Alanah Bergman

Monte Carlo based, patient-specific RapidArc QA using Linac log files

PURPOSEnA Monte Carlo (MC) based QA process to validate the dynamic beam delivery accuracy for Varian RapidArc (Varian Medical Systems, Palo Alto, CA) using Linac delivery log files (DynaLog) is presented. Using DynaLog file analysis and MC simulations, the goal of this article is to (a) confirm that adequate sampling is used in the RapidArc optimization algorithm (177 static gantry angles) and (b) to assess the physical machine performance [gantry angle and monitor unit (MU) delivery accuracy].nnnMETHODSnTen clinically acceptable RapidArc treatment plans were generated for various tumor sites and delivered to a water-equivalent cylindrical phantom on the treatment unit. Three Monte Carlo simulations were performed to calculate dose to the CT phantom image set: (a) One using a series of static gantry angles defined by 177 control points with treatment planning system (TPS) MLC control files (planning files), (b) one using continuous gantry rotation with TPS generated MLC control files, and (c) one using continuous gantry rotation with actual Linac delivery log files. Monte Carlo simulated dose distributions are compared to both ionization chamber point measurements and with RapidArc TPS calculated doses. The 3D dose distributions were compared using a 3D gamma-factor analysis, employing a 3%/3 mm distance-to-agreement criterion.nnnRESULTSnThe dose difference between MC simulations, TPS, and ionization chamber point measurements was less than 2.1%. For all plans, the MC calculated 3D dose distributions agreed well with the TPS calculated doses (gamma-factor values were less than 1 for more than 95% of the points considered). Machine performance QA was supplemented with an extensive DynaLog file analysis. A DynaLog file analysis showed that leaf position errors were less than 1 mm for 94% of the time and there were no leaf errors greater than 2.5 mm. The mean standard deviation in MU and gantry angle were 0.052 MU and 0.355 degrees, respectively, for the ten cases analyzed.nnnCONCLUSIONSnThe accuracy and flexibility of the Monte Carlo based RapidArc QA system were demonstrated. Good machine performance and accurate dose distribution delivery of RapidArc plans were observed. The sampling used in the TPS optimization algorithm was found to be adequate.

A Monte Carlo approach to validation of FFF VMAT treatment plans for the TrueBeam linac

PURPOSEnTo commission and benchmark a vendor-supplied (Varian Medical Systems) Monte Carlo phase-space data for the 6 MV flattening filter free (FFF) energy mode on a TrueBeam linear accelerator for the purpose of quality assurance of clinical volumetric modulated arc therapy (VMAT) treatment plans. A method for rendering the phase-space data compatible with BEAMnrc/DOSXYZnrc simulation software package is presented.nnnMETHODSnMonte Carlo (MC) simulations were performed to benchmark the TrueBeam 6 MV FFF phase space data that have been released by the Varian MC Research team. The simulations to benchmark the phase space data were done in three steps. First, the original phase space which was created on a cylindrical surface was converted into a format that was compatible with BEAMnrc. Second, BEAMnrc was used to create field size specific phase spaces located underneath the jaws. Third, doses were calculated with DOSXYZnrc in a water phantom for fields ranging from 1 × 1 to 40 × 40 cm(2). Calculated percent depth doses (PDD), transverse profiles, and output factors were compared with measurements for all the fields simulated. After completing the benchmarking study, three stereotactic body radiotherapy (SBRT) VMAT plans created with the Eclipse treatment planning system (TPS) were calculated with Monte Carlo. Ion chamber and film measurements were also performed on these plans. 3D gamma analysis was used to compare Monte Carlo calculation with TPS calculations and with film measurement.nnnRESULTSnFor the benchmarking study, MC calculated and measured values agreed within 1% and 1.5% for PDDs and in-field transverse profiles, respectively, for field sizes >1 × 1 cm(2). Agreements in the 80%-20% penumbra widths were better than 2u2009mm for all the fields that were compared. With the exception of the 1 × 1 cm(2) field, the agreement between measured and calculated output factors was within 1%. It is of note that excellent agreement in output factors for all field sizes including highly asymmetric fields was achieved without accounting for backscatter into the beam monitor chamber. For the SBRT VMAT plans, the agreement between Monte Carlo and ion chamber point dose measurements was within 1%. Excellent agreement between Monte Carlo, treatment planning system and Gafchromic film dose distribution was observed with over 99% of the points in the high dose volume passing the 3%, 3 mm gamma test.nnnCONCLUSIONSnThe authors have presented a method for making the Varian IAEA compliant 6 MV FFF phase space file of the TrueBeam linac compatible with BEAMnrc/DOSXYZnrc. After benchmarking the modified phase space against measurement, they have demonstrated its potential for use in MC based quality assurance of complex delivery techniques.

Monte Carlo modeling of HD120 multileaf collimator on Varian TrueBeam linear accelerator for verification of 6X and 6X FFF VMAT SABR treatment plans

A Monte Carlo (MC) validation of the vendor‐supplied Varian TrueBeam 6 MV flattened (6X) phase‐space file and the first implementation of the Siebers‐Keall MC MLC model as applied to the HD120 MLC (for 6X flat and 6X flattening filterfree (6X FFF) beams) are described. The MC model is validated in the context of VMAT patient‐specific quality assurance. The Monte Carlo commissioning process involves: 1) validating the calculated open‐field percentage depth doses (PDDs), profiles, and output factors (OF), 2) adapting the Siebers‐Keall MLC model to match the new HD120‐MLC geometry and material composition, 3) determining the absolute dose conversion factor for the MC calculation, and 4) validating this entire linac/MLC in the context of dose calculation verification for clinical VMAT plans. MC PDDs for the 6X beams agree with the measured data to within 2.0% for field sizes ranging from 2 × 2 to 40 × 40 cm2. Measured and MC profiles show agreement in the 50% field width and the 80%‐20% penumbra region to within 1.3 mm for all square field sizes. MC OFs for the 2 to 40 cm2 square fields agree with measurement to within 1.6%. Verification of VMAT SABR lung, liver, and vertebra plans demonstrate that measured and MC ion chamber doses agree within 0.6% for the 6X beam and within 2.0% for the 6X FFF beam. A 3D gamma factor analysis demonstrates that for the 6X beam, > 99% of voxels meet the pass criteria (3%/3 mm). For the 6X FFF beam, > 94% of voxels meet this criteria. The TrueBeam accelerator delivering 6X and 6X FFF beams with the HD120 MLC can be modeled in Monte Carlo to provide an independent 3D dose calculation for clinical VMAT plans. This quality assurance tool has been used clinically to verify over 140 6X and 16 6X FFF TrueBeam treatment plans. PACS number: 87.55.K‐

Monte Carlo validation of the TrueBeam 10XFFF phase–space files for applications in lung SABR

PURPOSEnTo establish the clinical acceptability of universal Monte Carlo phase-space data for the 10XFFF (flattening filter free) photon beam on the Varian TrueBeam Linac, including previously unreported data for small fields, output factors, and inhomogeneous media. The study was particularly aimed at confirming the suitability for use in simulations of lung stereotactic ablative radiotherapy treatment plans.nnnMETHODSnMonte Carlo calculated percent depth doses (PDDs), transverse profiles, and output factors for the TrueBeam 10 MV FFF beam using generic phase-space data that have been released by the Varian MC research team were compared with in-house measurements and published data from multiple institutions (ten Linacs from eight different institutions). BEAMnrc was used to create field size specific phase-spaces located underneath the jaws. Doses were calculated with DOSXYZnrc in a water phantom for fields ranging from 1 × 1 to 40 × 40 cm(2). Particular attention was paid to small fields (down to 1 × 1 cm(2)) and dose per pulse effects on dosimeter response for high dose rate 10XFFF beams. Ion chamber measurements were corrected for changes in ion collection efficiency (P(ion)) with increasing dose per pulse. MC and ECLIPSE ANISOTROPIC ANALYTICAL ALGORITHM (AAA) calculated PDDs were compared to Gafchromic film measurement in inhomogeneous media (water, bone, lung).nnnRESULTSnMeasured data from all machines agreed with Monte Carlo simulations within 1.0% and 1.5% for PDDs and in-field transverse profiles, respectively, for field sizes >1 × 1 cm(2) in a homogeneous water phantom. Agreements in the 80%-20% penumbra widths were better than 2 mm for all the fields that were compared. For all the field sizes considered, the agreement between their measured and calculated output factors was within 1.1%. Monte Carlo results for dose to water at water/bone, bone/lung, and lung/water interfaces as well as within lung agree with film measurements to within 2.8% for 10 × 10 and 3 × 3 cm(2) field sizes. This represents a significant improvement over the performance of the ECLIPSE AAA.nnnCONCLUSIONSnThe 10XFFF phase-space data offered by the Varian Monte Carlo research team have been validated for clinical use using measured, interinstitutional beam data in water and with film dosimetry in inhomogeneous media.

Treatment planning for spinal radiosurgery

PurposeTo investigate the quality of treatment plans of spinal radiosurgery derived from different planning and delivery systems. The comparisons include robotic delivery and intensity modulated arc therapy (IMAT) approaches. Multiple centers with equal systems were used to reduce axa0bias based on individual’s planning abilities. The study used axa0series of three complex spine lesions to maximize the difference in plan quality among the various approaches.MethodsInternationally recognized experts in the field of treatment planning and spinal radiosurgery from 12xa0centers with various treatment planning systems participated. For axa0complex spinal lesion, the results were compared against axa0previously published benchmark plan derived for CyberKnife radiosurgery (CKRS) using circular cones only. For two additional cases, one with multiple small lesions infiltrating three vertebrae and axa0single vertebra lesion treated with integrated boost, the results were compared against axa0benchmark plan generated using axa0best practice guideline for CKRS. All plans were rated based on axa0previously established ranking system.ResultsAll 12xa0centers could reach equality (nu202f=u20094) or outperform (nu202f=u20098) the benchmark plan. For the multiple lesions and the single vertebra lesion plan only 5 and 3 of the 12xa0centers, respectively, reached equality or outperformed the best practice benchmark plan. However, the absolute differences in target and critical structure dosimetry were small and strongly planner-dependent rather than system-dependent. Overall, gantry-based IMAT with simple planning techniques (two coplanar arcs) produced faster treatments and significantly outperformed static gantry intensity modulated radiation therapy (IMRT) and multileaf collimator (MLC) or non-MLC CKRS treatment plan quality regardless of the system (mean rank out of 4 was 1.2 vs. 3.1, pu202f=u20090.002).ConclusionsHigh plan quality for complex spinal radiosurgery was achieved among all systems and all participating centers in this planning challenge. This study concludes that simple IMAT techniques can generate significantly better plan quality compared to previous established CKRS benchmarks.ZusammenfassungZielsetzungUntersuchung der Qualität von Behandlungsplänen für die Wirbelsäulen-Stereotaxie, die durch verschiedene Planungs- und Bestrahlungssysteme generiert wurden. Die Arbeit umfasst die robotergestützte Radiochirurgie sowie die intensitätsmodulierte Rotationstherapie (IMAT). Multiple Zentren mit gleichen Systemen wurden eingesetzt, um eine Verzerrung aufgrund individueller Planungsqualitäten zu reduzieren. Die Studie verwendete drei Fälle mit komplexen Wirbelsäulenläsionen, um den Unterschied in der Planqualität zwischen den verschiedenen Ansätzen zu untersuchen.MethodenInternational anerkannte Experten auf dem Gebiet der Behandlungsplanung und der Wirbelsäulen-Radiochirurgie aus 12xa0Zentren nahmen mit unterschiedlichen Planungssystemen teil. Für eine komplexe Wirbelsäulenläsion wurden die Ergebnisse mit einem zuvor publizierten Referenzplan verglichen, der für die CyberKnife-Radiochirurgie (CKRS) erstellt wurde und ausschließlich runde Kegelstrahlen verwendete. Für zwei weitere Fälle – einer mit mehreren kleinen Läsionen, die drei Wirbel infiltrierten, und einer mit einer einzelnen Wirbelkörperläsion, die mit integriertem Boost behandelt wurde – wurden die Ergebnisse mit einem Referenzplan verglichen, der unter Verwendung einer „Best-Practice“-Richtlinie entstanden ist. Alle Pläne wurden auf Basis eines zuvor etablierten Rankingsystems bewertet.ErgebnisseAlle 12xa0Zentren erreichen entweder die gleiche Planqualität (nu202f=u20094) oder übertrafen den Referenzplan (nu202f=u20098). Für die multiplen Läsionen und den Einzelwirbel-Läsionsplan erreichten jeweils nur 5 bzw. 3 der 12xa0Zentren eine gleiche oder bessere Planqualität als der „Best-Practice“-Referenzplan. Die absoluten Unterschiede in der Dosisverteilung im Zielvolumen und in den kritischen Strukturen waren jedoch klein und stark vom individuellen Planer und nicht vom System abhängig. Insgesamt führte die Gantry-basierte IMAT mit einfachen Planungstechniken (zwei koplanare Arcs) zu schnelleren Behandlungen und deutlich besseren Ergebnissen in der Planqualität als die statische Gantry-basierte Intensitätsmodulierte Strahlentherapie (IMRT) oder die Multilamellenkollimator (MLC)- und nicht-MLC-basierte CKRS unabhängig vom System (mittlerer Rank 1,2 vs. 3,1 von 4; pu202f=u20090,002).SchlussfolgerungBei dieser Planungsherausforderung für komplexe Wirbelsäulenradiochirurgie wurde eine hohe Planqualität unter allen Systemen und allen beteiligten Zentren erreicht. Die Studie schlussfolgert, dass einfache IMAT-Techniken deutlich bessere Planqualitäten im Vergleich zu früheren etablierten CKRS-Benchmarks generieren können.

SU‐E‐T‐411: Monte Carlo Simulations for Quality Assurance of Varian TrueBeam 10XFFF VMAT SABR Treatments

PURPOSEnTo establish feasibility of performing quality assurance on 10X Flattening Filter Free (FFF) VMAT stereotactic ablative radiotherapy (SABR) treatments on the TrueBeam LINAC with Monte Carlo simulations using vendor-supplied phase space data.nnnMETHODSnMonte Carlo simulations were performed with BEAMnrc and DOSXYZnrc using the TrueBeam 10XFFF photon beam phase space data that were made available by the Varian Monte Carlo research team. To establish validity of the phase space, dose calculations in a water phantom for fields ranging from 2×2 cm2 to 40×40 cm2 were performed using DOSXYZnrc. Percent depth doses (PDDs), transverse profiles and output factors were calculated and compared with measurements for all the fields simulated. Monte Carlo simulations of 10XFFF SABR VMAT plans were performed on a homogeneous cylindrical phantom and on the Quasar™ phantom with a lung-equivalent insert. Simulations were compared with both ion chamber measurement and Eclipse Treatment Planning System (TPS) dose calculations. 3D gamma analysis comparing Monte Carlo and TPS results was performed.nnnRESULTSnMonte Carlo simulations and measured values agreed within 1% and 2% for PDDs and profiles respectively. The agreement between measured and calculated output factors was within 1% including for highly asymmetric fields. These results indicate that the 10XFFF phase space data is sufficiently accurate for use in simulations for quality assurance purposes in radiation therapy. For the VMAT plans the point dose agreement between MC and both measured and TPS calculations was within 1.2%. The 3%, 3mm Gamma test pass rates were 95% and 92% for plans calculated on the homogeneous phantom and on the Quasar phantom respectively.nnnCONCLUSIONnWe have demonstrated the feasibility of performing patient specific quality assurance on 10XFFF SABR treatment plans using Monte Carlo simulations of the TrueBeam LINAC. This is the first independent validation on the 10XFFF Varian phase space data presented to date. The work of Tony Teke, Cheryl Duzenli and Ermais Gete has been supported by Varian under a Master Research Agreement.

Poster — Thur Eve — 30: Comparison of treatment planning and delivery performance of VMAT versus IMRT

The purpose of this study was to determine whether VMAT (Varian RapidArc ™) treatment planning and delivery performance is in compliance with accepted quality assurance tolerances developed for sliding window IMRT. We present an analysis of data for over 1300 patients treated with VMAT and IMRT over a period of three years. Data was acquired on 6 dosimetrically matched linacs for sites including head and neck, brain, gynaecological, and a variety of other cancer cases treated with 6 MV. We have demonstrated that it is possible to dosimetrically match multiple Varian iX linacs with the millennium series MLC using a sliding gap and intercept test. QA is performed by Monte Carlo simulation and ion chamber measurement comparisons with Varian Eclipse TPS as well as linac log file analysis of MLC positions, gantry angles and monitor units on each patient. Point dose and 3D gamma analysis indicate that agreement between Eclipse and measurement or Monte Carlo calculation is site specific, with the dosimetric leaf gap parameter in Eclipse optimized for the most frequently treated site Point dose agreement within 2% and gamma pass rate of > 95% (3%/ 3 mm) is achievable for all sites for both IMRT and VMAT. Linac log file analysis indicates that planned MLC positions are achieved within 2 mm >99.7% of the time for both sliding window IMRT and VMAT. Planned gantry angles are achieved within 0.6 mm 99.8% of the time and planned MUs within 0.1 mm are achieved 99.8% of the time for VMAT.

Sci—Wed PM: Delivery—08: Monte Carlo Based RapidArc QA Using LINAC Log Files

Purpose/Objective(s): To present our Monte Carlo based RapidArc quality assurance (QA) process to validate both the dose calculation and dynamic beam delivery accuracy using the planning MLCcontrol files and the post‐delivery MLC diagnostic files. Materials/Methods: Ten clinically acceptable RapidArc treatment plans were generated with a clinical version of the planning system for various tumor sites. Monte Carlo dose calculations were performed in a water equivalent phantom for each plan using both DynaLog files and the planning control (DVA) files. Results were compared to measurements using a calibrated Farmer ionization chamber with an active volume of 0.6 cm 3 . Comparison of RapidArc and Monte Carlo 3D doses was performed using a 3 dimensional Gamma‐factor analysis with a 3%/3mm DTA criteria. A thorough analysis of the DynaLog files was performed to evaluate the treatment delivery accuracy. Results: Good agreement was observed between chamber measurements and MC dose calculations and between RapidArc and MC dose distributions with Gamma values below 1 in over 90% of the points considered for all plans. The analysis of the MLC DynaLog files indicated that the leaf position errors were lower than 1 mm in more than 94% of the time with none above 2.5 mm and that few beam hold‐offs occurred Conclusions The accuracy and flexibility of our Monte Carlo based RapidArc QA system was demonstrated. Good machine performance and accurate dose distributions delivery of RapidArc plans was observed.

Automatic, Patient-Specific, Monte Carlo Based IMRT QA, without Ionization Chamber or Film

We present a Monte Carlo based QA process for IMRT that uses both the planning MLC control files and the post-delivery MLC diagnostic files, thereby opening the possibility of performing highly accurate, automatic, patient- specific IMRT QA, without any measuring devices. This method also allows the accumulation of the actually delivered dose to the irradiated volume of the patient over the entire course of treatment.