Gamma Index Comparisons Research Articles

Yet anOther Dose Algorithm (YODA) for independent computations of dose and dose changes due to anatomical changes

Objective. To assess the viability of a physics-based, deterministic and adjoint-capable algorithm for performing treatment planning system independent dose calculations and for computing dosimetric differences caused by anatomical changes. Approach. A semi-numerical approach is employed to solve two partial differential equations for the proton phase-space density which determines the deposited dose. Lateral hetereogeneities are accounted for by an optimized (Gaussian) beam splitting scheme. Adjoint theory is applied to approximate the change in the deposited dose caused by a new underlying patient anatomy. Main results. The dose engine’s accuracy was benchmarked through three-dimensional gamma index comparisons against Monte Carlo simulations done in TOPAS. For a lung test case, the worst passing rate with (1 mm, 1%, 10% dose cut-off) criteria is 94.55%. The effect of delivering treatment plans on repeat CTs was also tested. For non-robustly optimized plans the adjoint component was accurate to 5.7% while for a robustly optimized plan it was accurate to 4.8%. Significance. Yet anOther Dose Algorithm is capable of accurate dose computations in both single and multi spot irradiations when compared to TOPAS. Moreover, it is able to compute dosimetric differences due to anatomical changes with small to moderate errors thereby facilitating its use for patient-specific quality assurance in online adaptive proton therapy.

RapidBrachyTG43: A Geant4-based TG-43 parameter and dose calculation module for brachytherapy dosimetry.

The AAPM TG-43U1 formalism remains the clinical standard for dosimetry of low- and high-energy -emitting brachytherapy sources. TG-43U1 and related reports provide consensus datasets of TG-43 parameters derived from various published measured data and Monte Carlo simulations. These data are used to perform standardized and fast dose calculations for brachytherapy treatmentplanning. Monte Carlo TG-43 dosimetry parameters are commonly derived to characterize novel brachytherapy sources. RapidBrachyTG43 is a module of RapidBrachyMCTPS, a Monte Carlo-based treatment planning system, designed to automate this process, requiring minimal user input to prepare Geant4-based Monte Carlo simulations for a source. RapidBrachyTG43 may also perform a TG-43 dose to water-in-water calculation for a plan, substantially accelerating the same calculation performed using RapidBrachyMCTPS's Monte Carlo dose calculationengine. TG-43 parameters , , , and were calculated using three commercial source models, one each of I, Ir, and Co, and were benchmarked to published data. TG-43 dose to water was calculated for a clinical breast brachytherapy plan and was compared to a Monte Carlo dose calculation with all patient tissues, air, and catheters set towater. TG-43 parameters for the three simulated sources agreed with benchmark datasets within tolerances specified by the High Energy Brachytherapy Dosimetry working group. A gamma index comparison between the TG-43 and Monte Carlo dose-to-water calculations with a dose difference and difference to agreement criterion of 1%/1 mm yielded a 98.9% pass rate, with all relevant dose volume histogram metrics for the plan agreeing within 1%. Performing a TG-43-based dose calculation provided an acceleration of dose-to-water calculation by a factor of165. Determination of TG-43 parameter data for novel brachytherapy sources may now be facilitated by RapidBrachyMCTPS. These parameter datasets and existing consensus or published datasets may also be used to determine the TG-43 dose for a plan inRapidBrachyMCTPS.

An open-source software for calculating 1D gamma index in radiation therapy

PurposeThis study was developed to create computer software for performing the gamma index comparison between measurement and Monte Carlo (MC) simulation for the percentage depth dose (PDD) and beam off-center ratio profile (OCR). Materials and methodsThe gamma software was built in the matrix laboratory (MATLAB) software environment. The developed software was compared with ScanDosematch and Bistromath software’s gamma evaluation to assess its accuracy. A set of reference and evaluated dose distribution, which were obtained from measurement and MC simulation, was input to the software to calculate the 1D gamma index using different criteria (i.e. 3%/3 mm, 2%/3 mm, and 2%/2 mm). ResultsWe compared the two results of gamma index at 3%/3 mm, 2%/3 mm, and 2%/2 mm criteria, one calculated by the proposed software and one manually. The comparison showed high agreement between the proposed software and theoretical calculation. ConclusionsBased on the results, we concluded that our developed software has high accuracy, compared to theoretical calculation. This software could serve as a non-commercial and open-source tool for researchers and students.

Validation of Monte Carlo multithreading TOpas 3.6 software in order to simulate the 6 MV Elekta Synergy MLCi2 platform linac.

Monte Carlo simulation is generally appreciated as an extraordinary technique to investigate particle physics processes and interactions in nuclear medicine and Radiation Therapy. The present task validates a new methodology of Monte Carlo simulation based on the Multithreading technique to reduce CPU time to simulate a 6 MV photon beam provided by the Elekta Synergy MLCi2 platform medical linear accelerator treatment head utilizing TOpas version 3.6 Monte Carlo software and the Slurm Marwan cluster. The simulation includes the linear accelerator (LINAC) major components. Calculations are performed for the photon beam with several treatment field sizes varying from 3 × 3 to 10 × 10 cm2 at a 100 cm of distance from the source to the surface of the IBA dosimetry water box. The simulation was wholly approved by comparison with experimental distributions. To evaluate simulation accuracy, gamma index formalism for (2%/2mm) and (3%/2mm) criteria, Distance To Agreement DTA, and the estimator standard error ɛ and ɛ max are used. Good agreement between simulations and measurements was observed for depth doses and lateral dose profiles, respectively. The gamma index comparisons also highlighted this agreement; more than 97% of the points for all simulations satisfy the quality assurance criteria of (2%/2mm). Regarding calculation performance, the event processing speed is faster using Gate-[mp] compared to TOpas-[mt] mode when running the identical simulation code for both. Consequently, according to the achieved results, the proposed methodology shows the first validation of TOpas in radiotherapy linacs simulations and a reduction in calculation time, capping simulation accuracy as much as possible. For this reason, this software is recommended to be serviceable for Treatment Planning Systems (TPS) purposes.

Experimental validation of a linac head Geant4 model under a grid computing environment

Background and purpose . This work aims to present the strategy to simulate a clinical linear accelerator based on the geometry provided by the manufacturer and summarize the corresponding experimental validation. Simulations were performed with the Geant4 Monte Carlo code under a grid computing environment. The objective of this contribution is reproducing therapeutic dose distributions in a water phantom with an accuracy less than 2%. Materials and methods. A Geant4 Monte Carlo model of an Elekta Synergy linear accelerator has been established, the simulations were launched in a large grid computing platform. Dose distributions are calculated for a 6 MV photon beam with treatment fields ranging from 5 × 5 cm2 to 20 × 20 cm2 at a source—surface distance of 100 cm. Results. A high degree of agreement is achieved between the simulation results and the measured data, with dose differences of about 1.03% and 1.96% for the percentage depth dose curves and lateral dose profiles, respectively. This agreement is evaluated by the gamma index comparisons. Over 98% of the points for all simulations meet the restrictive acceptability criteria of 2%/2 mm. Conclusion. We have demonstrated the possibility to establish an accurate linac head Monte Carlo model for dose distribution simulations and quality assurance. Percentage depth dose curves and beam quality indices are in perfect agreement with the measured data with an accuracy of better than 2%.

Characterization of a novel 3D printed patient specific phantom for quality assurance in cranial stereotactic radiosurgery applications

In single-isocenter stereotactic radiosurgery/radiotherapy (SRS/SRT) intracranial applications, multiple targets are being treated concurrently, often involving non-coplanar arcs, small photon beams and steep dose gradients. In search for more rigorous quality assurance protocols, this work presents and evaluates a novel methodology for patient-specific pre-treatment plan verification, utilizing 3D printing technology.In a patient’s planning CT scan, the external contour and bone structures were segmented and 3D-printed using high-density bone-mimicking material. The resulting head phantom was filled with water while a film dosimetry insert was incorporated. Patient and phantom CT image series were fused and inspected for anatomical coherence. HUs and corresponding densities were compared in several anatomical regions within the head. Furthermore, the level of patient-to-phantom dosimetric equivalence was evaluated both computationally and experimentally. A single-isocenter multi-focal SRS treatment plan was prepared, while dose distributions were calculated on both CT image series, using identical calculation parameters. Phantom- and patient-derived dose distributions were compared in terms of isolines, DVHs, dose-volume metrics and 3D gamma index (GI) analysis. The phantom was treated as if the real patient and film measurements were compared against the patient-derived calculated dose distribution.Visual inspection of the fused CT images suggests excellent geometric similarity between phantom and patient, also confirmed using similarity indices. HUs and densities agreed within one standard deviation except for the skin (modeled as ‘bone’) and sinuses (water-filled). GI comparison between the calculated distributions resulted in passing rates better than 97% (1%/1 mm). DVHs and dose-volume metrics were also in satisfying agreement. In addition to serving as a feasibility proof-of-concept, experimental absolute film dosimetry verified the computational study results. GI passing rates were above 90%.Results of this work suggest that employing the presented methodology, patient-equivalent phantoms (except for the skin and sinuses areas) can be produced, enabling literally patient-specific pre-treatment plan verification in intracranial applications.

Clinical Evaluation of a Two-dimensional Liquid-Filled Ion chamber Detector Array for Verification of High Modulation Small Fields in Radiotherapy.

Introduction:Clinical evaluation of a two-dimensional (2D) liquid-filled ion chamber detector array used in the verification of highly modulated small beams of stereotactic body radiation therapy (SBRT) has been conducted.Materials and Methods:Measurements with the Octavius 1000 SRS (PTW, Freiburg, Germany) detector with 977 liquid-filled ion chambers were compared against EDR2 film and PTW Octavius Seven29. The performance of detector array has been evaluated on ten SBRT patient plans. Dose profiles of individual and composite fields' calculated using Pinnacle3 treatment planning system were compared against measurements with Octavius 1000 SRS detector array, EDR2 film, and Octavius Seven29 detector. Gamma index and profile comparison were used in the evaluation and assessment of the detector's performance.Results:The Gamma index measurements show agreement between Pinnacle3 computations and Octavius 1000 SRS array, PTW Octavius Seven29, and EDR2 film for >90% of the points using 2%, 2 mm tolerance criteria. Profiles obtained with the Octavius 1000 SRS were in agreement with the EDR2 film profiles, demonstrating the detector's superior sampling rate.Conclusions:The Octavius 1000 SRS is a dosimetrically accurate device to perform quality assurance checks in SBRT treatments. The broad range of measurements performed in this study quantified the dosimetric accuracy of Octavius 1000 SRS detector in the clinical setup of the small fields in radiotherapy.

P249] Influence of gamma index comparison on differences in dose calculation algorithm deposition method

Purpose Phantomless patient specific QA saves time and is able to detect errors by comparing the TPS dose distribution with a secondary independent dose calculation. TG 218 recommends that gamma analysis should be performed on metrics such as, mean gamma, γ > 1 and γ > 1.5, per structure. The aim of this study is to investigate how gamma index comparison is affected by differences on dose calculation algorithms when treating bone metastases. Methods 10 patients treated for bone metastases in different sites were chosen and 3D and VMAT plans were created using Elekta Monaco TPS in a Versa linac and 10 MV photons. Both 3D and VMAT dose distributions were calculated with Monaco XVMC Monte Carlo (MC) calculation algorithm using dose to medium in medium MC(Dm,m) and dose to water in medium MC(Dw,m) dose deposition modes. All plans were also calculated and evaluated using IBA Compass incorporating a Collapsed Cone convolution superposition algorithm calculating dose to water in medium CC(Dw,m). 3D dose distribution evaluation was performed in terms of gamma metrics such as mean gamma values and % PTV with γ > 1 and γ > 1.5 using 3%, 3 mm criteria for the following combinations: (a) MC(Dw,m) vs CC(Dw,m) and (b) MC(Dm,m) vs CC(Dw,m). Results For the 3D plans, average mean gamma values for MC(Dw,m) vs CC(Dw,m) and MC(Dm,m) vs CC(Dw,m) were found 0.37 ± 0.1 and 0.52 ± 0.12 respectively. The average % PTV points with γ > 1 were found 6.34 ± 5.7% and 12.6 ± 7.5% and the average % PTV points with γ > 1.5 1.45 ± 1.87% and 2.21 ± 2.28%, respectively. Corresponding values for the VMAT plans were 0.46 ± 0.12 and 0.68 ± 0.09 (mean gamma values), 9.48 ± 7.26% and 21.5 ± 7.3% (% PTV points with γ > 1) and 2.37 ± 2.8% and 7.95 ± 7.4% (% PTV points with γ > 1.5). Conclusions Gamma index comparison revealed that both MC and CC calculation algorithms are in fair agreement in dose calculation determination providing that the same dose deposition mode (i.e. Dw,m or Dm,m) is used. The use of different dose deposition mode, results in increased differences in dose calculation determination in structures/areas where materials different than water, such as bone, is included. The observed differences are also increased as the complexity of the plan increases (e.g. VMAT vs 3D).

Treatment plan quality and delivery accuracy assessments on 3 IMRT delivery methods of stereotactic body radiotherapy for spine tumors

Stereotactic body radiotherapy (SBRT) for spine tumors has demonstrated clinical effectiveness. The treatment planning and delivery techniques have evolved from dynamic conformal arc therapy, to fixed gantry angle intensity modulated radiotherapy (IMRT), and most recently to volumetric modulated arc therapy (VMAT). A hybrid-arc (HARC) planning and delivery method combining dynamic conformal arc therapy delivery with a number of equally spaced IMRT beams is proposed. In this study we investigated plan quality, delivery accuracy, and efficiency of 3 delivery techniques: IMRT, HARC, and VMAT. Patients who underwent spine SBRT treatments were randomly selected from an Institutional Review Board-approved registry. For each patient, the prescription dose was 14 to 16 Gy in a single fraction to cover >90% of the tumor (without planning margin) while constraining V10Gy ≤ 10% of the spinal cord and the maximum point dose (MPD) of the spinal cord ≤ 14 Gy. All cases were clinically treated with fixed gantry step-shoot IMRT plans and then re-planned with VMAT using Pinnacle 9.0 and with HARC using Brainlab iPlan 4.5. Student t-test was used to compare the dosimetric end points, including V16Gy to the planning target volume, homogeneity index, MPDPTV, the conformity index, V10Gy of the spinal cord, and MPDcord. To compare the accuracy of delivery, we delivered all plans on a phantom and conducted gamma index (GI) comparisons with 3 mm/3% and 2 mm/2% criteria. All plans met our clinical requirements. Among 3 techniques, there were no differences on dose coverage to the tumor volume, maximum dose to the spinal cord, and plan homogeneity index (p > 0.05). The average V10Gy of the spinal cord was 6.66 ± 0.03%, 5.49 ± 0.03%, and 4.76 ± 0.02% for IMRT, HARC, and VMAT plans, respectively. Accordingly, the conformity indices were 1.30 ± 0.11 and 1.29 ± 0.20, 1.53 ± 0.29, respectively. VMAT plans were significantly (p < 0.05) less conformal but significantly (p < 0.05) lower V10Gy of the spinal cord than those from HARC and IMRT plans. With delivery accuracy measured by GIs, the average GIs of 3%/3 mm were 92.6 ± 1.1%, 96.5 ± 2.7%, 99.0 ± 1.1% for IMRT, HARC, and VMAT plans, respectively. The differences were significant (p < 0.05). Accordingly, the average monitor units were 9238 ± 2242, 9853 ± 2548 and 5091 ± 910. The plan quality created from the 3 planning techniques can meet the clinical requirement. Adding arc beams in delivery such as in HARC and VMAT plans improves the delivery accuracy. VMAT is the most efficient delivery method.

12. Comparative study of two methods of Patient Quality Control: PDIP vs Delta4

Introduction Modern irradiation techniques such as IMRT and Volumetric Modulated Arctherapy require a quality assurance plan for the systematic control of treatment plans. In 2010, IMRT began in our radiotherapy department, two controls was set up, one with the Delta4 (Scandidos©) and the other with the PDIP module (VarianMedicalSystem©). This study compares the results of the quality tests carried out by means of two detectors. The objective is to lighten the quality assurance procedure implemented in order to reduce the delivery time in charge of patients as well as the duration of immobilization of accelerators dedicated to quality controls. Methods The Delta4 is a 3D detector measuring the dose according to two perpendicular planes. The PDIP, an algorithm developed by Varian, measures a dose image with the portal imager. These two detectors use a Gamma-index comparison between the calculated dose and the measured dose. The analysis carried out on 245 patients and 719 beams. 186 patients were treated by VMAT: 51% for head and neck cancer, 24% for pelvic cancer, 19% for prostate cancer, and 6% for other sites. IMRT was studied for 53 patients treated for a brain tumor. Gamma index criteria of 3% 3 mm with a passing threshold of 97% was used for PDIP and Delta4 analysis The implementation time (preparation, measurement, analysis) of these two controls was evaluated in a second stage. Results The beams analyzed for Gamma-Index met the criterion 3% 3 mm in 89% of the cases for the Delta4 and in 95% of the cases for the PDIP. The boxplot (Fig. 1) shows that there are no significant differences between the two detectors. Dispersion of the results is similar. The duration of the Delta4 control is estimated to 24 min versus 8 min for PDIP. Download : Download high-res image (92KB) Download : Download full-size image Conclusions In this work, there is no difference between the two detectors. As a result, it was decided to conserve double-checking to locations without sufficient experience and for doses per fraction greater than routine practice. Only PDIP is maintained for IMRT and VMAT conventional treatments. This modification reduces the times of control and optimizes the patient treatment workflow.

Monte Carlo dose calculation in presence of low-density media: Application to lung SBRT treated during DIBH

PurposeCommercial algorithms used in Radiotherapy include approximations that are generally acceptable. However their limits can be seen when confronted with small fields and low-density media. These conditions exist during the treatment of lung cancers with Stereotactic Body Radiation Therapy (SBRT) achieved with the “Deep Inspiration Breath Hold” (DIBH) technique. A Monte Carlo (MC) model of a linear accelerator was used to assess the performance of two algorithms (Varian Acuros and AAA) in these conditions. This model is validated using phantoms with different densities. Lastly, results for SBRT cases are compared to both Acuros and AAA. MethodsA Varian TrueBeam linac was modeled using GATE/Geant4 and validated by comparing dose distributions for simple fields to measurements in water and in heterogeneous phantoms composed of PMMA and two types of cork (corresponding to lung densities during free-breathing and DIBH). Experimental measurements are also compared to AAA and Acuros. Finally, results of Acuros/AAA are compared to MC for a clinical case (SBRT during DIBH). ResultsBased on 1D gamma index comparisons with measurements in water, the TrueBeam model was validated (>97% of points passed this test). In heterogeneous phantoms, and in particular for small field sizes, very low density (0.12g.cm−3) and at the edge of the field, MC model was still in good agreement with measurements whilst AAA and Acuros showed discrepancies. With the patient CT, similar differences between MC and AAA/Acuros were observed for static fields but disappeared using an SBRT arc field. ConclusionsOur MC model is validated and limits of commercial algorithms are shown in very low densities.

Dose warping performance in deformable image registration in lung

PurposeIt is unclear that spatial accuracy can reflect the impact of deformed dose distribution. In this study, we used dosimetric parameters to compare an in-house deformable image registration (DIR) system using NiftyReg, with two commercially available systems, MIM Maestro (MIM) and Velocity AI (Velocity). MethodsFor 19 non-small-cell lung cancer patients, the peak inspiration (0%)-4DCT images were deformed to the peak expiration (50%)-4DCT images using each of the three DIR systems, which included computation of the deformation vector fields (DVF). The 0%-gross tumor volume (GTV) and the 0%-dose distribution were also then deformed using the DVFs. The agreement in the dose distributions for the GTVs was evaluated using generalized equivalent uniform dose (gEUD), mean dose (Dmean), and three-dimensional (3D) gamma index (criteria: 3mm/3%). Additionally, a Dice similarity coefficient (DSC) was used to measure the similarity of the GTV volumes. ResultsDmean and gEUD demonstrated good agreement between the original and deformed dose distributions (differences were generally less than 3%) in 17 of the patients. In two other patients, the Velocity system resulted in differences in gEUD of 50.1% and 29.7% and in Dmean of 11.8% and 4.78%. The gamma index comparison showed statistically significant differences for the in-house DIR vs. MIM, and MIM vs. Velocity. ConclusionsThe finely tuned in-house DIR system could achieve similar spatial and dose accuracy to the commercial systems. Care must be taken, as we found errors of more than 5% for Dmean and 30% for gEUD, even with a commercially available DIR tool.

A portal dosimetry dose prediction method based on collapsed cone algorithm using the clinical beam model.

Amorphous silicon electronical portal imaging devices (EPIDs) are widely used for dosimetric measurements in Radiation Therapy. The purpose of this work was to determine if a portal dose prediction method can be utilized for dose map calculations based on the linear accelerator model within a commercial treatment planning system (Pinnacle3 v8.0m). The method was developed for a 6MV photon beam on the Varian Clinac 21-EX, at a nominal dose rate of 400MU/min. The Varian aS1000 EPID was unmounted from the linear accelerator and scanned to acquire CT images of the EPID. The CT images were imported into Pinnacle3 and were used as a quality assurance phantom to calculate dose on the EPID setup at a source to detector distance of 105cm. The best match of the dose distributions was obtained considering the image plane located at 106cm from the source to detector plane. The EPID was calibrated according to the manufacturer procedure and corrections were made for output factors. Arm-backscattering effect, based on profile correction curves, has been introduced. Five low-modulated and three high-modulated clinical planned treatments were predicted and measured with the method presented here and with MatriXX (IBA Dosimetry, Schwarzenbruck, Germany). A portal dose prediction method based on Pinnacle3 was developed without modifying the commissioned parameters of the model in use in the clinic. CT images of the EPID were acquired and used as a quality assurance phantom. The CT images indicated a mean density of 1.16g/cm3 for the sensitive area of the EPID. Output factor measured with the EPID were lower for small fields and larger for larger fields (beyond 10×10cm2 ). Arm-backscatter correction showed a better agreement at the target side of the EPID. Analysis of Gamma index comparison (3%, 3mm) indicated a minimum of 97.4% pass rate for low modulated and 98.3% for high modulated treatments. Pass rates were similar for MatriXX measurements. The method developed here can be easily implemented into clinic, as neither additional modeling of the clinical energy nor an independent image prediction algorithm are necessary. The main advantage of this method is that portal dose prediction is calculated with the same algorithm and beam model used for patient dose distribution calculation. This method was independently validated with an ionization chamber matrix.

A method to enhance 2D ion chamber array patient specific quality assurance for IMRT.

Gamma index comparison has been established as a method for patient specific quality assurance in IMRT. Detector arrays can replace radiographic film systems to record 2D dose distributions and fulfill quality assurance requirements. These electronic devices present spatial resolution disadvantages with respect to films. This handicap can be partially overcome with a multiple acquisition sequence of adjacent 2D dose distributions. The detector spatial response influence can also be taken into account through the convolution of the calculated dose with the detector spatial response. A methodology that employs both approaches could allow for enhancements of the quality assurance procedure. 35 beams from different step and shoot IMRT plans were delivered on a phantom. 2D dose distributions were recorded with a PTW-729 ion chamber array for individual beams, following the multiple acquisition methodology. 2D dose distributions were also recorded on radiographic films. Measured dose distributions with films and with the PTW-729 array were processed with the software RITv5.2 for Gamma index comparison with calculated doses. Calculated dose was also convolved with the ion chamber 2D response and the Gamma index comparisons with the 2D dose distribution measured with the PTW-729 array was repeated. 3.7±2.7% of points surpassed the accepted Gamma index when using radiographic films compared with calculated dose, with a minimum of 0.67 and a maximum of 13.27. With the PTW-729 multiple acquisition methodology compared with calculated dose, 4.1±1.3% of points surpassed the accepted Gamma index, with a minimum of 1.44 and a maximum of 11.26. With the PTW- multiple acquisition methodology compared with convolved calculated dose, 2.7±1.3% of points surpassed the accepted Gamma index, with a minimum of 0.42 and a maximum of 5.75. The results obtained in this work suggest that the comparison of merged adjacent dose distributions with convolved calculated dose represents an enhancement in the methodology for IMRT patient specific quality assurance with the PTW-729 ion chamber array.

Monte carlo simulation of a 6 MV varian LINAC photon beam using GEANT4-GATE code

Introduction Monte Carlo simulation of radiation transport is considered to be one of the most accurate methods of radiation therapy dose calculation. Various codes have been used during the years and have been validated for clinical photon beams. GEANT4 Monte Carlo simulation code is originally created for high energy physics and GATE package for the simulation of SPECT/PET systems. New platform of GATE incorporate simulation processes for clinical photon beams by linear accelerators. Purpose The aim of this work is the validation of a GEANT4-based GATE Monte Carlo (MC) 6 MV photon beam delivered by a medical linear accelerator. Materials and methods The head of the medical linear accelerator (VARIAN CLINAC 23 EX) of the Metropolitan Hospital was simulated based on the manufacturer’s detailed information using the Gate v.7.0 GEANT4 MC code. Two-phase simulation process has been followed: calculation of the phase space (PH) from the linac head and PH transport to a water phantom. The results were compared with measured PDD and OAR data for different field sizes and the simulated model was validated. Results The simulated and measured dose distributions were in good agreement, in the order of 1%. The statistical relative uncertainty was below of 0.6% for a 10 × 10 cm 2 field in all dosels. Gamma index comparisons were performed: more than 90% simulated points passed the clinical 3%/3 mm gamma criterion. Discussion and Conclusion The GATE v.7.0 Monte Carlo simulation code has been validated for 6 MV photon beam of a VARIAN medical linear accelerator.

Detection of IMRT delivery errors based on a simple constancy check of transit dose by using an EPID

Beam delivery errors during intensity modulated radiotherapy (IMRT) were detected based on a simple constancy check of the transit dose by using an electronic portal imaging device (EPID). Twenty-one IMRT plans were selected from various treatment sites, and the transit doses during treatment were measured by using an EPID. Transit doses were measured 11 times for each course of treatment, and the constancy check was based on gamma index (3%/3 mm) comparisons between a reference dose map (the first measured transit dose) and test dose maps (the following ten measured dose maps). In a simulation using an anthropomorphic phantom, the average passing rate of the tested transit dose was 100% for three representative treatment sites (head & neck, chest, and pelvis), indicating that IMRT was highly constant for normal beam delivery. The average passing rate of the transit dose for 1224 IMRT fields from 21 actual patients was 97.6% ± 2.5%, with the lower rate possibly being due to inaccuracies of patient positioning or anatomic changes. An EPIDbased simple constancy check may provide information about IMRT beam delivery errors during treatment.

Gamma index comparison of three VMAT QA systems and evaluation of their sensitivity to delivery errors

PurposeTo compare detectors for dosimetric verification before VMAT treatments and evaluate their sensitivity to errors. Methods and materialsMeasurements using three detectors (ArcCheck, 2d array 729 and EPID) were used to validate the dosimetric accuracy of the VMAT delivery. Firstly, performance of the three devices was studied. Secondly, to assess the reliability of the detectors, 59 VMAT treatment plans from a variety of clinical sites were considered. Thirdly, systematic variations in collimator, couch and gantry angle plus MLC positioning were applied to four clinical treatments (two prostate, two head and neck cases) in order to establish the detection sensitivity of the three devices. Measurements were compared with TPS computed doses via gamma analysis (3%/3 mm and 2%/2 mm) with an agreement of at least 95% and 90% respectively in all pixels. Effect of the errors on the dose distributions was analyzed. ResultsRepeatability and reproducibility were excellent for the three devices. The average pass rate for the 59 cases was superior to 98% for all devices with 3%/3 mm criteria. It was found that for the plans delivered with errors, the sensitivity was quite similar for all devices. Devices were able to detect a 2 mm opened or closed MLC error with 3%/3 mm tolerance level. An error of 3° in collimator, gantry or couch rotation was detected by the three devices using 2%/2 mm criteria. ConclusionsAll three devices have the potential to detect errors with more or less the same threshold. Nevertheless, there is no guarantee that pretreatment QA will catch delivery errors.

Simulation of the 6 MV Elekta Synergy Platform linac photon beam using Geant4 Application for Tomographic Emission.

The present work validates the Geant4 Application for Tomographic Emission Monte Carlo software for the simulation of a 6 MV photon beam given by Elekta Synergy Platform medical linear accelerator treatment head. The simulation includes the major components of the linear accelerator (LINAC) with multi-leaf collimator and a homogeneous water phantom. Calculations were performed for the photon beam with several treatment field sizes ranging from 5 cm × 5 cm to 30 cm × 30 cm at 100 cm distance from the source. The simulation was successfully validated by comparison with experimental distributions. Good agreement between simulations and measurements was observed, with dose differences of about 0.02% and 2.5% for depth doses and lateral dose profiles, respectively. This agreement was also emphasized by the Kolmogorov–Smirnov goodness-of-fit test and by the gamma-index comparisons where more than 99% of the points for all simulations fulfill the quality assurance criteria of 2 mm/2%.

SU‐E‐T‐06: A Comparison of IMRT Treatment of Esophageal Carcinoma in Elekta‐Precise and Varian23EX Linac

Purpose:To compare and analyze the characteristics of static intensity‐modulated radiotherapy (IMRT) plans designed on Elekta and Varian Linac in different esophageal cancer(EC), exploring advantages and disadvantages of different vendor Linac, thus can be better serve for clinical.Methods:Twenty‐four patients with EC were selected, including 6 cases located in the cervical, upper, middle and the lower thorax, respectively. Two IMRT plans were generated with the Oncentra planning system: in Elekta and Varian Linac, prescription dose of 60Gy in 30 fractions to the PTV. We examined the dose‐volume histogram parameters of PTV and the organs at risk (OAR) such as lungs, spinal cord and heart, and additional Monitor units(MU), treatment time, Homogeneity index(HI), Conformity index(CI) and Gamma index comparisons were performed.Results:All plans resulted in abundant dose coverage of PTV for EC of different locations. The doses to PTV, HI and OAR in Elekta plans were not statistically different in comparison with Varian plans, with the following exceptions: in cervical, upper and lower thoracic EC the PTV's CI, and in middle thorax EC PTV's D2, D50, V105 and PTV‐average were better in Elekta plans than in Varian plans. In the cervical, upper and the middle thorax EC, treatment time were significantly decreased in Varian plans as against Elekta plans, while in the lower thoracic EC treatment time were no striking difference. MUs and gamma index were similar between the two Linac plans.Conclusion:For the the middle thorax EC Varian plans is better than Elekta plans, not only in treatment time but in the PTV dose; while for the lower thorax EC Elekta plans is the first choice for better CI; for the other part of the EC usually Elekta plans can increase the CI, while Varian plans can reduce treatment time, can be selected according to the actual situation of the patient treatment.

A methodology for dosimetry audit of rotational radiotherapy using a commercial detector array

PurposeTo develop a methodology for the use of a commercial detector array in dosimetry audits of rotational radiotherapy. Materials and methodsThe methodology was developed as part of the development of a national audit of rotational radiotherapy. Ten cancer centres were asked to create a rotational radiotherapy treatment plan for a three-dimensional treatment-planning-system (3DTPS) test and audited. Phantom measurements using a commercial 2D ionisation chamber (IC) array were compared with measurements using 0.125cm3 IC, Gafchromic film and alanine pellets in the same plane. Relative and absolute gamma index (γ) comparisons were made for Gafchromic film and 2D-Array planes, respectively. ResultsComparisons between individual detectors within the 2D-Array against the corresponding IC and alanine measurement showed a statistically significant concordance correlation coefficient (both ρc>0.998, p<0.001) with mean difference of −1.1±1.1% and −0.8±1.1%, respectively, in a high dose PTV. In the γ comparison between the 2D-Array and film it was that the 2D-Array was more likely to fail planes where there was a dose discrepancy due to the absolute analysis performed. ConclusionsIt has been found that using a commercial detector array for a dosimetry audit of rotational radiotherapy is suitable in place of standard systems of dosimetry.