Monte Carlo Dose Calculation Algorithm Research Articles

Influence of multiple brain metastases’ size and number on the quality of SRS – VMAT dose delivery

Stereotactic radiosurgery with volumetric modulated arc therapy (SRS-VMAT) has recently been introduced for treatment of multiple brain metastases with a single isocenter. The technique’s high efficiency is nevertheless dependent of metastatic tumors’ characteristics such as size and number. In this work the impact of the metastases’ size and number on the plan quality indices clinically used for plan evaluation and acceptance is investigated. Fifteen targets with a diameter of 1 cm and average volume of 0.7 cm3 and ten targets with a diameter of 2 cm and average volume of 6.5 cm3 were contoured on an anonymized patient CT dataset, in Monaco (Elekta) treatment planning system. VMAT plans for different target volumes (1 and 2 cm in diameter) and various target numbers (1–15) were generated using four non-coplanar arcs and the Agility (Elekta) linear accelerator (5 mm MLC width) using a Monte Carlo dose calculation algorithm and 1mm dose calculation grid resolution. Conformity index (CI), gradient index (GI) and heterogeneity index (HI) were determined for each target. High quality plans were created for both 1 cm and 2 cm in diameter targets for limited (<6) number of targets per plan. For increased number of irradiated targets (>6) both CI and GI, clinically used for plan evaluation and acceptance, were found to deteriorate.

Abstract ID: 220 Evaluating the effect of dental implants in the treatment of head and neck cancer using Monte Carlo dose calculation

Purpose To quantify and compare the effect of metallic dental implants (MDI) on dose distributions calculated using Collapsed Cone Convolution Superposition (CCCS) algorithm or a Monte Carlo algorithm (with and without correcting for the density of the MDI). Methods Seven previously treated patients to the head and neck region were included in this study. The MDI and the streaking artifacts on the CT images were carefully contoured. For each patient a plan was optimized and calculated using the Pinnacle treatment planning system (TPS). For each patient two dose calculations were performed, a) with the densities of the MDI and CT artifacts overridden (12 g/cc and 1 g/cc respectively) and b) without density overrides. The plans were then exported to the Monaco TPS and recalculated using Monte Carlo dose calculation algorithm. The changes in dose to PTVs and surrounding Regions of Interest (ROIs) were examined between all plans. Results The Monte Carlo dose calculation indicated that PTVs received 6% lower dose than the CCCS algorithm predicted. In some cases, the Monte Carlo algorithm indicated that surrounding ROIs received higher dose (up to a factor of 2). Conclusion Not properly accounting for dental implants can impact both the high dose regions (PTV) and the low dose regions (OAR). This study implies that if MDI and the artifacts are not appropriately contoured and given the correct density, there is potential significant impact on PTV coverage and OAR maximum doses.

Abstract ID: 116 Comparison of beam output factors in MCNP6 and Geant4 based IAEA phase-space files

Monte Carlo (MC) dose calculation algorithms demand an accurate characterization of the radiation beam. At present, three MC-based beam models are commonly used for dose calculation; namely, full MC simulation, virtual source model, and phase space (phsp) files. The first two require detailed information of the LINAC head: information that is not always available from vendors. Therefore, properly validated phase-space data for external beam radiotherapy, available from the IAEA Nuclear Data Services section, remains a valuable alternative for MC beam simulation. The aim of this work is to compare the beam output factors obtained by MC-based phsp simulations in MCNP6 and Geant4. The simulations were performed with the photon mode energies of 6, 10, and 25 MV on the ELEKTA Precise. The simulations are divided into two steps: (1) the development of a method for directly reading the phase-space files provided by the IAEA in MCNP6, and (2) the determination of cut-off values, variation reduction techniques, and field size. Patient-specific beam line devices (blocks, jaws, wedges, etc.) were not simulated. The comparison of the output factors as percentage depth dose (PDD), lateral dose profiles, and dose distributions for different field sizes were computed using Geant4 and MCNP6. Both calculations generate matching PDD for a range of open-field sizes within the statistical uncertainties. Variation in lateral dose profiles varies up to 5% in the 25 MV energy mode. Comparison of dose distributions for an open 10 × 10 field by the gamma evaluation test returns a value of 0.93. Despite the lack of a user-friendly MCNP6 interface, this work shows that MCNP6 is suitable for beam simulation based IAEA-phsp.

Dose Comparison between Proton Pencil Beam and Monte Carlo Dose Calculation Algorithm in Lung Cancer Patients

Proton pencil beam dose calculation algorithms (PBA) have limitations in modeling proton scatter dose and its impact on depth dose properties through heterogeneous tissues present when treating lung cancer patients. Monte Carlo dose calculations (MC) may provide more accurate dose calculations when compared with the current algorithms that are in clinical use. This study investigates clinical pencil beam scanning (PBS) planned dose differences between PB and MC in lung cancer patients. With IRB approval, a comparative dosimetric analysis between RaystationTM Monte Carlo v4.0 (pending FDA 510 clearance) and RaystationTM pencil beam v4.0 dose algorithms was performed on 10 patient plans. PBS gantry plans were generated using single-field optimization technique to maintain target coverage under 3% range and 3 mm setup uncertainty. Dose prescriptions ranged from 60 Gy(RBE) to 66.6 Gy(RBE). Dose differences between PBA and MC were recorded for the following plan metrics: CTV V95, CTV homogeneity index (HI), total lung V20, total lung VRX (relative volume of lung receiving the prescribed dose or higher), and global maximum dose. Non-parametric Wilcoxon signed-rank testing was carried out to evaluate pairwise differences in planned dose and dose-volume parameters between PBA and MC. In this 10 patient sample, CTV V95 ranged from 99% to 100% with a median of 100% for PBA. When recalculated with MC, CTV V95 ranged from 77% to 94% with a median of 90% (p=0.002). MC treatment plans showed a reduction of dose homogeneity within the target volume. The median CTV HI (D95/D5) was 0.98 for PBA and 0.91 for MC (p = 0.002). Total lung V20 distributions were statistically identical to within 1% between dose calculation algorithms (p=1.0). Median lung VRX was 7% and 1% for PBA and MC, respectively (p = 0.002), indicating a reduction in lung volume receiving high dose when using MC. Lastly, the global maximum patient dose increased from a median value of 106% for PBA (range 101-113%) to 109% for MC (range 104-122%, p = 0.002). A retrospective analysis of 10 lung cancer PBS plans revealed a planned dose reduction in coverage to the CTV when recalculated using MC. This suggests that the use of PBA dose calculation in this disease site may lead to underdosing of the target volume and may support the clinical integration of MC dose calculation during treatment planning.Abstract 3654Pencil BeamMonte CarloMedianRangeMedianRangeSign Rank pCTV V95100%99% - 100%90%77% - 94%0.002CTV HI0.980.95 - 1.000.910.87-0.940.002Lung V2025%15% - 35%25%16% - 34%1.000Lung VRX7%2% - 11%1%0% - 6%0.002Max Dose106%101% - 113%109%104% - 122%0.002 Open table in a new tab

Commercial Analytic Dose Calculation Algorithms Dramatically Overpredict Proton Therapy Dose to Thoracic Tumors in Lung Credentialing Phantom

To better understand the poor agreement between calculated and measured doses in the IROC Houston lung phantom. IROC’s anthropomorphic lung phantom is used for proton approval and credentialing for NCI-funded trials. The phantom contains TLD and film to measure the delivered dose. A motor moves the phantom target insert within the thorax, mimicking a human breathing cycle. This phantom provides a consistent QA tool that assesses comparability in dose delivery between participating proton centers. The overall pass rate for this lung phantom is only 63%, and those irradiations that do pass, do so marginally, despite the generous 7%/4mm acceptance criteria. To understand the role of the dose calculation algorithm in this disagreement, five institutions that previously irradiated the phantom with their clinical analytic dose calculation algorithms were asked to recalculate dose to the phantom with a Monte Carlo algorithm. These recalculations were collected and compared to measurements. Monte Carlo recalculations showed a striking improvement in agreement with the physical measurements, compared to the clinical analytic algorithm calculations. The analytic algorithms calculate, on average, the dose to the center of the target to be 7% higher than TLD measurements, whereas Monte Carlo algorithms are, on average, only 1.5% higher. The difference is even greater at the edge of the GTV, where analytic algorithms overpredict dose by up to 27% as compared to physical measurements. Monte Carlo recalculations agreed within 5% of the measurements. Within the PTV, one case showed a 41% overpredition of dose by the analytic algorithm. All comparisons showed a region of at least 10% dose discrepancy between the analytic and Monte Carlo algorithms inside the target volume. Previous studies comparing in-house Monte Carlo to a commercial analytic algorithm also showed a systematic overestimation of dose by the analytic algorithm, both in lung and in other disease sites. Analytic algorithms are doing a poor job predicting proton dose in lung tumors. There are many uncertainties in proton dose delivery, but eliminating this one uncertainty will reduce the overall uncertainty. For the safety of patients and the efficacy of treatment, proton therapy centers should switch to Monte Carlo or other advanced dose calculation algorithms for treatment planning for thoracic tumors.

Characterisation of small photon field outputs in a heterogeneous medium using X-ray voxel Monte Carlo dose calculation algorithm

AbstractAimTo characterise small photon beams using the Monte Carlo dose calculation algorithm for small field ranges in a heterogeneous medium.Materials and methodAn in-house phantom constructed with three different mediums, foam, polymethyl methacrylate and delrin resembling the densities of lung, soft tissue and bone respectively, was used in this study. Photon beam energies of 6 and 15 MV and field sizes of 8×8, 16×16, 24×24, 32×32 and 40×40 mm using X-ray voxel Monte Carlo (XVMC) algorithm using different detectors were validated. The relative output factor was measured in three different mediums having six different tissue interfaces; at the depth of 0, 1, 2 and 3 cm. The planar dose verification was undertaken using gafchromic films and considered dose at the lung and bone medium interfaces. For all the measurements, 104×104 mm was taken as the reference field size. The relative output factor for all other field sizes was taken and compared with planning system calculated values.ResultsFrom field size 16×16 mm and above, the relative output factors were analysed in bone and soft tissue medium having lung as first medium. The maximum deviations were observed as 1·8 and 1·3% for 6 MV and 2·5 and 1·1% for 15 MV photon beams for bone and soft tissue, respectively. For lung as measurement medium, the maximum deviation of 14·8 and 19·2% were observed and having bone as first medium with 8×8 mm for 6 and 15 MV photon beams, respectively. The fluence verification of dose spectrum for the lung–bone interface scenarios with smaller field sizes were found within 2% of deviation with treatment planning system (TPS).ConclusionThe accuracy of dose calculations for small field sizes in XVMC-based treatment planning algorithm was studied in different inhomogeneous mediums. It was found that the results correlated with measurement data for field size 16×16 mm and above. Noticeable deviation was observed for the smallest field size of 8×8 mm with interfaces of significant change in density. The observed results demands further analysis of work with smaller field sizes.

SP-0559: Basics, implementations, applications and limitations of Monte Carlo dose calculation algorithms

SP-0559: Basics, implementations, applications and limitations of Monte Carlo dose calculation algorithms

40. Evaluation of the Incise2™ multileaf collimator for stereotactic radiotherapy on Cyberknife®

Introduction The CyberKnife® (Accuray) is a stereotactic robotic system using historically circular collimators to define the beam apertures. A new multileaf collimator (MLC) is now available on the Cyberknife® M6 version. The purpose of this study was to evaluate the quality of treatment plans using MLC compared to treatment plans using a variable circular aperture collimator IRIS™ for radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Methods The study was performed on a Cyberknife® M6™ v10.6. A total of 17 cases including cranial, liver, spinal and prostatic cases were investigated. For each case, two treatment plans were generated with Multiplan® v5.3 treatment planning system: one plan using the Incise2™ MLC associated with the Finite Size Pencil Beam (FSPB) dose calculation algorithm and one using the IRIS™ collimator associated with Ray Tracing or Monte Carlo dose calculation algorithm. The dose was prescribed near the 80% isodose level relative to max dose. Plans were normalized to have the same target coverage at ±0.5%. The quality of the MLC and IRIS™ treatment plans was assessed using two criteria: - Dose distribution by reporting the conformal index (COIN), Homogeneity Index (HI) and dose gradient index (GI). - Efficiency by reporting the number of delivered monitor units (MUs) and the treatment time. To evaluate the accuracy of dose calculation algorithm within heterogeneity, a typical lung treatment plan for each collimator was created on a QUASAR™ Respiratory phantom (Modus Medical Inc) with a lung target insert. Calculated planar dose was compared with measurements by radiochromic EBT3 films (Ashland). The analysis was done using gamma analysis 3% (local) / 2 mm (threshold: 30% of the max dose) Results Results are summarized in Table 1. Dose conformality and homogeneity were equivalent for both collimator systems (mean differences less than 3%) except for targets with volumes smaller than 2 cm3 for which dose conformality was better of 14% on average with the IRIS collimator. The dose gradient index was improved by 10.5% on average with the MLC and up to 26% on average for volume larger than 100 cm3. The number of MUs decreased by 25% on average (up to 70%) and the treatment times were systematically reduced with a mean reduction of 28% (max 54%) with the MLC. The comparison between calculated and measured dose in lung phantom showed a gamma passing rate of 51.6 %, 45.5 % and 98.7% for FSPB, Ray Tracing and Monte Carlo respectively. Conclusion The MLC Incise2™ on Cyberknife® allows a significant reduction of MUs and treatment time compared to circular collimators while maintaining a high degree of conformality and a steep dose gradient. For treatment of target volumes smaller than 2 cm3, circular collimators should be still used. The use of MLC for SBRT of lung targets should not be recommended currently due to the absence of type B dose calculation algorithm.

Accuracy of the small field dosimetry using the monte carlo and sequential dose calculation algorithms of Multiplan treatment planning system within and beyond heterogeneous media for Cyberknife M6 unit

Purpose In this study, dosimetric accuracy of Monte Carlo (MC) advanced dose calculation and sequential algorithms of Multiplan treatment planning system were investigated for small radiation fields incident on homogeneous and heterogeneous geometries. Methods Small open fields of fixed cones of Cyberknife M6 unit 10 × 10 to 50 × 50 mm 2 were used for this study. The fields were incident on phantom containing lung, air and bone inhomogeneities and also homogeneous phantom. Using the same film batch, the net OD to dose calibration curve was obtained using CK with the 60 mm fixed cone by delivering 0–800 cGy. Films were scanned 48 h after irradiation using an Epson 1000XL flatbed scanner. The dosimetric accuracy of MC and sequential algorithms in the presence of the inhomogeneities was compared against EBT3 film dosimetry Results Open field tests in a homogeneous phantom showed good agreement between two algorithms and film measurement For MC algorithm, the minimum gamma analysis passing rates between measured and calculated dose distributions were 99.7% and 98.3% for homogeneous and inhomogeneous fields in the case of lung and bone respectively. For sequential algorithm, the minimum gamma analysis passing rates were 98.9% and 92.5% for homogeneous and inhomogeneous fields respectively for used all cone sizes. In the case of the air heterogeneity, the differences were larger for both calculation algorithms. Overall, when compared to measurement, the MC had better agreement than sequential algorithm. Conclusions The MC calculation algorithm in the Multiplan TPS is an improvement over the existing sequential algorithm. Dose discrepancies were observed for in the presence of air inhomogeneities.

Commissioning and validation of the electron Monte Carlo dose calculation at extended source to surface distance from a medical linear accelerator

Introduction: Radiotherapy is one of the major modality for cancer management playing curative, adjuvant, and palliative and sometimes has an alternative role to chemotherapy. Radiotherapy is practiced in two ways viz. External beam therapy and Brachytherapy. Electron beam therapy is widely used in the management of cancers. An electron beam is characterized by a finite range of penetration with a rapid dose fall off towards a slowly decaying x-ray background as the electrons traverse through tissues. The electron monte carlo (eMC) dose calculation algorithm for eclipse treatment system has been introduced by Varian Medical systems. The algorithm is commissioned and validated by comparing percentage depth dose (PDD) and gamma index. Methods: Percentage depth dose curves were generated for all the energies for 4x4 cm2 and 10x10 cm2 field sizes. The depth of maximum dose (R100), therapeutic depth (R85), depth of 50% isodose (R50) and the relative surface (Ds) were compared with the measured and calculated PDD curves. Results: The eMC calculated fluence and measured fluence were compared for all the energies and cones at nominal source to surface distance and extended distances. For 4x4 cm2 field size the maximum shift in R100 was 5 mm, R85 was 1.9 mm, R50 was 0.9 mm and the variation in the relative surface (DS) was about 25Gamma analysis shows excellent agreements with greater than 98% of the pixels passing the gamma requirements. Conclusion: We have successfully commissioned and validated the electron monte carlo dose calculation at extended source to surface distance.

SU-F-T-370: A Fast Monte Carlo Dose Engine for Gamma Knife

Purpose: To develop a fast Monte Carlo dose calculation algorithm for Gamma Knife. Methods: To make the simulation more efficient, we implemented the track repeating technique on GPU. We first use EGSnrc to pre-calculate the photon and secondary electron tracks in water from two mono-energy photons of 60Co. The total photon mean free paths for different materials and energies are obtained from NIST. During simulation, each entire photon track was first loaded to shared memory for each block, the incident original photon was then splitted to Nthread sub-photons, each thread transport one sub-photon, the Russian roulette technique was applied for scattered and bremsstrahlung photons. The resultant electrons from photon interactions are simulated by repeating the recorded electron tracks. The electron step length is stretched/shrunk proportionally based on the local density and stopping power ratios of the local material. Energy deposition in a voxel is proportional to the fraction of the equivalent step length in that voxel. To evaluate its accuracy, dose deposition in a 300mm*300mm*300mm water phantom is calculated, and compared to EGSnrc results. Results: Both PDD and OAR showed great agreements (within 0.5%) between our dose engine result and the EGSnrc result. It only takes less than 1 min for every simulation, being reduced up to ∼40 times compared to EGSnrc simulations. Conclusion: We have successfully developed a fast Monte Carlo dose engine for Gamma Knife.

SU‐F‐T‐444: Quality Improvement Review of Radiation Therapy Treatment Planning in the Presence of Dental Implants

Purpose:To quantify and compare the effect of metallic dental implants (MDI) on dose distributions calculated using Collapsed Cone Convolution Superposition (CCCS) algorithm or a Monte Carlo algorithm (with and without correcting for the density of the MDI).Methods:Seven previously treated patients to the head and neck region were included in this study. The MDI and the streaking artifacts on the CT images were carefully contoured. For each patient a plan was optimized and calculated using the Pinnacle3 treatment planning system (TPS). For each patient two dose calculations were performed, a) with the densities of the MDI and CT artifacts overridden (12 g/cc and 1 g/cc respectively) and b) without density overrides. The plans were then exported to the Monaco TPS and recalculated using Monte Carlo dose calculation algorithm. The changes in dose to PTVs and surrounding Regions of Interest (ROIs) were examined between all plans.Results:The Monte Carlo dose calculation indicated that PTVs received 6% lower dose than the CCCS algorithm predicted. In some cases, the Monte Carlo algorithm indicated that surrounding ROIs received higher dose (up to a factor of 2).Conclusion:Not properly accounting for dental implants can impact both the high dose regions (PTV) and the low dose regions (OAR). This study implies that if MDI and the artifacts are not appropriately contoured and given the correct density, there is potential significant impact on PTV coverage and OAR maximum doses.

SU‐F‐T‐430: Validation of IBEAM Evo Couch Top for Different Relative Electron Density (RED) Combination During Photon Beam Dose Calculation in Monaco− Treatment Planning System

Purpose:Validation of iBEAM™ evo couch‐top for different relative electron density (RED) combination during photon beam dose calculation in Monaco− TPS.Methods:The iBEAM™ evo couch‐top has two layers:outer carbon fiber (CF) and inner foam core (FC). To study the beam intensity attenuation of couch‐top, measured doses were compared with doses calculated for different REDs. Measurements were performed in solid water phantom with PTW‐0.125cc ion‐chamber positioned at center of the phantom with 5.3cm thickness slabs placed above and below the chamber. Similarly, in TPS, iBEAM™ evo couch‐top was simulated and doses were calculated for different RED combinations (0.2CF‐0.2FC, 0.4CF‐0.2FC, 0.6CF‐0.2FC, 0.8CF‐0.2FC, and 1.0CF‐0.2FC) by using Monte Carlo dose calculation algorithm in Monaco TPS (V5.1). Doses were measured for every 10 degree gantry angle separation, 10×10cm2 field size and 6MV photons. Then, attenuation is defined as the ratio of output at posterior gantry angle to output of its opposed anterior gantry angle (e.g.225°/45°). output fluctuation with different gantry angle was within ±0.21%. To confirm above results, dose‐planes were measured for five pelvic VMAT plans (360°arc) in PTW two‐dimensional array and compared with different calculated dose‐planes of above‐mentioned couch REDs. Gamma pass rates&lt;1.00) were analyzed for 3%/2mm criteria.Results:Measured and calculated attenuation was in good agreement for the RED combination of 0.2CF‐0.2FC and difference was within ±0.515%. However, other density combination showed difference of ±0.9841%, ±1.667%, ±2.9241% and ±2.8832% for 0.4CF‐0.2FC, 0.6CF‐0.2FC, 0.8CF‐0.2FC, and 1.0CF‐0.2FC, respectively. Maximum couch‐top attenuation was observed at 110°–120° and 240°–250° and decreases linearly as the gantry angle approaches 180°. Moreover, gamma pass rate confirmed the above results and showed maximum pass rate of 96.23% for 0.2CF‐0.2FC, whereas others were 95.72%, 95.12%, 94.31% and 93.24%.Conclusion:RED value of 0.2CF‐0.2FC was found to be suitable for accurate couch‐top modeling for 6MV photon beam Monte Carlo calculations in Monaco TPS.

SU-F-T-373: Monte Carlo Versus Pencil Beam Dose Calculation for Spine SBRT Treatments Using HybridARC and Sliding Windows IMRT

Purpose: The purpose of this work was to quantify the dosimetric impact of Monte Carlo (MC) dose calculation algorithm compared to Pencil Beam (PB) on Spine SBRT with HybridARC (HA) and sliding windows IMRT (dMLC) treatment modality. Methods: A 6MV beam (1000MU/min) produced by a Novalis TX (BrainLAB-Varian) equipped with HDMLC was used. HA uses 1 arc plus 8 IMRT beams (arc weight between 60–40%) and dIMRT 15 beams. Plans were calculated using iPlan v.4.5.3 (BrainLAB) and the treatment dose prescription was 27Gy in 3 fractions. Dose calculation was done by PB (4mm spatial resolution) with heterogeneity correction and MC dose to water (4mm spatial resolution and 4% mean variance). PTV and spinal cord dose comparison were done. Study was done on 12 patients. IROC Spine Phantom was used to validate HA and quantify dose variation using PB and MC algorithm. Results: The difference between PB and MC for PTV D98%, D95%, Dmean, D2% were 2.6% [−5.1, 6.8], 0.1% [−4.2, 5.4], 0.9% [−1.5, 3.8] and 2.4% [−0.5, 8.3]. The difference between PB and MC for spinal cord Dmax, D1.2cc and D0.35cc were 5.3% [−6.4, 18.4], 9% [−7.0, 17.0] and 7.6% [−0.6, 14.8] respectively. IROC spine phantom shows PTV TLD dose variation of 0.98% for PB and 1.01% for MC. Axial and sagittal film plane gamma index (5%-3mm) was 95% and 97% for PB and 95% and 99% for MC. Conclusion: PB slightly underestimates the dose for the PTV. For the spinal cord PB underestimates the dose and dose differences could be as high as 18% which could have unexpected clinical impact. CI shows no variation between PB and MC for both treatment modalities Treatment modalities have no impact with the dose calculation algorithms used. Following the IROC pass-fail criteria, treatment acceptance requirement was fulfilled for PB and MC.

SU‐F‐SPS‐02: Accuracy of the Small Field Dosimetry Using the Monte Carlo and Sequential Dose Calculation Algorithms of Multiplan Treatment Planning System Within and Beyond Heterogeneous Media for Cyberknife M6 Unit

Purpose:In small field geometries, the electronic equilibrium can be lost, making it challenging for the dose‐calculation algorithm to accurately predict the dose, especially in the presence of tissue heterogeneities. In this study, dosimetric accuracy of Monte Carlo (MC) advanced dose calculation and sequential algorithms of Multiplan treatment planning system were investigated for small radiation fields incident on homogeneous and heterogeneous geometries.Methods:Small open fields of fixed cones of Cyberknife M6 unit 100 to 500 mm2 were used for this study. The fields were incident on in house phantom containing lung, air, and bone inhomogeneities and also homogeneous phantom. Using the same film batch, the net OD to dose calibration curve was obtained using CK with the 60 mm fixed cone by delivering 0‐ 800 cGy. Films were scanned 48 hours after irradiation using an Epson 1000XL flatbed scanner. The dosimetric accuracy of MC and sequential algorithms in the presence of the inhomogeneities was compared against EBT3 film dosimetryResults:Open field tests in a homogeneous phantom showed good agreement between two algorithms and film measurement For MC algorithm, the minimum gamma analysis passing rates between measured and calculated dose distributions were 99.7% and 98.3% for homogeneous and inhomogeneous fields in the case of lung and bone respectively. For sequential algorithm, the minimum gamma analysis passing rates were 98.9% and 92.5% for for homogeneous and inhomogeneous fields respectively for used all cone sizes. In the case of the air heterogeneity, the differences were larger for both calculation algorithms. Overall, when compared to measurement, the MC had better agreement than sequential algorithm.Conclusion:The Monte Carlo calculation algorithm in the Multiplan treatment planning system is an improvement over the existing sequential algorithm. Dose discrepancies were observed for in the presence of air inhomogeneities.

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 × 1.5 cm2, 5 × 5 cm2, 10 × 10 cm2) 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.

SU‐E‐T‐219: Comprehensive Validation of the Electron Monte Carlo Dose Calculation Algorithm in RayStation Treatment Planning System for An Elekta Linear Accelerator with AgilityTM Treatment Head

Purpose:This study evaluated the performance of the electron Monte Carlo dose calculation algorithm in RayStation v4.0 for an Elekta machine with Agility™ treatment head.Methods:The machine has five electron energies (6–8 MeV) and five applicators (6×6 to 25×25 cm 2). The dose (cGy/MU at dmax), depth dose and profiles were measured in water using an electron diode at 100 cm SSD for nine square fields ≥2×2 cm2 and four complex fields at normal incidence, and a 14×14 cm2 field at 15° and 30° incidence. The dose was also measured for three square fields ≥4×4 cm2 at 98, 105 and 110 cm SSD. Using selected energies, the EBT3 radiochromic film was used for dose measurements in slab‐shaped inhomogeneous phantoms and a breast phantom with surface curvature. The measured and calculated doses were analyzed using a gamma criterion of 3%/3 mm.Results:The calculated and measured doses varied by &lt;3% for 116 of the 120 points, and &lt;5% for the 4×4 cm2 field at 110 cm SSD at 9–18 MeV. The gamma analysis comparing the 105 pairs of in‐water isodoses passed by &gt;98.1%. The planar doses measured from films placed at 0.5 cm below a lung/tissue layer (12 MeV) and 1.0 cm below a bone/air layer (15 MeV) showed excellent agreement with calculations, with gamma passing by 99.9% and 98.5%, respectively. At the breast‐tissue interface, the gamma passing rate is &gt;98.8% at 12–18 MeV. The film results directly validated the accuracy of MU calculation and spatial dose distribution in presence of tissue inhomogeneity and surface curvature ‐ situations challenging for simpler pencil‐beam algorithms.Conclusion:The electron Monte Carlo algorithm in RayStation v4.0 is fully validated for clinical use for the Elekta Agility™ machine. The comprehensive validation included small fields, complex fields, oblique beams, extended distance, tissue inhomogeneity and surface curvature.

SU‐E‐T‐579: On the Relative Sensitivity of Monte Carlo and Pencil Beam Dose Calculation Algorithms to CT Metal Artifacts in Volumetric‐Modulated Arc Spine Radiosurgery (RS)

Purpose:Investigating the relative sensitivity of Monte Carlo (MC) and Pencil Beam (PB) dose calculation algorithms to low‐Z (titanium) metallic artifacts is important for accurate and consistent dose reporting in post¬operative spinal RS.Methods:Sensitivity analysis of MC and PB dose calculation algorithms on the Monaco v.3.3 treatment planning system (Elekta CMS, Maryland Heights, MO, USA) was performed using CT images reconstructed without (plain) and with Orthopedic Metal Artifact Reduction (OMAR; Philips Healthcare system, Cleveland, OH, USA). 6MV and 10MV volumetric‐modulated arc (VMAT) RS plans were obtained for MC and PB on the plain and OMAR images (MC‐plain/OMAR and PB‐plain/OMAR).Results:Maximum differences in dose to 0.2cc (D0.2cc) of spinal cord and cord +2mm for 6MV and 10MV VMAT plans were 0.1Gy between MC‐OMAR and MC‐plain, and between PB‐OMAR and PB‐plain. Planning target volume (PTV) dose coverage changed by 0.1±0.7% and 0.2±0.3% for 6MV and 10MV from MC‐OMAR to MC‐plain, and by 0.1±0.1% for both 6MV and 10 MV from PB‐OMAR to PB‐plain, respectively. In no case for both MC and PB the D0.2cc to spinal cord was found to exceed the planned tolerance changing from OMAR to plain CT in dose calculations.Conclusion:Dosimetric impacts of metallic artifacts caused by low‐Z metallic spinal hardware (mainly titanium alloy) are not clinically important in VMAT‐based spine RS, without significant dependence on dose calculation methods (MC and PB) and photon energy ≥ 6MV. There is no need to use one algorithm instead of the other to reduce uncertainty for dose reporting. The dose calculation method that should be used in spine RS shall be consistent with the usual clinical practice.

SU‐E‐T‐416: Experimental Evaluation of a Commercial GPU‐Based Monte Carlo Dose Calculation Algorithm

Purpose:A new commercial GPU‐based Monte Carlo dose calculation algorithm (GPUMCD) developed by the vendor Elekta™ to be used in the Monaco Treatment Planning System (TPS) is capable of modeling dose for both a standard linear accelerator and for an Elekta MRI‐Linear accelerator (modeling magnetic field effects). We are evaluating this algorithm in two parts: commissioning the algorithm for an Elekta Agility linear accelerator (the focus of this work) and evaluating the algorithm's ability to model magnetic field effects for an MRI‐linear accelerator.Methods:A beam model was developed in the Monaco TPS (v.5.09.06) using the commissioned beam data for a 6MV Agility linac. A heterogeneous phantom representing tumor‐in‐lung, lung, bone‐in‐tissue, and prosthetic was designed/built. Dose calculations in Monaco were done using the current clinical algorithm (XVMC) and the new GPUMCD algorithm (1 mm3 voxel size, 0.5% statistical uncertainty) and in the Pinnacle TPS using the collapsed cone convolution (CCC) algorithm. These were compared with the measured doses using an ionization chamber (A1SL) and Gafchromic EBT3 films for 2×2 cm2, 5×5 cm2, and 10×10 cm2 field sizes.Results:The calculated central axis percentage depth doses (PDDs) in homogeneous solid water were within 2% compared to measurements for XVMC and GPUMCD. For tumor‐in‐lung and lung phantoms, doses calculated by all of the algorithms were within the experimental uncertainty of the measurements (±2% in the homogeneous phantom and ±3% for the tumor‐in‐lung or lung phantoms), except for 2×2 cm2 field size where only the CCC algorithm differs from film by 5% in the lung region. The analysis for bone‐in‐tissue and the prosthetic phantoms are ongoing.Conclusion:The new GPUMCD algorithm calculated dose comparable to both the XVMC algorithm and to measurements in both a homogeneous solid water medium and the heterogeneous phantom representing lung or tumor‐in‐lung for 2×2 cm2‐10×10 cm2 field sizes.Funding support was obtained from Elekta.

TU-F-CAMPUS-T-05: A Cloud-Based Monte Carlo Dose Calculation for Electron Cutout Factors

Purpose: For electron cutouts of smaller sizes, it is necessary to verify electron cutout factors due to perturbations in electron scattering. Often, this requires a physical measurement using a small ion chamber, diode, or film. The purpose of this study is to develop a fast Monte Carlo based dose calculation framework that requires only a smart phone photograph of the cutout and specification of the SSD and energy to determine the electron cutout factor, with the ultimate goal of making this cloud-based calculation widely available to the medical physics community. Methods: The algorithm uses a pattern recognition technique to identify the corners of the cutout in the photograph as shown in Figure 1. It then corrects for variations in perspective, scaling, and translation of the photograph introduced by the user’s positioning of the camera. Blob detection is used to identify the portions of the cutout which comprise the aperture and the portions which are cutout material. This information is then used define physical densities of the voxels used in the Monte Carlo dose calculation algorithm as shown in Figure 2, and select a particle source from a pre-computed library of phase-spaces scored above the cutout. The electron cutout factor is obtained by taking a ratio of the maximum dose delivered with the cutout in place to the dose delivered under calibration/reference conditions. Results: The algorithm has been shown to successfully identify all necessary features of the electron cutout to perform the calculation. Subsequent testing will be performed to compare the Monte Carlo results with a physical measurement. Conclusion: A simple, cloud-based method of calculating electron cutout factors could eliminate the need for physical measurements and substantially reduce the time required to properly assure accurate dose delivery.