Pencil Beam Dose Calculation Algorithm Research Articles

Dose calculation accuracy in particle therapy: Comparing carbon ions with protons.

PurposeThis work presents the validation of an analytical pencil beam dose calculation algorithm in a commercial treatment planning system (TPS) for carbon ions by measurements of dose distributions in heterogeneous phantom geometries. Additionally, a comparison study of carbon ions versus protons is performed considering current best solutions in commercial TPS.MethodsAll treatment plans were optimized and calculated using the RayStation TPS (RaySearch, Sweden). The dose distributions calculated with the TPS were compared with measurements using a 24‐pinpoint ionization chamber array (T31015, PTW, Germany). Tissue‐like inhomogeneities (bone, lung, and soft tissue) were embedded in water, while a target volume of 4 x 4 x 4 cm3 was defined at two different depths behind the heterogeneities. In total, 10 different test cases, with and without range shifter as well as different air gaps, were investigated. Dose distributions inside as well as behind the target volume were evaluated.ResultsInside the target volume, the mean dose difference between calculations and measurements, averaged over all test cases, was 1.6% for carbon ions. This compares well to the final agreement of 1.5% obtained in water at the commissioning stage of the TPS for carbon ions and is also within the clinically acceptable interval of 3%. The mean dose difference and maximal dose difference obtained outside the target area were 1.8% and 13.4%, respectively. The agreement of dose distributions for carbon ions in the target volumes was comparable or better to that between Monte Carlo (MC) dose calculations and measurements for protons. Percentage dose differences of more than 10% were present outside the target area behind bone–lung structures, where the carbon ion calculations systematically over predicted the dose. MC dose calculations for protons were superior to carbon ion beams outside the target volumes.ConclusionThe pencil beam dose calculations for carbon ions in RayStation were found to be in good agreement with dosimetric measurements in heterogeneous geometries for points of interest located within the target. Large local discrepancies behind the target may contribute to incorrect dose predictions for organs at risk.

Impact of dose engine algorithm in pencil beam scanning proton therapy for breast cancer

PurposeProton therapy for the treatment of breast cancer is acquiring increasing interest, due to the potential reduction of radiation-induced side effects such as cardiac and pulmonary toxicity. While several in silico studies demonstrated the gain in plan quality offered by pencil beam scanning (PBS) compared to passive scattering techniques, the related dosimetric uncertainties have been poorly investigated so far. MethodsFive breast cancer patients were planned with Raystation 6 analytical pencil beam (APB) and Monte Carlo (MC) dose calculation algorithms. Plans were optimized with APB and then MC was used to recalculate dose distribution. Movable snout and beam splitting techniques (i.e. using two sub-fields for the same beam entrance, one with and the other without the use of a range shifter) were considered. PTV dose statistics were recorded. The same planning configurations were adopted for the experimental benchmark. Dose distributions were measured with a 2D array of ionization chambers and compared to APB and MC calculated ones by means of a γ analysis (agreement criteria 3%, 3 mm). ResultsOur results indicate that, when using proton PBS for breast cancer treatment, the Raystation 6 APB algorithm does not allow obtaining sufficient accuracy, especially with large air gaps. On the contrary, the MC algorithm resulted into much higher accuracy in all beam configurations tested and has to be recommended. ConclusionsCenters where a MC algorithm is not yet available should consider a careful use of APB, possibly combined with a movable snout system or in any case with strategies aimed at minimizing air gaps.

Advanced proton beam dosimetry part II: Monte Carlo vs. pencil beam-based planning for lung cancer.

Proton pencil beam (PB) dose calculation algorithms have limited accuracy within heterogeneous tissues of lung cancer patients, which may be addressed by modern commercial Monte Carlo (MC) algorithms. We investigated clinical pencil beam scanning (PBS) dose differences between PB and MC-based treatment planning for lung cancer patients. With IRB approval, a comparative dosimetric analysis between RayStation MC and PB dose engines was performed on ten patient plans. PBS gantry plans were generated using single-field optimization technique to maintain target coverage under range and setup uncertainties. Dose differences between PB-optimized (PBopt), MC-recalculated (MCrecalc), and MC-optimized (MCopt) plans were recorded for the following region-of-interest metrics: clinical target volume (CTV) V95, CTV homogeneity index (HI), total lung V20, total lung VRX (relative lung volume receiving prescribed dose or higher), and global maximum dose. The impact of PB-based and MC-based planning on robustness to systematic perturbation of range (±3% density) and setup (±3 mm isotropic) was assessed. Pairwise differences in dose parameters were evaluated through non-parametric Friedman and Wilcoxon sign-rank testing. In this ten-patient sample, CTV V95 decreased significantly from 99-100% for PBopt to 77-94% for MCrecalc and recovered to 99-100% for MCopt (P<10-5). The median CTV HI (D95/D5) decreased from 0.98 for PBopt to 0.91 for MCrecalc and increased to 0.95 for MCopt (P<10-3). CTV D95 robustness to range and setup errors improved under MCopt (ΔD95 =-1%) compared to MCrecalc (ΔD95 =-6%, P=0.006). No changes in lung dosimetry were observed for large volumes receiving low to intermediate doses (e.g., V20), while differences between PB-based and MC-based planning were noted for small volumes receiving high doses (e.g., VRX). Global maximum patient dose increased from 106% for PBopt to 109% for MCrecalc and 112% for MCopt (P<10-3). MC dosimetry revealed a reduction in target dose coverage under PB-based planning that was regained under MC-based planning along with improved plan robustness. MC-based optimization and dose calculation should be integrated into clinical planning workflows of lung cancer patients receiving actively scanned proton therapy.

Analytical probabilistic modeling of RBE-weighted dose for ion therapy

Particle therapy is especially prone to uncertainties. This issue is usually addressed with uncertainty quantification and minimization techniques based on scenario sampling. For proton therapy, however, it was recently shown that it is also possible to use closed-form computations based on analytical probabilistic modeling (APM) for this purpose. APM yields unique features compared to sampling-based approaches, motivating further research in this context.This paper demonstrates the application of APM for intensity-modulated carbon ion therapy to quantify the influence of setup and range uncertainties on the RBE-weighted dose. In particular, we derive analytical forms for the nonlinear computations of the expectation value and variance of the RBE-weighted dose by propagating linearly correlated Gaussian input uncertainties through a pencil beam dose calculation algorithm. Both exact and approximation formulas are presented for the expectation value and variance of the RBE-weighted dose and are subsequently studied in-depth for a one-dimensional carbon ion spread-out Bragg peak. With V and B being the number of voxels and pencil beams, respectively, the proposed approximations induce only a marginal loss of accuracy while lowering the computational complexity from order to for the expectation value and from to for the variance of the RBE-weighted dose. Moreover, we evaluated the approximated calculation of the expectation value and standard deviation of the RBE-weighted dose in combination with a probabilistic effect-based optimization on three patient cases considering carbon ions as radiation modality against sampled references. The resulting global γ-pass rates (2 mm,2%) are 99.15% for the expectation value and 94.95% for the standard deviation of the RBE-weighted dose, respectively. We applied the derived analytical model to carbon ion treatment planning, although the concept is in general applicable to other ion species considering a variable RBE.

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

Performance Characteristics of an Independent Dose Verification Program for Helical Tomotherapy.

Helical tomotherapy with its advanced method of intensity-modulated radiation therapy delivery has been used clinically for over 20 years. The standard delivery quality assurance procedure to measure the accuracy of delivered radiation dose from each treatment plan to a phantom is time-consuming. RadCalc®, a radiotherapy dose verification software, has released specifically for beta testing a module for tomotherapy plan dose calculations. RadCalc®'s accuracy for tomotherapy dose calculations was evaluated through examination of point doses in ten lung and ten prostate clinical plans. Doses calculated by the TomoHDA™ tomotherapy treatment planning system were used as the baseline. For lung cases, RadCalc® overestimated point doses in the lung by an average of 13%. Doses within the spinal cord and esophagus were overestimated by 10%. Prostate plans showed better agreement, with overestimations of 6% in the prostate, bladder, and rectum. The systematic overestimation likely resulted from limitations of the pencil beam dose calculation algorithm implemented by RadCalc®. Limitations were more severe in areas of greater inhomogeneity and less prominent in regions of homogeneity with densities closer to 1 g/cm3. Recommendations for RadCalc® dose calculation algorithms and anatomical representation were provided based on the results of the study.

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.

A pencil beam dose calculation model for CyberKnife system.

CyberKnife system is initially equipped with fixed circular cones for stereotactic radiosurgery. Two dose calculation algorithms, Ray-Tracing and Monte Carlo, are available in the supplied treatment planning system. A multileaf collimator system was recently introduced in the latest generation of system, capable of arbitrarily shaped treatment field. The purpose of this study is to develop a model based dose calculation algorithm to better handle the lateral scatter in an irregularly shaped small field for the CyberKnife system. A pencil beam dose calculation algorithm widely used in linac based treatment planning system was modified. The kernel parameters and intensity profile were systematically determined by fitting to the commissioning data. The model was tuned using only a subset of measured data (4 out of 12 cones) and applied to all fixed circular cones for evaluation. The root mean square (RMS) of the difference between the measured and calculated tissue-phantom-ratios (TPRs) and off-center-ratio (OCR) was compared. Three cone size correction techniques were developed to better fit the OCRs at the penumbra region, which are further evaluated by the output factors (OFs). The pencil beam model was further validated against measurement data on the variable dodecagon-shaped Iris collimators and a half-beam blocked field. Comparison with Ray-Tracing and Monte Carlo methods was also performed on a lung SBRT case. The RMS between the measured and calculated TPRs is 0.7% averaged for all cones, with the descending region at 0.5%. The RMSs of OCR at infield and outfield regions are both at 0.5%. The distance to agreement (DTA) at the OCR penumbra region is 0.2 mm. All three cone size correction models achieve the same improvement in OCR agreement, with the effective source shift model (SSM) preferred, due to their ability to predict more accurately the OF variations with the source to axis distance (SAD). In noncircular field validation, the pencil beam calculated results agreed well with the film measurement of both Iris collimators and the half-beam blocked field, fared much better than the Ray-Tracing calculation. The authors have developed a pencil beam dose calculation model for the CyberKnife system. The dose calculation accuracy is better than the standard linac based system because the model parameters were specifically tuned to the CyberKnife system and geometry correction factors. The model handles better the lateral scatter and has the potential to be used for the irregularly shaped fields. Comprehensive validations on MLC equipped system are necessary for its clinical implementation. It is reasonably fast enough to be used during plan optimization.

Comparison of the effects of two different dose algorithms in cardiac dose parameters for hodgkin lymphoma patients receiving mediastinal radiotheraphy

Objective: This study aimed to compare pencil beam (PB) and convolution superposition (CS) dose calculation algorithms with respect to cardiac dose-volume parameters V5, V25, V30 and MHD (median heart dose) in Hodgkin lymphoma (HL) patients who received mediastinal radiotherapy (RT). Material and Methods: Ten eligible cases with the diagnosis of HL who completed mediastinal RT at Dokuz Eylul University Department of Radiation Oncology before 01.01.2011 formed the study population. Related 3D conformal RT treatment plans were obtained from the Treatment Planning System (TPS) (Nucletron Oncentra Master Plan Version 3.3 SP3 program). Calculations were done for 6 and 18 MV-X beam energy, respectively by using PB and CS algorithms in the TPS. Two algorithms were compared in terms of cardiac dose-volume parameters (V5, V25, V30 and MHD). Results: Mean relative difference for both V25 and V30 was found to be significantly higher in favor of PB algorithm in both 6 MV-X and 18 MV-X photon energy. Mean relative differences were 1.75% (p=0.012) and 6.76% (p=0.028) for the V25 and V30 parameters, respectively in 6 MV-X and 3.25% (p=0.021) and 13.95% (p=0.007) for the V25 and V30 parameters, respectively in 18 MV-X energy. Conclusion: When PB algorithm is used in RT treatment planning of HL patients for the mediastinal region where tissue inhomogeneity is relatively high, heart V25 and V30 parameters were shown to result in higher values compared to CS algorithm in our study. It is more appropriate to prefer CS algorithm instead of PB algorithm since it increases accuracy of calculations in the treatment planning of mediastinal RT.

Effect of the normalized prescription isodose line on the magnitude of Monte Carlo vs. pencil beam target dose differences for lung stereotactic body radiotherapy

In lung stereotactic body radiotherapy (SBRT) cases, the pencil beam (PB) dose calculation algorithm is known to overestimate target dose as compared to the more accurate Monte Carlo (MC) algorithm. We investigated whether changing the normalized prescription isodose line affected the magnitude of MC vs. PB target dose differences. Forty‐eight patient plans and twenty virtual‐tumor phantom plans were studied. For patient plans, four alternative plans prescribed to 60%, 70%, 80%, and 90% isodose lines were each created for 12 patients who previously received lung SBRT treatments. Using 6 MV dynamic conformal arcs, the plans were individually optimized to achieve similar dose coverage and conformity for all plans of the same patient, albeit at the different prescription levels. These plans, having used a PB algorithm, were all recalculated with MC to compare the target dose differences. The relative MC vs. PB target dose variations were investigated by comparing PTV D95, Dmean, and D5 loss at the four prescription levels. The MC‐to‐PB ratio of the plan heterogeneity index (HI) was also evaluated and compared among different isodose levels. To definitively demonstrate the cause of the isodose line dependence, a simulated phantom study was conducted using simple, spherical virtual tumors planned with uniform block margins. The tumor size and beam energy were also altered in the phantom study to investigate the interplay between these confounding factors and the isodose line effect. The magnitude of the target dose overestimation by PB was greater for higher prescription isodose levels. The MC vs. PB reduction in the target dose coverage indices, D95 and V100 of PTV, were found to monotonically increase with increasing isodose lines from 60% to 90%, resulting in more pronounced target dose coverage deficiency at higher isodose prescription levels. No isodose level‐dependent trend was observed for the dose errors in the target mean or high dose indices, Dmean or D5. The phantom study demonstrated that the observed isodose level dependence was caused by different beam margins used for the different isodose levels: a higher prescription line required a larger beam margin, leading to more low‐density lung tissues in the field and, therefore, larger dose errors at the target periphery (when calculated with PB). The phantom study also found that the observed isodose level dependence was greater for smaller targets and for higher beam energies. We hereby characterized the effect of normalized prescription isodose line on magnitude of PTV dose coverage as calculated by MC vs. PB. When comparing reported MC dose deficiency values for different patients, the selection of prescription isodose line should be considered in addition to other factors known to affect differences in calculated doses between various algorithms.PACS number(s): 87.55.kh, 87.55.dk, 87.55.de

SU-G-TeP1-08: LINAC Head Geometry Modeling for Cyber Knife System

Purpose: Knowledge of the LINAC head information is critical for model based dose calculation algorithms. However, the geometries are difficult to measure precisely. The purpose of this study is to develop linac head models for Cyber Knife system (CKS). Methods: For CKS, the commissioning data were measured in water at 800mm SAD. The measured full width at half maximum (FWHM) for each cone was found greater than the nominal value, this was further confirmed by additional film measurement in air. Diameter correction, cone shift and source shift models (DCM, CSM and SSM) are proposed to account for the differences. In DCM, a cone-specific correction is applied. For CSM and SSM, a single shift is applied to the cone or source physical position. All three models were validated with an in-house developed pencil beam dose calculation algorithm, and further evaluated by the collimator scatter factor (Sc) correction. Results: The mean square error (MSE) between nominal diameter and the FWHM derived from commissioning data and in-air measurement are 0.54mm and 0.44mm, with the discrepancy increasing with cone size. Optimal shift for CSM and SSM is found to be 9mm upward and 18mm downward, respectively. The MSE in FWHM is reduced to 0.04mm and 0.14mm for DCM and CSM (SSM). Both DCM and CSM result in the same set of Sc values. Combining all cones at SAD 600–1000mm, the average deviation from 1 in Sc of DCM (CSM) and SSM is 2.6% and 2.2%, and reduced to 0.9% and 0.7% for the cones with diameter greater than 15mm. Conclusion: We developed three geometrical models for CKS. All models can handle the discrepancy between vendor specifications and commissioning data. And SSM has the best performance for Sc correction. The study also validated that a point source can be used in CKS dose calculation algorithms.

SU‐F‐T‐622: Comparative Analysis of Pencil Beam and Anisotropic Analytical Algorithm (AAA) for Stereotactic Body Radiation Therapy (SBRT) of Thoracic Spine

Purpose:To compare the impact of Pencil Beam(PB) and Anisotropic Analytic Algorithm(AAA) dose calculation algorithms on OARs and planning target volume (PTV) in thoracic spine stereotactic body radiation therapy (SBRT).Methods:Ten Spine SBRT patients were planned on Brainlab iPlan system using hybrid plan consisting of 1–2 non‐coplanar conformal‐dynamic arcs and few IMRT beams treated on NovalisTx with 6MV photon. Dose prescription varied from 20Gy to 30Gy in 5 fractions depending on the situation of the patient. PB plans were retrospectively recalculated using the Varian Eclipse with AAA algorithm using same MUs, MLC pattern and grid size(3mm).Differences in dose volume parameters for PTV, spinal cord, lung, and esophagus were analyzed and compared for PB and AAA algorithms. OAR constrains were followed per RTOG‐0631.Results:Since patients were treated using PB calculation, we compared all the AAA DVH values with respect to PB plan values as standard, although AAA predicts the dose more accurately than PB. PTV(min), PTV(Max), PTV(mean), PTV(D99%), PTV(D90%) were overestimated with AAA calculation on average by 3.5%, 1.84%, 0.95%, 3.98% and 1.55% respectively as compared to PB. All lung DVH parameters were underestimated with AAA algorithm mean deviation of lung V20, V10, V5, and 1000cc were 42.81%,19.83%, 18.79%, and 18.35% respectively. AAA overestimated Cord(0.35cc) by mean of 17.3%; cord (0.03cc) by 12.19% and cord(max) by 10.5% as compared to PB. Esophagus max dose were overestimated by 4.4% and 5cc by 3.26% for AAA algorithm as compared to PB.Conclusion:AAA overestimated the PTV dose values by up to 4%.The lung DVH had the greatest underestimation of dose by AAA versus PB. Spinal cord dose was overestimated by AAA versus PB. Given the critical importance of accuracy of OAR and PTV dose calculation for SBRT spine, more accurate algorithms and validation of calculated doses in phantom models are indicated.

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‐252: Developing a Pencil Beam Dose Calculation Algorithm for CyberKnife System

Purpose:Currently there are two dose calculation algorithms available in the Cyberknife planning system: ray‐tracing and Monte Carlo, which is either not accurate or time‐consuming for irregular field shaped by the MLC that was recently introduced. The purpose of this study is to develop a fast and accurate pencil beam dose calculation algorithm which can handle irregular field.Methods:A pencil beam dose calculation algorithm widely used in Linac system is modified. The algorithm models both primary (short range) and scatter (long range) components with a single input parameter: TPR20/10. The TPR20/20/10 value was first estimated to derive an initial set of pencil beam model parameters (PBMP). The agreement between predicted and measured TPRs for all cones were evaluated using the root mean square of the difference (RMSTPR), which was then minimized by adjusting PBMPs. PBMPs are further tuned to minimize OCR RMS (RMSocr) by focusing at the outfield region. Finally, an arbitrary intensity profile is optimized by minimizing RMSocr difference at infield region. To test model validity, the PBMPs were obtained by fitting to only a subset of cones (4) and applied to all cones (12) for evaluation.Results:With RMS values normalized to the dmax and all cones combined, the average RMSTPR at build‐up and descending region is 2.3% and 0.4%, respectively. The RMSocr at infield, penumbra and outfield region is 1.5%, 7.8% and 0.6%, respectively. Average DTA in penumbra region is 0.5mm. There is no trend found in TPR or OCR agreement among cones or depths.Conclusion:We have developed a pencil beam algorithm for Cyberknife system. The prediction agrees well with commissioning data. Only a subset of measurements is needed to derive the model. Further improvements are needed for TPR buildup region and OCR penumbra. Experimental validations on MLC shaped irregular field needs to be performed.This work was partially supported by the National Natural Science Foundation of China (61171005) and the China Scholarship Council (CSC).

Development of a golden beam data set for the commissioning of a proton double-scattering system in a pencil-beam dose calculation algorithm.

The purpose of this investigation is to determine if a single set of beam data, described by a minimal set of equations and fitting variables, can be used to commission different installations of a proton double-scattering system in a commercial pencil-beam dose calculation algorithm. The beam model parameters required to commission the pencil-beam dose calculation algorithm (virtual and effective SAD, effective source size, and pristine-peak energy spread) are determined for a commercial double-scattering system. These parameters are measured in a first room and parameterized as function of proton energy and nozzle settings by fitting four analytical equations to the measured data. The combination of these equations and fitting values constitutes the golden beam data (GBD). To determine the variation in dose delivery between installations, the same dosimetric properties are measured in two additional rooms at the same facility, as well as in a single room at another facility. The difference between the room-specific measurements and the GBD is evaluated against tolerances that guarantee the 3D dose distribution in each of the rooms matches the GBD-based dose distribution within clinically reasonable limits. The pencil-beam treatment-planning algorithm is commissioned with the GBD. The three-dimensional dose distribution in water is evaluated in the four treatment rooms and compared to the treatment-planning calculated dose distribution. The virtual and effective SAD measurements fall between 226 and 257 cm. The effective source size varies between 2.4 and 6.2 cm for the large-field options, and 1.0 and 2.0 cm for the small-field options. The pristine-peak energy spread decreases from 1.05% at the lowest range to 0.6% at the highest. The virtual SAD as well as the effective source size can be accurately described by a linear relationship as function of the inverse of the residual energy. An additional linear correction term as function of RM-step thickness is required for accurate parameterization of the effective SAD. The GBD energy spread is given by a linear function of the exponential of the beam energy. Except for a few outliers, the measured parameters match the GBD within the specified tolerances in all of the four rooms investigated. For a SOBP field with a range of 15 g/cm2 and an air gap of 25 cm, the maximum difference in the 80%-20% lateral penumbra between the GBD-commissioned treatment-planning system and measurements in any of the four rooms is 0.5 mm. The beam model parameters of the double-scattering system can be parameterized with a limited set of equations and parameters. This GBD closely matches the measured dosimetric properties in four different rooms.

The photon dose calculation algorithm used in breast radiotherapy has significant impact on the parameters of radiobiological models.

The comparison of the pencil beam dose calculation algorithm with modified Batho heterogeneity correction (PBC‐MB) and the analytical anisotropic algorithm (AAA) and the mutual comparison of advanced dose calculation algorithms used in breast radiotherapy have focused on the differences between the physical dose distributions. Studies on the radiobiological impact of the algorithm (both on the tumor control and the moderate breast fibrosis prediction) are lacking. We, therefore, investigated the radiobiological impact of the dose calculation algorithm in whole breast radiotherapy. The clinical dose distributions of 30 breast cancer patients, calculated with PBC‐MB, were recalculated with fixed monitor units using more advanced algorithms: AAA and Acuros XB. For the latter, both dose reporting modes were used (i.e., dose‐to‐medium and dose‐to‐water). Next, the tumor control probability (TCP) and the normal tissue complication probability (NTCP) of each dose distribution were calculated with the Poisson model and with the relative seriality model, respectively. The endpoint for the NTCP calculation was moderate breast fibrosis five years post treatment. The differences were checked for significance with the paired t‐test. The more advanced algorithms predicted a significantly lower TCP and NTCP of moderate breast fibrosis then found during the corresponding clinical follow‐up study based on PBC calculations. The differences varied between 1% and 2.1% for the TCP and between 2.9% and 5.5% for the NTCP of moderate breast fibrosis. The significant differences were eliminated by determination of algorithm‐specific model parameters using least square fitting. Application of the new parameters on a second group of 30 breast cancer patients proved their appropriateness. In this study, we assessed the impact of the dose calculation algorithms used in whole breast radiotherapy on the parameters of the radiobiological models. The radiobiological impact was eliminated by determination of algorithm specific model parameters.PACS numbers: 87.55.dh, 87.55.dk

TH‐A‐19A‐01: An Open Source Software for Proton Treatment Planning in Heterogeneous Medium

Purpose:Due to its success in Radiation Oncology during the last decade, interest in proton therapy is on the rise. Unfortunately, despite the global enthusiasm in the field, there is presently no free, multiplatform and customizable Treatment Planning System (TPS) providing proton dose distributions in heterogenous medium. This restricts substantially the progress of clinical research for groups without access to a commercial Proton TPS. The latest implementation of our pencil beam dose calculation algorithm for proton beams within the 3D Slicer open‐source environment fulfills all the conditions described above.Methods:The core dose calculation algorithm is based on the Hong algorithm (1), which was upgraded with the Kanematsu theory describing the evolution of the lateral scattering of proton beamlets in heterogeneous medium. This algorithm deals with both mono‐energetic beams and Spread Out Bragg Peak (SOBP). In order to be user‐friendly, we provide a graphical user interface implemented with the Qt libraries, and visualization with the 3D Slicer medical image analysis software. Two different pencil beam algorithms were developed, and the clinical proton beam line at our facility was modeled.Results:The dose distributions provided by our algorithms were compared to dose distributions coming from both commercialized XiO TPS and literature (dose measurements, GEANT4 and MCNPx) and turned out to be in a good agreement, with maximum dose discrepancies of 5% in homogeneous phantoms and 10% in heterogeneous phantoms. The algorithm of SOBP creation from an optimized weigthing of mono‐energetic beams results in flat SOBP.Conclusion:We hope that our efforts in implementing this new, open‐source proton TPS will help the research groups to have a free access to a useful, reliable proton dose calculation software.(1) L. Hong et al., A pencil beam algorithm for proton dose calculations, Phys. Med. Biol. 41 (1996) 1305–1330.This project is paid for by NCI/MGH Federal share as a research grant.

TH-C-BRD-02: Analytical Modeling and Dose Calculation Method for Asymmetric Proton Pencil Beams

Purpose: A dynamic collimation system (DCS), which consists of two pairs of orthogonal trimmer blades driven by linear motors has been proposed to decrease the lateral penumbra in pencil beam scanning proton therapy. The DCS reduces lateral penumbra by intercepting the proton pencil beam near the lateral boundary of the target in the beam's eye view. The resultant trimmed pencil beams are asymmetric and laterally shifted, and therefore existing pencil beam dose calculation algorithms are not capable of trimmed beam dose calculations. This work develops a method to model and compute dose from trimmed pencil beams when using the DCS. Methods: MCNPX simulations were used to determine the dose distributions expected from various trimmer configurations using the DCS. Using these data, the lateral distribution for individual beamlets was modeled with a 2D asymmetric Gaussian function. The integral depth dose (IDD) of each configuration was also modeled by combining the IDD of an untrimmed pencil beam with a linear correction factor. The convolution of these two terms, along with the Highland approximation to account for lateral growth of the beam along the depth direction, allows a trimmed pencil beam dose distribution to be analytically generated. The algorithm was validated by computingmore » dose for a single energy layer 5×5 cm{sup 2} treatment field, defined by the trimmers, using both the proposed method and MCNPX beamlets. Results: The Gaussian modeled asymmetric lateral profiles along the principal axes match the MCNPX data very well (R{sup 2}≥0.95 at the depth of the Bragg peak). For the 5×5 cm{sup 2} treatment plan created with both the modeled and MCNPX pencil beams, the passing rate of the 3D gamma test was 98% using a standard threshold of 3%/3 mm. Conclusion: An analytical method capable of accurately computing asymmetric pencil beam dose when using the DCS has been developed.« less

SU-E-T-202: Impact of Monte Carlo Dose Calculation Algorithm On Prostate SBRT Treatments

Purpose: The purpose of this work was to quantify the dosimetric impact of using Monte Carlo algorithm on pre calculated SBRT prostate treatment with pencil beam dose calculation algorithm. Methods: A 6MV photon beam produced by a Novalis TX (BrainLAB-Varian) linear accelerator equipped with HDMLC was used. Treatment plans were done using 9 fields with Iplanv4.5 (BrainLAB) and dynamic IMRT modality. Institutional SBRT protocol uses a total dose to the prostate of 40Gy in 5 fractions, every other day. Dose calculation is done by pencil beam (2mm dose resolution), heterogeneity correction and dose volume constraint (UCLA) for PTV D95%=40Gy and D98%>39.2Gy, Rectum V20Gy 39.2Gy) and should have been renormalized. Dose volume constraint differences for rectum, bladder, femoral heads and penile bulb were les than 2% and within tolerances. Urethra region and overlapping between PTV and PRV Rectum shows increment of dose in all plans. The average difference for urethra region was 2.1% with a maximum of 7.8% and for the overlapping region 2.5% with a maximum of 8.7%. Conclusion: Monte Carlo dose calculation on dynamic IMRT treatments could affects on plan normalization. Dose increment in critical region of urethra and PTV overlapping region with PTV could have clinical consequences which need to be studied. The use of Monte Carlo dose calculation algorithm is limited because inverse planning dose optimization use only pencil beam.

TH‐E‐BRE‐11: Adaptive‐Beamlet Based Finite Size Pencil Beam (AB‐FSPB) Dose Calculation Algorithm for Independent Verification of IMRT and VMAT

Purpose:In current IMRT and VMAT settings, the use of sophisticated dose calculation procedure is inevitable in order to account complex treatment field created by MLCs. As a consequence, independent volumetric dose verification procedure is time consuming which affect the efficiency of clinical workflow. In this study, the authors present an efficient Pencil Beam based dose calculation algorithm that minimizes the computational procedure while preserving the accuracy.Methods:The computational time of Finite Size Pencil Beam (FSPB) algorithm is proportional to the number of infinitesimal identical beamlets that constitute the arbitrary field shape. In AB‐FSPB, the dose distribution from each beamlet is mathematically modelled such that the sizes of beamlets to represent arbitrary field shape are no longer needed to be infinitesimal nor identical. In consequence, it is possible to represent arbitrary field shape with combinations of different sized and minimal number of beamlets.Results:On comparing FSPB with AB‐FSPB, the complexity of the algorithm has been reduced significantly. For 25 by 25 cm2 squared shaped field, 1 beamlet of 25 by 25 cm2 was sufficient to calculate dose in AB‐FSPB, whereas in conventional FSPB, minimum 2500 beamlets of 0.5 by 0.5 cm2 size were needed to calculate dose that was comparable to the Result computed from Treatment Planning System (TPS). The algorithm was also found to be GPU compatible to maximize its computational speed. On calculating 3D dose of IMRT (∼30 control points) and VMAT plan (∼90 control points) with grid size 2.0 mm (200 by 200 by 200), the dose could be computed within 3∼5 and 10∼15 seconds.Conclusion:Authors have developed an efficient Pencil Beam type dose calculation algorithm called AB‐FSPB. The fast computation nature along with GPU compatibility has shown performance better than conventional FSPB. This completely enables the implantation of AB‐FSPB in the clinical environment for independent volumetric dose verification.