Clinical Volumetric Modulated Arc Therapy Research Articles

EPID dosimetry for IMRT and VMAT PSQA

Aim The aim of this project is to investigate if the Electronic Portal Imaging Device (EPID) would be suitable for Patient Specific Quality Assurance (PSQA) of intensity modulated radiation therapy (IMRT) & volumetric modulated arc therapy (VMAT). Materials and methods The EPID was compared against the MapCHECK detector using 5 clinical IMRT and VMAT plans. All plans were measured using an EPID from an Elekta synergy machine and the MapCHECK diode array. Sun nuclear software (SNC patient) was used to acquire and process measured dose distributions from the EPID and MapCHECK and comparisons were made using pass rate ⩾90% for 2%/2 mm and ⩾95% for 3%/2 mm with 10% threshold, for Gamma evaluation, distance to agreement (DTA) and isocentric dose to treatment planning system (TPS). A physics model (EPIDose) was created following manufacturer recommendations in order to convert the EPID panel pixel information to dose using a four step approach. Results Clinical cases for IMRT plans shows the average pass rate of all fields is 98.92% for 2%/2 mm and 99.58% for 3%/2 mm for EPID, and 99.08% for 2%/2 mm and 99.78% for 3%/2 mm for MapCHECK. Conclusion The PSQA results for MapCHECK and EPID provides acceptable agreement for clinical criteria. The sensitivity of EPID panel provide more information for EPIDose dosimetry with relative short time for setup and friendly use, this is make the EPID panel more usable as dosimetry technique, compare to MapCHECK that required to more time for setup.

Feasibility of magnetic resonance imaging-only rectum radiotherapy with a commercial synthetic computed tomography generation solution.

Background and purposeSynthetic computed tomography (sCT) images enable magnetic resonance (MR)-based dose calculations. This work investigated whether a commercially available sCT generation solution was suitable for accurate dose calculations and position verification on patients with rectal cancer.Material and methodsFor twenty rectal cancer patients computed tomography (CT) images were rigidly registered to sCT images. Clinical volumetric modulated arc therapy plans were recalculated on registered CT and sCT images. Dose deviations were determined through gamma and voxelwise analysis. The impact on position verification was investigated by identifying differences in translations and rotation between cone-beam CT (CBCT) to CT and CBCT to sCT registrations.ResultsAcross twenty patients, within a threshold of 90% of the prescription dose, a gamma analysis (2%, 2 mm) mean pass rate of 95.2 ± 4.0% (±1) and mean dose deviation of −0.3 ± 0.2% of prescription dose were obtained. The mean difference of translations and rotations over ten patients (76 CBCTs) was <1 mm and <0.5° in all directions. In the sole posterior-anterior direction a mean systematic shift of 0.7 ± 0.6 mm was found.ConclusionsAccurate MR-based dose calculations using a commercial sCT generation method were clinically feasible for treatment of rectal cancer patients. The accuracy of position verification was clinically acceptable. However, before clinical implementation future investigations will be performed to determine the origin of the systematic shift.

Fully automated, multi-criterial planning for Volumetric Modulated Arc Therapy – An international multi-center validation for prostate cancer

Background and purposeReported plan quality improvements with autoplanning of radiotherapy of the prostate and seminal vesicles are poor. A system for automated multi-criterial planning has been validated for this treatment in a large international multi-center study. The system is configured with training plans using a mechanism that strives for quality improvements relative to those plans. Material and methodsEach of the four participating centers included thirty manually generated clinical Volumetric Modulated Arc Therapy prostate plans (manVMAT). Ten plans were used for autoplanning training. The other twenty were compared with an automatically generated plan (autoVMAT). Plan evaluations considered dosimetric plan parameters and blinded side-by-side plan comparisons by clinicians. ResultsWith equivalent Planning Target Volume (PTV) V95%, D2%, D98%, and dose homogeneity autoVMAT was overall superior for rectum with median differences of 3.4 Gy (p < 0.001) in Dmean, 4.0% (p < 0.001) in V60Gy, and 1.5% (p = 0.001) in V75Gy, and for bladder Dmean (0.9 Gy, p < 0.001). Also the clinicians’ plan comparisons pointed at an overall preference for autoVMAT. Advantages of autoVMAT were highly treatment center- and patient-specific with overall ranges for differences in rectum Dmean and V60Gy of [−4,12] Gy and [−2,15]%, respectively. ConclusionObserved advantages of autoplanning were clinically relevant and larger than reported in the literature. The latter is likely related to the multi-criterial nature of the applied autoplanning algorithm, with for each center a dedicated configuration that aims at plan improvements relative to its (clinical) training plans. Large variations among patients in differences between manVMAT and autoVMAT point at inconsistencies in manual planning.

To propose adding index of achievement (IOA) to IMRT QA process

BackgroundIn intensity modulated radiation therapy (IMRT) quality assurance (QA), evaluation of QA result using a pass/non-pass strategy under an acceptance criterion often suffers from lack of information on how good the plan is in absolute manner. In this study, we suggested adding an index system, previously developed for dose painting technique, to current IMRT QA process for better understanding of QA result.MethodsThe index system consists of three indices, index of achievement (IOA), index of hotness (IOH) and index of coldness (IOC). As indicated by its name, IOA does measure the level of agreement. IOH and IOC, on the other hand, measure the magnitude of overdose and underdose, respectively. A systematic analysis was performed with three 1-dimensional hypothetical dose distributions to investigate the characteristics of the index system. The feasibility of the system was also assessed with clinical volumetric modulated arc therapy (VMAT) QA cases from 8 head & neck and 5 prostate patients. In both simulation studies, certain amount of errors was intentionally induced to each dose distribution. Furthermore, we applied the proposed system to compare calculated with actual measured data for a total of 60 patients (30 head & neck and 30 prostate cases). QA analysis was made using both the index system and gamma method, and results were compared.ResultsWhile the gamma evaluation showed limited sensitivity in evaluating QA result depending on the level of tolerance criteria used, the proposed indices tended to better distinguish plans in terms of the amount of errors. Hotness and coldness of prescribed dose in the plan could be evaluated quantitatively by the indices.ConclusionsThe proposed index system provides information with which IMRT QA result would be better evaluated, especially when gamma pass rates are identical or similar among multiple plans. In addition, the independency of the index system on acceptance criteria would help making clear communications among readers of published articles and researchers in multi-institutional studies.

A novel approach to EPID-based 3D volumetric dosimetry for IMRT and VMAT QA

Intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) are relatively complex treatment delivery techniques and require quality assurance (QA) procedures. Pre-treatment dosimetric verification represents a fundamental QA procedure in daily clinical routine in radiation therapy. The purpose of this study is to develop an EPID-based approach to reconstruct a 3D dose distribution as imparted to a virtual cylindrical water phantom to be used for plan-specific pre-treatment dosimetric verification for IMRT and VMAT plans. For each depth, the planar 2D dose distributions acquired in air were back-projected and convolved by depth-specific scatter and attenuation kernels. The kernels were obtained by making use of scatter and attenuation models to iteratively estimate the parameters from a set of reference measurements. The derived parameters served as a look-up table for reconstruction of arbitrary measurements. The summation of the reconstructed 3D dose distributions resulted in the integrated 3D dose distribution of the treatment delivery.The accuracy of the proposed approach was validated in clinical IMRT and VMAT plans by means of gamma evaluation, comparing the reconstructed 3D dose distributions with Octavius measurement. The comparison was carried out using (3%, 3 mm) criteria scoring 99% and 96% passing rates for IMRT and VMAT, respectively. An accuracy comparable to the one of the commercial device for 3D volumetric dosimetry was demonstrated. In addition, five IMRT and five VMAT were validated against the 3D dose calculation performed by the TPS in a water phantom using the same passing rate criteria. The median passing rates within the ten treatment plans was 97.3%, whereas the lowest was 95%. Besides, the reconstructed 3D distribution is obtained without predictions relying on forward dose calculation and without external phantom or dosimetric devices. Thus, the approach provides a fully automated, fast and easy QA procedure for plan-specific pre-treatment dosimetric verification.

Validation of the Octavius 4D measuring system in verifying advanced external beams radiotherapy techniques

The development of advanced techniques as Intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) are characterized by their numerous steep dose gradient using the dynamic MLC modality and these requires specific quality assurance (QA) to meet this complexity. The conventional dosimetery techniques, such as ionization chamber, point dose measurements and film dosimetry, are gradually being replaced by detector arrays which can produce accurate results immediately. There are many commercial detector arrays one of them is 2D-ARRAY seven29 (PTW, Freiburg, Germany). We used the Octavius 4 D measuring system with its detector array, ionization chamber. The accuracy of the synchronizing motion of the phantom was validated. It was found that the detector array response is stable after warming up with 800 MU, the dose rate measurements and the dose response is linear with R2=0.929. The gamma index for the clinical IMRT plans were found to be 93.3%, 98.0 %, 95.5% and 96.2% for head and neck, prostate, breast and brain cases, and for VMAT plans are 96.3%, 99.4 %, 98.1% and 98.6% for head and neck, prostate, breast and brain, respectively, using gamma criteria 3%/3 mm. The aim of this work is to characterize and validate the Octavius 4D measuring system for 6MV beams. Also, to verify clinical IMRT and VMAT plans, the volumetric dose matrix of the clinical plans, dose line profiles and to study the factors affect the gamma index.

Commissioning and validation of fluence-based 3D VMAT dose reconstruction system using new transmission detector.

In this study, we evaluated the basic performance of the three-dimensional dose verification system COMPASS (IBA Dosimetry). This system is capable of reconstructing 3D dose distributions on the patient anatomy based on the fluence measured using a new transmission detector (Dolphin, IBA Dosimetry) during treatment. The stability of the absolute dose and geometric calibrations of the COMPASS system with the Dolphin detector were investigated for fundamental validation. Furthermore, multileaf collimator (MLC) test patterns and a complicated volumetric modulated arc therapy (VMAT) plan were used to evaluate the accuracy of the reconstructed dose distributions determined by the COMPASS. The results from the COMPASS were compared with those of a Monte Carlo simulation (MC), EDR2 film measurement, and a treatment planning system (TPS). The maximum errors for the absolute dose and geometrical position were -0.28% and 1.0mm for 3months, respectively. The Dolphin detector, which consists of ionization chamber detectors, was firmly mounted on the linear accelerator and was very stable. For the MLC test patterns, the TPS showed a > 5% difference at small fields, while the COMPASS showed good agreement with the MC simulation at small fields. However, the COMPASS produced a large error for complex small fields. For a clinical VMAT plan, COMPASS was more accurate than TPS. COMPASS showed real delivered-dose distributions because it uses the measured fluence, a high-resolution detector, and accurate beam modeling. We confirm here that the accuracy and detectability of the delivered dose of the COMPASS system are sufficient for clinical practice.

Improvement of off-axis SABR plan verification results by using adapted dose reconstruction algorithms for the Octavius 4D system.

Stereotactic ablative body radiotherapy (SABR) for lung patients can be performed with volumetric-modulated arc therapy (VMAT) plans using off-axis target geometry to allow treatment in their CBCT verified position. For patient-specific quality assurance measurements using the PTW Octavius 4D phantom (PTW, Freiburg, Germany) (OCT4D) in conjunction with an Octavius 1000SRS array (OCT1000) (PTW, Freiburg, Germany), repositioning the phantom off-axis is required to ensure the measurement area coincides with the tumor. The aim of this work is to quantify delivery errors using an array repositioned off-axis and evaluate new software which incorporates corrections for off-axis phantom measurements. Dynamic conformal arcs and 25 lung SABR plans were created with the isocenter at the patient midline and the target volume off-axis. Measurements were acquired with an OCT4D phantom in conjunction with a 729 array (PTW, Freiburg, Germany) (OCT729) placed at isocenter. These plans were recalculated and delivered to both the OCT729 and OCT1000 arrays repositioned so that the high-dose region was at the center of the phantom. Comparisons were made using VeriSoft v7.0 (PTW, Freiburg, Germany) and the newly implemented version 7.1 with 2%/2mm gamma criterion (10% threshold) and results correlated with off-axis distance to the tumor. Average pass rates for VeriSoft v7.0 significantly reduced from 92.7±2.4% to 84.9±4.1% when the phantom was repositioned compared to the isocenter setup for the OCT729. The gamma pass rates significantly decreased the further the phantom was moved off-axis. Significantly higher pass rates were observed for the OCT1000 of 95.7±3.6% and a significant decrease in gamma pass rate with off-axis phantom distance was again observed. In contrast, even with phantom repositioning, the pass rates for analysis with VeriSoft v7.1 were 93.7±2.1% and 99.4±1.1% for OCT729 and OCT1000, respectively. No significant difference in gamma pass rate was observed with off-axis phantom position irrespective of array type with the new software. The errors in QA phantom measurements due to dose reconstruction at off-axis target geometry have been demonstrated for conformal arcs and clinical VMAT SABR plans. A novel software solution implemented by the vendor to allow accurate pass rates has been tested. This solution enables high-resolution arrays with small active detection areas to be used for quality assurance of SABR treatment plans in the off-axis treatment position.

Comparison of complexity metrics for multi-institutional evaluations of treatment plans in radiotherapy.

It is known that intensity-modulated radiotherapy plans that are highly complex might be less accurate in dose calculation and treatment delivery. Multiple complexity metrics have been proposed, but the relationships between them have not been thoroughly investigated. This study investigated these relationships in multi-institutional comparisons of treatment plans, where plans from multiple treatment planning systems (TPSs) are typically evaluated. A program was developed to compute several complexity indices and provide analysis of dynamic plan parameters. This in-house software was used to analyse plans from a recent multi-institutional audit. Additionally, 100 clinical volumetric modulated arc therapy (VMAT) plans from two institutions using different TPSs were analysed. All plans produced satisfactory pre-treatment verification results and, hence, complexity metrics could not be used to predict plans failing QA. Regarding the relationship among complexity indices, some very strong correlations were found (r > 0.9 with p < 0.01). However, some relevant discrepancies between complexity indices were obtained, even with negative correlation coefficients (r ∼ -0.6) which were expected to be positive. These discrepancies could be explained because each complexity index focused on different features of the plan and different TPSs prioritised modulation of different plan parameters. Some complexity indices provided similar information and can be considered equivalent. However, indices that focused on different plan parameters yielded different results and it was unclear which complexity index should be used. Careful consideration should be given to the use of complexity metrics in multi-institutional studies.

Compass model-based quality assurance for stereotactic VMAT treatment plans

PurposeTo use Compass as a model-based quality assurance (QA) tool for stereotactic body radiation therapy (SBRT) and stereotactic radiation therapy (SRT) volumetric modulated arc therapy (VMAT) treatment plans calculated with Eclipse treatment planning system (TPS). Materials and methodsTwenty clinical stereotactic VMAT SBRT and SRT treatment plans were blindly selected for evaluation. Those plans included four different treatment sites: prostate, brain, lung and body. The plans were evaluated against dose-volume histogram (DVH) parameters and 2D and 3D gamma analysis. The dose calculated with Eclipse treatment planning system (TPS) was compared to Compass calculated dose (CCD) and Compass reconstructed dose (CRD). ResultsThe maximum differences in mean dose of planning target volume (PTV) were 2.7 ± 1.0% between AAA and Acuros XB calculation algorithm TPS dose, −7.6 ± 3.5% between Eclipse TPS dose and CCD dose and −5.9 ± 3.7% between Eclipse TPS dose and CRD dose for both Eclipse calculation algorithms, respectively. 2D gamma analysis was not able to identify all the cases that 3D gamma analysis specified for further verification. ConclusionsCompass is suitable for QA of SBRT and SRT treatment plans. However, the QA process should include wide set of DVH-based dose parameters and 3D gamma analysis should be the preferred method when performing clinical patient QA. The results suggest that the Compass should not be used for smaller field sizes than 3 × 3 cm2 or the beam model should be adjusted separately for both small (FS ≤ 3 cm) and large (FS > 3 cm) field sizes.

Couch modelling for volumetric modulated arc therapy (VMAT).

The aim of this work was to investigate the impact of the accuracy of a certain couch model on the overall accuracy of the dose calculation. For this purpose two models of the Varian Exact Couch (Varian Medical Systems, Palo Alto, CA, USA) for use in the planning of volumetric modulated arc therapy (VMAT) were created in Monaco 5.1 (Elekta, Stockholm, Sweden). The first was a simplistic model, with the relative electron densities (ED) of structures based entirely on CT Hounsfield units (HU). The second couch model was obtained by assigning the appropriate EDs to the relevant couch structures. Instead of getting EDs from the CT scan using the Hounsfield units to ED conversion, the second approach was to determine the optimal ED iteratively. The values accepted for the second couch model were the EDs that produced the best agreement between Monaco calculations and the measured couch attenuation. Finally, the performance of the two couch models in clinical prostate VMAT plans was compared.

An EPID‐based system for gantry‐resolved MLC quality assurance for VMAT

Multileaf collimator (MLC) positions should be precisely and independently measured as a function of gantry angle as part of a comprehensive quality assurance (QA) program for volumetric‐modulated arc therapy (VMAT). It is also ideal that such a QA program has the ability to relate MLC positional accuracy to patient‐specific dosimetry in order to determine the clinical significance of any detected MLC errors. In this work we propose a method to verify individual MLC trajectories during VMAT deliveries for use as a routine linear accelerator QA tool. We also extend this method to reconstruct the 3D patient dose in the treatment planning system based on the measured MLC trajectories and the original DICOM plan file. The method relies on extracting MLC positions from EPID images acquired at 8.41 fps during clinical VMAT deliveries. A gantry angle is automatically tagged to each image in order to obtain the MLC trajectories as a function of gantry angle. This analysis was performed for six clinical VMAT plans acquired at monthly intervals for three months. The measured trajectories for each delivery were compared to the MLC positions from the DICOM plan file. The maximum mean error detected was 0.07 mm and a maximum root‐mean‐square error was 0.8 mm for any leaf of any delivery. The sensitivity of this system was characterized by introducing random and systematic MLC errors into the test plans. It was demonstrated that the system is capable of detecting random and systematic errors on the range of 1–2 mm and single leaf calibration errors of 0.5 mm. The methodology developed in the work has potential to be used for efficient routine linear accelerator MLC QA and pretreatment patient‐specific QA and has the ability to relate measured MLC positional errors to 3D dosimetric errors within a patient volume.PACS number(s): 87.55.Qr

A study of parameters of a volumetric modulated arc therapy plan for cervical and upper esophageal cancer

Objective To figure out the optimal parameters of a volumetric modulated arc therapy (VMAT) plan for cervical and upper esophageal cancer by quality evaluation of VMAT plans with different parameters, and to provide a reference for the design of clinical VMAT treatment plan. Methods Ten patients with cervical esophageal cancer and ten patients with upper esophageal cancer were enrolled as subjects. The Nucletron Oncentra 4.3 treatment planning system was used to generate plans for Elekta Synergy VMAT accelerator. Six VMAT plans were made with variation in the gantry angle (2°, 3°, and 4°), the maximum delivery time (80 s, 110 s, and 150 s), and the collimator angle (0° and 45°). The doses to the planning target volume and organs at risk were analyzed by paired t test. Results For cervical and upper esophageal cancer, the quality of VMAT plans with a collimator angle of 45° was better than those with a collimator angle of 0°(P=0.003-0.007). For cervical esophageal cancer, there was no significant difference in quality between VMAT plans with a maximum delivery time of 110 s or 150 s and those with a maximum delivery time of 80 s (P>0.05); for upper esophageal cancer, there was also no significant difference in quality between VMAT plans with three different maximum delivery times (P>0.05). For cervical esophageal cancer, the VMAT plans with a gantry angle of 3° had a better quality than those with a gantry angle of 2° or 4°(P=0.010-0.048). For upper esophageal cancer, the VMAT plans with a gantry angle of 3°had a better quality than those with a gantry angle of 4°(P=0.010-0.048). Compared with those with a gantry angle of 2°, the VMAT plans with a gantry angle of 3°had a slightly better dose distribution in the target volume (P=0.046), but a slightly higher dose to lung tissue (V25 and V30, P=0.007 and 0.026). Conclusions The optimal initial parameters of a VMAT plan for cervical and upper esophageal cancer are a collimator angle of 45°, a maximum delivery time of 80 s, and a gantry angle of 3°. Key words: Esophageal neoplasms/volumetric modulated arc therapy; Collimator angle; Max delivery time; Gantry spacing

Limited Impact of Setup and Range Uncertainties, Breathing Motion, and Interplay Effects in Robustly Optimized Intensity Modulated Proton Therapy for Stage III Non-small Cell Lung Cancer

To investigate the impact of setup and range uncertainties, breathing motion, and interplay effects using scanning pencil beams in robustly optimized intensity modulated proton therapy (IMPT) for stage III non-small cell lung cancer (NSCLC). Three-field IMPT plans were created using a minimax robust optimization technique for 10 NSCLC patients. The plans accounted for 5- or 7-mm setup errors with ±3% range uncertainties. The robustness of the IMPT nominal plans was evaluated considering (1) isotropic 5-mm setup errors with ±3% range uncertainties; (2) breathing motion; (3) interplay effects; and (4) a combination of items 1 and 2. The plans were calculated using 4-dimensional and average intensity projection computed tomography images. The target coverage (TC, volume receiving 95% of prescribed dose) and homogeneity index (D2-D98, where D2 and D98 are the least doses received by 2% and 98% of the volume) for the internal clinical target volume, and dose indexes for lung, esophagus, heart and spinal cord were compared with that of clinical volumetric modulated arc therapy plans. The TC and homogeneity index for all plans were within clinical limits when considering the breathing motion and interplay effects independently. The setup and range uncertainties had a larger effect when considering their combined effect. The TC decreased to <98% (clinical threshold) in 3 of 10 patients for robust 5-mm evaluations. However, the TC remained >98% for robust 7-mm evaluations for all patients. The organ at risk dose parameters did not significantly vary between the respective robust 5-mm and robust 7-mm evaluations for the 4 error types. Compared with the volumetric modulated arc therapy plans, the IMPT plans showed better target homogeneity and mean lung and heart dose parameters reduced by about 40% and 60%, respectively. In robustly optimized IMPT for stage III NSCLC, the setup and range uncertainties, breathing motion, and interplay effects have limited impact on target coverage, dose homogeneity, and organ-at-risk dose parameters.

MO‐AB‐BRA‐01: A Global Level Set Based Formulation for Volumetric Modulated Arc Therapy

Purpose:The current clinical Volumetric Modulated Arc Therapy (VMAT) optimization is formulated as a non‐convex problem and various greedy heuristics have been employed for an empirical solution, jeopardizing plan consistency and quality. We introduce a novel global direct aperture optimization method for VMAT to overcome these limitations.Methods:The global VMAT (gVMAT) planning was formulated as an optimization problem with an L2‐norm fidelity term and an anisotropic total variation term. A level set function was used to describe the aperture shapes and adjacent aperture shapes were penalized to control MLC motion range. An alternating optimization strategy was implemented to solve the fluence intensity and aperture shapes simultaneously. Single arc gVMAT plans, utilizing 180 beams with 2° angular resolution, were generated for a glioblastoma multiforme (GBM), lung (LNG), and 2 head and neck cases—one with 3 PTVs (H&amp;N3PTV) and one with 4 PTVs (H&amp;N4PTV). The plans were compared against the clinical VMAT (cVMAT) plans utilizing two overlapping coplanar arcs.Results:The optimization of the gVMAT plans had converged within 600 iterations. gVMAT reduced the average max and mean OAR dose by 6.59% and 7.45% of the prescription dose. Reductions in max dose and mean dose were as high as 14.5 Gy in the LNG case and 15.3 Gy in the H&amp;N3PTV case. PTV coverages (D95, D98, D99) were within 0.25% of the prescription dose. By globally considering all beams, the gVMAT optimizer allowed some beams to deliver higher intensities, yielding a dose distribution that resembles a static beam IMRT plan with beam orientation optimization.Conclusions:The novel VMAT approach allows for the search of an optimal plan in the global solution space and generates deliverable apertures directly. The single arc VMAT approach fully utilizes the digital linacs’ capability in dose rate and gantry rotation speed modulation.Varian Medical Systems, NIH grant R01CA188300, NIH grant R43CA183390.

SU‐F‐T‐312: Identifying Distinct Radiation Therapy Plan Classes Through Multi‐Dimensional Analysis of Plan Complexity Metrics

Purpose:To determine whether body site‐specific treatment plans form unique “plan class” clusters in a multi‐dimensional analysis of plan complexity metrics such that a single beam quality correction determined for a representative plan could be universally applied within the “plan class”, thereby increasing the dosimetric accuracy of a detector's response within a subset of similarly modulated nonstandard deliveries.Methods:We collected 95 clinical volumetric modulated arc therapy (VMAT) plans from four body sites (brain, lung, prostate, and spine). The lung data was further subdivided into SBRT and non‐SBRT data for a total of five plan classes. For each control point in each plan, a variety of aperture‐based complexity metrics were calculated and stored as unique characteristics of each patient plan. A multiple comparison of means analysis was performed such that every plan class was compared to every other plan class for every complexity metric in order to determine which groups could be considered different from one another. Statistical significance was assessed after correcting for multiple hypothesis testing.Results:Six out of a possible 10 pairwise plan class comparisons were uniquely distinguished based on at least nine out of 14 of the proposed metrics (Brain/Lung, Brain/SBRT lung, Lung/Prostate, Lung/SBRT Lung, Lung/Spine, Prostate/SBRT Lung). Eight out of 14 of the complexity metrics could distinguish at least six out of the possible 10 pairwise plan class comparisons.Conclusion:Aperture‐based complexity metrics could prove to be useful tools to quantitatively describe a distinct class of treatment plans. Certain plan‐averaged complexity metrics could be considered unique characteristics of a particular plan. A new approach to generating plan‐class specific reference (pcsr) fields could be established through a targeted preservation of select complexity metrics or a clustering algorithm that identifies plans exhibiting similar modulation characteristics. Measurements and simulations will better elucidate potential plan‐class specific dosimetry correction factors.

SU-G-BRC-03: A Novel Multi-Criteria Optimization Method for Volumetric Modulated Arc Therapy (VMAT) Treatment Planning

Purpose: To present a novel multi-criteria optimization (MCO) solution approach that generates treatment plans for VMAT. Methods: We demonstrate our method with generating double-arc VMAT plans for 10 locally advanced head-and-neck cancer cases retrospectively. In our MCO formulation, we defined an objective function for each structure in the treatment volume. This resulted in 9 objective functions, including 3 distinct objectives for primary, high-risk and low-risk target volumes, 5 objectives for each of the organs-at-risk (OARs) (two parotid glands, spinal cord, brain stem and oral cavity), and one for the non-target non-OAR normal tissue. Conditional value-at-risk (CVaR) constraints were utilized to ensure at least certain fraction of the PTVs receiving the prescription doses. Besides formulating the problem as MCO, our approach directly generated weight and aperture for each control point considering MLC connectivity constraints, leaf speed, gantry speed, gantry speed change and dose rate utilizing column generation algorithm. Final dose distributions for all plans were compared after a Monte Carlo kernel-superposition dose calculation. Results: We compared our method with clinical VMAT plans and segment weight optimization (SWO) of the clinical plans. At least 95% target coverage was achieved by all plans. However, the average of the conformity indices for MCO was 1.28 comparing to clinical VMAT 1.83 (30% reduction, p<0.01) and SWO 1.63 (22% reduction, p<0.01). MCO method achieved reduction in right parotid mean dose of 17% (p<0.01) and 10% (p=0.03), oral cavity mean dose of 23% (p<0.01) and 6% (p=0.34) and spinal cord maximum dose of 20% (p<0.01) and 10% (p<0.01), brain stem maximum dose of 59% (p<0.01) and 57% (p<0.01), and normal tissue maximum dose of 4% (p<0.01) and 5% (p<0.01), comparing to clinical VMAT and SWO respectively. Conclusion: We demonstrated that the proposed MCO method was able to obtain VMAT plans with significant improvement in dosimetric plan quality.

SU‐F‐T‐412: Spatiotemporal Distribution of Dose Rates in VMAT Plans

Purpose:To assess dose rates in volumetric modulated arc therapy (VMAT) plans.Methods:For dose rate analysis, using Monte Carlo methods, dose distributions are calculated in sectors of consecutive DICOM control points and each multiplied with the MU‐rate obtaining dose rate distributions for every voxel at every time‐point during a treatment fraction. Dose rate distributions were calculated for a clinical head and neck, cranial (2 Gy fraction dose) and a SBRT liver treatment case (5 Gy per fraction). To assess optimizer influence on dose rate distributions, five conventionally fractionated head and neck cases were optimized using the progressive resolution optimizer (PRO) versions 2 and 3 (Varian Medical Systems, Palo Alto, CA).Results:Firstly, histograms of dose rates over the course of treatment delivery in volumes of interest were generated for the PTV and spine or brainstem of the clinical treatment plans. Secondly, dose rates were assessed as color wash overlay on top of CT slices, allowing the user to explore the dose rate variation over time. For the cases considered, dose rates were spread out over a continuous spectrum with a mean value in the lower half. PRO3 generated plans showed higher maximum dose rates compared to PRO2 (699.80 cGy/min vs. 543.46 cGy/min) and modulation of MU rates in blocks as opposed to gradual modulation.Conclusion:Calculation and visualization of dose rate distributions in VMAT have been performed. Dose rates in clinical relevant VMAT cases are spread out over a continuous spectrum with a mean in the lower part for both conventional and SBRT test cases. Different optimization algorithms may lead to substantially different dose rate distributions in the same cases. The development of the Swiss Monte Carlo Plan framework has been partially supported by Varian Medical Systems

SU‐F‐J‐63: Abdominal Diameter Changes in Children During Volumetric Modulated Arc Therapy (VMAT): Is Re‐Planning Needed?

Purpose:To investigate the dosimetric impact of daily changes in patient's diameter, due to weight gain/loss and air in the bowel, based on CBCT information during radiotherapy treatment of pediatric abdominal tumors.Methods:10 pediatric patients with neuroblastoma (n=6) and Wilms’ (n=4) tumors were included. Available CBCTs were affinely registered to the planning CT for daily set‐up variations corrections. A density override approach assigning air‐density to the random air pockets and water‐density to the remaining anatomy was used to determine the CBCT and CT dose. Clinical VMAT plans, with a PTV prescribed dose ranging between (14.4‐ 36) Gy, were re‐optimized on the density override CT and re‐calculated on each CBCT. Dose‐volume statistics of the PTV and kidneys, delineated on each CBCT, were used to compare the daily and cumulative CBCT dose with the reference CT dose.Results:The average patient diameter variation was (0.5 ± 0.7) cm (maximum daily difference of 2.3 cm). The average PTV mean dose difference (MDD) between the CT and the cumulative CBCT plans was (0.1 ± 1.1) % (maximum daily MDD of 2%). A reduction in target coverage up to 3% and 7% was observed for the cumulative and daily CBCT plans, respectively. The average kidneys’ cumulative MDD was (−2.7 ± 3.6) % (maximum daily MDD of −12%), corresponding to an overdosage.Conclusion:Due to patient's diameter changes, a target underdosage was assessed. Given the high local tumor control of neuroblastoma and Wilms’ diseases, the need of re‐planning might be discarded. However, the assessed kidneys overdosage could represent a problem when the normal tissue tolerance is reached. The necessity of re‐planning should then be considered to reduce the risk of long‐term renal complications. Due to the poor softtissue contrast on CBCT, MRI‐guidance is required to obtain a better assessment of the accumulated dose on the remaining OARs.

Dose rate mapping of VMAT treatments

Human tissues exhibit a varying response to radiation dose depending on the dose rate and fractionation scheme used. Dose rate effects have been reported for different radiations, and tissue types. The literature indicates that there is not a significant difference in response for low-LET radiation when using dose rates between 1 Gy min−1 and 12 Gy min−1 but lower dose rates have an observable sparing effect on tissues and a differential effect between tissues. In intensity-modulated radiotherapy such as volumetric modulated arc therapy (VMAT) the dose can be delivered with a wide range of dose rates.In this work we developed a method based on time-resolved Monte Carlo simulations to quantify the dose rate frequency distribution for clinical VMAT treatments for three cancer sites, head and neck, lung, and pelvis within both planning target volumes (PTV) and normal tissues.The results show a wide range of dose rates are used to deliver dose in VMAT and up to 75% of the PTV can have its dose delivered with dose rates <1 Gy min−1. Pelvic plans on average have a lower mean dose rate within the PTV than lung or head and neck plans but a comparable mean dose rate within the organs at risk. Two VMAT plans that fulfil the same dose objectives and constraints may be delivered with different dose rate distributions, particularly when comparing single arcs to multiple arc plans.It is concluded that for dynamic plans, the dose rate range used varies to a larger degree than previously assumed. The effect of the dose rate range in VMAT on clinical outcome is unknown.