risk. Conclusion: In this work, SMCP was used for in‐silico verification of clinical RApidArc treatment fields. The results demonstrate the usefulness and the efficiency of this approach. Conflict of Interest: This work was supported by Varian Medical Systems.

Fast, accurate photon beam accelerator modeling using BEAMnrc: A systematic investigation of efficiency enhancing methods and cross‐section data

In this work, an investigation of efficiency enhancing methods and cross-section data in the BEAMnrc Monte Carlo (MC) code system is presented. Additionally, BEAMnrc was compared with VMC++, another special-purpose MC code system that has recently been enhanced for the simulation of the entire treatment head. BEAMnrc and VMC++ were used to simulate a 6 MV photon beam from a Siemens Primus linear accelerator (linac) and phase space (PHSP) files were generated at 100 cm source-to-surface distance for the 10 x 10 and 40 x 40 cm2 field sizes. The BEAMnrc parameters/techniques under investigation were grouped by (i) photon and bremsstrahlung cross sections, (ii) approximate efficiency improving techniques (AEITs), (iii) variance reduction techniques (VRTs), and (iv) a VRT (bremsstrahlung photon splitting) in combination with an AEIT (charged particle range rejection). The BEAMnrc PHSP file obtained without the efficiency enhancing techniques under study or, when not possible, with their default values (e.g., EXACT algorithm for the boundary crossing algorithm) and with the default cross-section data (PEGS4 and Bethe-Heitler) was used as the "base line" for accuracy verification of the PHSP files generated from the different groups described previously. Subsequently, a selection of the PHSP files was used as input for DOSXYZnrc-based water phantom dose calculations, which were verified against measurements. The performance of the different VRTs and AEITs available in BEAMnrc and of VMC++ was specified by the relative efficiency, i.e., by the efficiency of the MC simulation relative to that of the BEAMnrc base-line calculation. The highest relative efficiencies were approximately 935 (approximately 111 min on a single 2.6 GHz processor) and approximately 200 (approximately 45 min on a single processor) for the 10 x 10 field size with 50 million histories and 40 x 40 cm2 field size with 100 million histories, respectively, using the VRT directional bremsstrahlung splitting (DBS) with no electron splitting. When DBS was used with electron splitting and combined with augmented charged particle range rejection, a technique recently introduced in BEAMnrc, relative efficiencies were approximately 420 (approximately 253 min on a single processor) and approximately 175 (approximately 58 min on a single processor) for the 10 x 10 and 40 x 40 cm2 field sizes, respectively. Calculations of the Siemens Primus treatment head with VMC++ produced relative efficiencies of approximately 1400 (approximately 6 min on a single processor) and approximately 60 (approximately 4 min on a single processor) for the 10 x 10 and 40 x 40 cm2 field sizes, respectively. BEAMnrc PHSP calculations with DBS alone or DBS in combination with charged particle range rejection were more efficient than the other efficiency enhancing techniques used. Using VMC++, accurate simulations of the entire linac treatment head were performed within minutes on a single processor. Noteworthy differences (+/- 1%-3%) in the mean energy, planar fluence, and angular and spectral distributions were observed with the NIST bremsstrahlung cross sections compared with those of Bethe-Heitler (BEAMnrc default bremsstrahlung cross section). However, MC calculated dose distributions in water phantoms (using combinations of VRTs/AEITs and cross-section data) agreed within 2% of measurements. Furthermore, MC calculated dose distributions in a simulated water/air/water phantom, using NIST cross sections, were within 2% agreement with the BEAMnrc Bethe-Heitler default case.

Kilovoltage beam Monte Carlo dose calculations in submillimeter voxels for small animal radiotherapy

Small animal conformal radiotherapy (RT) is essential for preclinical cancer research studies and therefore various microRT systems have been recently designed. The aim of this paper is to efficiently calculate the dose delivered using our microRT system based on a microCT scanner with the Monte Carlo (MC) method and to compare the MC calculations to film measurements. Doses from 2-30 mm diameter 120 kVp photon beams deposited in a solid water phantom with 0.2 x 0.2 x 0.2 mm3 voxels are calculated using the latest versions of the EGSnrc codes BEAMNRC and DOSXYZNRC. Two dose calculation approaches are studied: a two-step approach using phase-space files and direct dose calculation with BEAMNRC simulation sources. Due to the small beam size and submillimeter voxel size resulting in long calculation times, variance reduction techniques are studied. The optimum bremsstrahlung splitting number (NBRSPL in BEAMNRC) and the optimum DOSXYZNRC photon splitting (Nsplit) number are examined for both calculation approaches and various beam sizes. The dose calculation efficiencies and the required number of histories to achieve 1% statistical uncertainty--with no particle recycling--are evaluated for 2-30 mm beams. As a final step, film dose measurements are compared to MC calculated dose distributions. The optimum NBRSPL is approximately 1 x 10(6) for both dose calculation approaches. For the dose calculations with phase-space files, Nsplit varies only slightly for 2-30 mm beams and is established to be 300. Nsplit for the DOSXYZNRC calculation with the BEAMNRC source ranges from 300 for the 30 mm beam to 4000 for the 2 mm beam. The calculation time significantly increases for small beam sizes when the BEAMNRC simulation source is used compared to the simulations with phase-space files. For the 2 and 30 mm beams, the dose calculations with phase-space files are more efficient than the dose calculations with BEAMNRC sources by factors of 54 and 1.6, respectively. The dose calculation efficiencies converge for beams with diameters larger than 30 mm. A very good agreement of MC calculated dose distributions to film measurements is found. The mean difference of percentage depth dose curves between calculated and measured data for 2, 5, 10, and 20 mm beams is 1.8%.

A parameter study of pencil beam proton dose distributions for the treatment of ocular melanoma utilizing spot scanning

The results of Monte Carlo calculated dose distributions of proton treatment of ocular melanoma are presented. An efficient spot scanning method utilizing active energy modulation, which also minimizes the number of target spots was developed. We simulated various parameter values for the particle energy spread and the pencil beam diameter in order to determine values suitable for medical treatment. We found that a 2.5-mm-diameter proton beam with a 5% Gaussian energy spread was suitable for treatment of ocular melanoma while preserving vision for the typical case that we simulated. The energy spectra and the required proton current were also calculated and are reported. The results are intended to serve as a guideline for a new class of low-cost, compact accelerators.

SU‐FF‐T‐405: Dose to Water Versus Dose to Medium in Proton Beam Therapy

Purpose: Dose in radiation therapy is traditionally reported as dose‐to‐water, Dw. Monte Carlo (MC) dose calculations report dose‐to‐medium, Dm. A methodology is needed to convert Dm into Dw for comparing MC and planning system results. This work addressed the following questions for proton therapy: What formalism allows the conversion from Dm to Dw?1. Can we convert MC calculated Dm into Dw retroactively or do we have to convert during particle tracking?2. What is the difference between Dm and Dw when analyzing patient data in terms of dose distributions, dose volume histograms and absolute doses?3. Is there a difference in the predicted beam range between Dm and Dw?Method: We did develop a formalism to convert Dm into Dw for proton beam dose calculations. Three different methods are introduced, an approximate method based on relative stopping power which allows retroactive conversion, a method considering energy dependent relative stopping powers, and a method incorporating nuclear interaction events as well. A total of 33 patient fields were analyzed. MC calculated dose distributions, dose volume histograms and absolute doses to assess the clinical significance of differences between Dm and Dw were compared. Results: We found that the difference between the three conversion methods was within 1% in most cases when analyzing mean doses to contoured structures. Further, we found that the difference between Dm and Dw can be up to 10% for high CT numbers. The difference is clinically insignificant for soft tissues. For proton beams stopping in bony anatomy, the predicted beam range can differ by 2–3 mm when comparing Dm and Dw. Conclusion: For comparison between MC and analytical dose distributions in proton therapy, a dose conversion is required. However, retroactive conversion seems to be sufficiently accurate in most cases, except for the end of range.

SU‐FF‐T‐395: The Effect of Lung Heterogeneity and Respiration On Proton Therapy Treatment Margins

Purpose: A critical parameter in lung proton therapy is the range determination, which is related to the treatment margins required to build the PTV. Factors affecting treatment margins include intrafractional tumor motion, interfractional tumor deformation, and anatomic changes. This work investigates the effect of lung heterogeneity and respiratory motion on proton dose distributions using CT scans at different respiratory phases. Method and Materials: Ten previously treated lung patients were recruited for this study. For each patient 4D CT scans were obtained with 10 respiratory phases. The variation of lung density at different respiratory phases and the effective distance from the patient surface to the back of the target volume were analyzed using specially designed software. Monte Carlo simulations were performed to calculate proton dose distributions with different energies and field sizes. The average lung density for a 1cm beamlet was computed for different target locations and beam orientations to provide useful information for advanced proton therapy treatment planning. The density calculation was performed in lung regions without major heterogeneities. Tumor motion and setup uncertainties were not included in this study. Results: Results showed a density difference between the inhale and the exhale phases of up to 0.25g/cm3 (the lung average density is 0.17g/cm3 for the inhale phase). Taking into account the real lateral size of the lung for each patient, this difference leads to a 1.3cm difference between the two extreme phases. The effects of lung heterogeneity vary based on the target location and beam orientation. Monte Carlo calculated dose distributions showed variable distal edge shifts and SOBP plateau deformation. Conclusion: The effects of lung heterogeneity and respiration vary significantly, which require precise margin determination for individual patients in advanced proton therapy with beam scanning and intensity modulation. Additional margins will be needed for population‐based PTV.