Ante Mestrovic

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

PURPOSE To 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. METHODS Monte 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. RESULTS For 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 2 mm 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. CONCLUSIONS The 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.

Integration of on-line imaging, plan adaptation and radiation delivery: proof of concept using digital tomosynthesis

The main objective of this manuscript is to propose a new approach to on-line adaptive radiation therapy (ART) in which daily image acquisition, plan adaptation and radiation delivery are integrated together and performed concurrently. A method is described in which on-line ART is performed based on intra-fractional digital tomosynthesis (DTS) images. Intra-fractional DTS images were reconstructed as the gantry rotated between treatment positions. An edge detection algorithm was used to automatically segment the DTS images as the gantry arrived at each treatment position. At each treatment position, radiation was delivered based on the treatment plan re-optimized for the most recent DTS image contours. To investigate the feasibility of this method, a model representing a typical prostate, bladder and rectum was used. To simulate prostate deformations, three clinically relevant, non-rigid deformations (small, medium and large) were modeled by systematically deforming the original anatomy. Using our approach to on-line ART, the original treatment plan was successfully adapted to arrive at a clinically acceptable plan for all three non-rigid deformations. In conclusion, we have proposed a new approach to on-line ART in which plan adaptation is performed based on intra-fractional DTS images. The study findings indicate that this approach can be used to re-optimize the original treatment plan to account for non-rigid anatomical deformations. The advantages of this approach are 1) image acquisition and radiation delivery are integrated in a single gantry rotation around the patient, reducing the treatment time, and 2) intra-fractional DTS images can be used to detect and correct for patient motion prior to the delivery of each beam (intra-fractional patient motion).

Population-based phase II trial of stereotactic ablative radiotherapy (SABR) for up to 5 oligometastases: SABR-5

BackgroundOligometastases refer to a state of disease where cancer has spread beyond the primary site, but is not yet widely metastatic, often defined as 1–3 or 1–5 metastases in number. Stereotactic ablative radiotherapy (SABR) is an emerging radiotherapy technique to treat oligometastases that require further prospective population-based toxicity estimates.MethodsThis is a non-randomized phase II trial where all participants will receive experimental SABR treatment to all sites of newly diagnosed or progressing oligometastatic disease. We will accrue 200 patients to assess toxicity associated with this experimental treatment. The study was powered to give a 95% confidence on the risk of late grade 4 toxicity, anticipating a < 5% rate of grade 4 toxicity.DiscussionSABR treatment of oligometastases is occurring off-trial at a high rate, without sufficient evidence of its efficacy or toxicity. This trial will provide necessary toxicity data in a population-based cohort, using standardized doses and organ at risk constraints, while we await data on efficacy from randomized phase III trials.Trial RegistrationRegistered through clinicaltrials.gov NCT02933242 on October 14, 2016 prospectively before patient accrual.

Sci—Sat AM: Stereo — 08: Stereotactic Ablative Radiotherapy (SABR) for low, intermediate and high risk prostate cancer using Volumetric Modulated Arc Therapy (VMAT) with a 10x Flattening Filter Free (FFF) beam

Purpose: To determine the feasibility of using Volumetric Modulated Arc Therapy (VMAT) with a 10x Flattening Filter Free (FFF) beam for Stereotactic Ablative Radiotherapy (SABR) for low, intermediate and high risk prostate cancer. Methods and Materials: Ten anonymized patient CT data sets were used in this planning study. For each patient CT data set, three sets of contours were generated: 1) low risk, 2) intermediate risk, and 3) high risk scenarios. For each scenario, a single-arc and a double-arc VMAT treatment plans were created. Plans were generated with the Varian Eclipse™ treatment planning system for a Varian TrueBeam™ linac equipped with Millenium 120 MLC. Plans were created using a 10x-FFF beam with a maximum dose rate of 2400 MU/min. Dose prescription was 36.25Gy/5 fractions with the planning objective of covering 99% of the Planning Target Volume with the 95% of the prescription dose. Normal tissue constraints were based on provincial prostate SABR planning guidelines, derived from national and international prostate SABR protocols. Plans were evaluated and compared in terms of: 1) dosimetric plan quality, and 2) treatment delivery efficiency. Results: Both single-arc and double-arc VMAT plans were able to meet the planning goals for low, intermediate and high risk scenarios. No significant dosimetric differences were observed between the plans. However, the treatment time was significantly lower for a single-arc VMAT plans. Conclusions: Prostate SABR treatments are feasible with 10x-FFF VMAT technique. A single-arc VMAT offers equivalent dosimetric plan quality and a superior treatment delivery efficiency, compared to a double-arc VMAT.

Sci-Thur PM: Planning & Delivery - 06: Real-Time Interactive Treatment Planning

Purpose: To describe and evaluate a novel system for generalized Real-Time Interactive Planning (RTIP) applied to head & neck (H&N) VMAT. Methods: The clinician interactively manipulates dose distributions using DVHs, isodoses, or rate of dose fall-off, which may be subjected to user-defined constraints. Dose is calculated using a fast Achievable Dose Estimate (ADE) algorithm, which simulates the limits of what can be achieved during treatment. After each manipulation contributing fluence elements are modified and the dose distribution updates in effectively real-time. For H&N VMAT planning, structure sets for 11 patients were imported into RTIP. Each dose distribution was interactively modified to minimize OAR dose while constraining target DVHs. The resulting RTIP DVHs were transferred to the Eclipse™ VMAT optimizer, and conventional VMAT optimization was performed. Results: Dose calculation and update times for the ADE algorithm ranged from 2.4 to 22.6 milliseconds, thus facilitating effectively real-time manipulation of dose distributions. For each of the 11 H&N VMAT cases, the RTIP process took ∼2–10 minutes. All RTIP plans exhibited acceptable PTV coverage, mean dose, and max dose. 10 of 11 RTIP plans achieved substantially improved sparing of one or more OARs without compromising dose to targets or other OARs. Importantly, 10 of the 11 RTIP plans required only one or two post-RTIP optimizations. Conclusions: RTIP is a novel system for manipulating and updating achievable dose distributions in real-time. H&N VMAT plans generated using RTIP demonstrate improved OAR sparing and planning efficiency. Disclosures: One author has a commercial interest in the presented materials.

TH‐C‐BRB‐05: Monte Carlo Simulations for Quality Assurance of Varian TrueBeam 6MV FFF RapidArc SBRT Treatments

Purpose: To establish feasibility of performing quality assurance for Flattening Filter Free (FFF) RapidArc stereotactic body radiotherapy treatments (SBRT) on a TrueBeam LINAC using Monte Carlo simulations.Methods: Phase‐space files for TrueBeam FFF photon beams were made available by Varian in IAEA‐compliant format. Monte Carlo simulations were performed using BEAMnrc and DOSXYZnrc to validate the 6MV FFF phase space files for use in this study. The phase space data provided by Varian is in cylindrical geometry and required conversion into a format that was compatible with BEAMnrc prior to use. To establish validity of the phase space data, dose calculations in a water phantom for fields ranging from 3×3 cm2 to 40 × 40 cm2 were performed using DOSXYZnrc. Percent depth doses (PDDs), transverse profiles and output factors were calculated and compared with measurements. Monte Carlo simulations of 6MV FFF SBRT RapidArc plans were performed using a 2mm3 voxel size and compared with both ion chamber measurement and Eclipse Treatment Planning System (TPS) dose calculations. 3D gamma analysis (3%,3mm) comparing Monte Carlo and TPS results was performed. Results:Monte Carlo simulations and measured values agreed within 1% and 1.5% for PDDs and profiles respectively for all fields. The agreement between measured and calculated output factors was within 1 % including for highly asymmetric fields. These results indicate that the 6MV FFF phase space data is sufficiently accurate for use in quality assurance in radiation therapy. For the 6 MV FFF RapidArc plans the agreement between MC and both measured and TPS dose calculations was within 2%. Over 95% of the points passed the 3D Gamma test Conclusions: We have demonstrated the feasibility of performing patient specific quality assurance for 6 MV FFF SBRT RapidArc treatments using Monte Carlo simulations for a TrueBeam linac. This project is funded by Varian Medical Systems

Sci—Thur AM: Planning ‐ 05: Lung SBRT: Dosimetric accuracy of the Analytical Anisotropic Algorithm (AAA) for 6MV FFF RapidArc planning

PURPOSE Stereotactic Body Radiation Therapy (SBRT) requires the delivery of a high biologically effective dose in only a few fractions. These large doses per fraction can necessitate long treatment times. The Varian Truebeam is capable of RapidArc delivery and also has the optional Flattening Filter Free (FFF) modes which greatly increase the dose rate. We have commissioned the 6MV FFF beam (1400 MU/min) for RapidArc lung SBRT, and verified heterogeneous dose calculations with Monte Carlo (MC). METHODS The standard commissioning data was acquired for Varians Analytical Anisotropic Algorithm (AAA) beam model. Measurements were acquired with the IBA Blue Phantom, using the CC13 and CC01 ion chambers and PTW diode. MLC-defined fields were also acquired for model verification. The Dosimetric Leaf Gap (DLG) was measured and then optimized using RapidArc lung SBRT plans, matching Eclipse with ion chamber measurements. Heterogeneous dose calculations were independently verified using MC. RESULTS There were some discrepancies regarding leaf transmission and penumbra, but the AAA model was generally well within 2% and 2 mm. A nominal DLG value of 1.6 mm was chosen. A representative lung SBRT case utilizing FFF RapidArc was calculated with MC. For the high dose region, 99% matched Eclipse within 3% and 3 mm. The mean dose difference of the PTV was 0.7%. CONCLUSIONS Although we have observed some minor infield discrepancies between the AAA and Monte Carlo calculations in heterogeneous media, the Eclipse AAA is reasonably accurate for complex FFF, RapidArc, SBRT lung planning.

Sci—Fri PM: Delivery — 05: A complete set of VMAT machine‐specific QA tests using EPID

During a VMAT (Volumetric Modulated Arc Therapy) radiation delivery, various machine parameters (gantry position and speed, MLC leaves position, dose rate, etc) have to be highly synchronized and coordinated. To ensure that all of the machine components are effectively operating together requires unique machine-specific quality assurance (QA) tests. The purpose of this work is to describe a complete set of VMAT machine-specific QA tests using Electronic Portal Imaging Device (EPID). To address the need for VMAT machine-specific QA tests, Varian has recommended 6 EPID-based tests for commissioning and QA of RapidArc™ capable machines. Recent studies have identified that these Varian tests do not define a complete set of tests needed for adequate commissioning and machine-specific QA of RapidArc machines. The goal of this work is twofold: a) to report and discuss the results of the original 6 Varians EPID-based tests, and b) to extend the original set by adding 2 more EPID-based tests, which results in a more complete and comprehensive set of tests. We have demonstrated that the Varian EPID-based QA tests can be used to verify that the synchronization of Dose Rate, Gantry Speed and MLC Leaves speed is generally to within 1.5% during a RapidArc delivery. Also, the gantry rotation does not have any significant effect on MLC leaves positional accuracy during a RapidArc delivery. Finally, we have introduced a new EPID-based QA test, which can be used to verify that the angular accuracy of the gantry position during a RapidArc delivery is to within ∼1 degree.

TU‐FF‐A2‐04: Geometric Parameter Analysis to Predetermine Optimal Radiosurgery Technique for the Treatment of Arteriovenous Malformation

Purpose: To develop a method of predicting the values of dose distribution parameters of different radiosurgery techniques for treatment of arteriovenous malformations (AVM) based on internal geometric parameters. Method and Materials: For each of eighteen previously treated AVM patients, four treatment plans were created: circular collimator arcs, dynamic conformal arcs, fixed conformal fields and intensity modulated radiosurgery (IMRS). An algorithm was developed to characterize the target and critical structure shape complexity and the position of the critical structures with respect to the target. Multiple regression was employed to establish the correlation between the internal geometric parameters and the dose distributions for different treatment techniques. The results were used develop a statistical model which predicts the values of dose distribution parameters based on internal geometric parameters. The model was applied to predict the dosimetric outcomes of different radiosurgery techniques and select the optimal radiosurgery technique for a random AVM patient. Results: Several internal geometric parameters showing statistically significant correlation (p < 0.05) with the treatment planning results for each technique were identified. The target volume and the average minimum distance between the target and the critical structures were the most effective predictors for normal tissue dose distribution. The structure overlap volume with the target and the mean distance between the target and the critical structure were the most effective predictors for critical structure dose distribution. When the model was applied to a random patient, the predicted treatment results were in close agreement with the original data. Conclusion: A statistical model has been described which successfully predicts the values of dose distribution parameters of different radiosurgery techniques and may be used to determine the optimal technique on patient‐to‐patient basis.