Volumetric Modulated Arc Therapy Cases Research Articles

Generating 3D images of VMAT plans for predictive models and activation maps associated with plan deliverability.

Intensity modulation with dynamic multi-leaf collimator (MLC) and monitor unit (MU) changes across control points (CPs) characterizes volumetric modulated arc therapy (VMAT). The increased uncertainty in plan deliverability required patient-specific quality assurance (PSQA), which remained inefficient upon Quality Assurance (QA) failure. To prevent waste before QA, plan complexity metrics (PCMs) and machine learning models with the metrics were generated, which were lack of providing CP-specific information upon QA failures. By generating 3D images from digital imaging and comminications in medicine in radiation therapy (DICOM RT) plan, we proposed a predictive model that can estimate the deliverability of VMAT plans and visualize CP-specific regions associated with plan deliverability. The patient cohort consisted of 259 and 190 cases for left- and right-breast VMAT treatments, which were split into 235 and 166 cases for training and 24 cases from each treatment for testing the networks. Three-channel 3D images generated from DICOM RT plans were fed into a DenseNet-based deep learning network. To reflect VMAT plan complexity as an image, the first two channels described MLC and MU variations between two consecutive CPs, while the last channel assigned the beam field size. The network output was defined as binary classified PSQA results, indicating deliverability. The predictive performance was assessed by accuracy, sensitivity, specificity, F1-score, and area under the curve (AUC). The gradient-weighted class activation map (Grad-CAM) highlighted the regions of CPs in VMAT plans associated with deliverability, compared against PCMs by Spearman correlation. The DenseNet-based predictive model yielded AUCs of 92.2% and 93.8%, F1-scores of 97.0% and 93.8% and accuracies of 95.8% and 91.7% for the left- and right-breast VMAT cases. Additionally, the specificity of 87.5% for both cases indicated that the predictive model accurately detected QA failing cases. The activation maps significantly differentiated QA failing-labeled from passing-labeled classes for the non-deliverable cases. The PCM with the highest correlation to the Grad-CAM varied from patient cases, implying that plan deliverability would be considered patient-specific. This work demonstrated that the deep learning-based network based on visualization of dynamic VMAT plan information successfully predicted plan deliverability, which also provided control-point specific planning parameter information associated with plan deliverability in a patient-specific manner.

Dosimetric comparison of four-dimensional computed tomography based internal target volume against variations in respiratory motion during treatment between volumetric modulated arc therapy and three-dimensional conformal radiotherapy in lung stereotactic body radiotherapy.

This study focused on the dosimetric impact of variations in respiratory motion during lung stereotactic body radiotherapy (SBRT). Dosimetric comparisons between volumetric modulated arc therapy (VMAT) and three-dimensional conformal radiotherapy (3DCRT) were performed using four-dimensional computed tomography (4DCT)-based internal target volumes (ITV). We created retrospective plans for ten patients with lung cancer who underwent SBRT using 3DCRT and VMAT techniques. A Delta4 Phantom + (ScandiDos, Uppsala, Sweden) was used to evaluate the dosimetric robustness of 4DCT-based ITV against variations in respiratory motion during treatment. We analyzed respiratory motion during treatment. Dose-volume histogram parameters were evaluated for the 95% dose (D95%) to the planning target volume (PTV) contoured on CT images obtained under free breathing. The correlations between patient respiratory parameters and dosimetric errors were also evaluated. In the phantom study, the average PTV D95% dose differences for all fractions were - 2.9 ± 4.4% (- 16.0 - 1.2%) and - 2.0 ± 2.8% (- 11.2 - 0.7%) for 3DCRT and VMAT, respectively. The average dose difference was < 3% for both 3DCRT and VMAT; however, in 5 out of 42 fractions in 3DCRT, the difference in PTV D95% was > 10%. Dosimetric errors were correlated with respiratory amplitude and velocity, and differences in respiratory amplitude between 4DCT and treatment days were the main factors causing dosimetric errors. The overall average dose error of the PTV D95% was small; however, both 3DCRT and VMAT cases exceeding 10% error were observed. Larger errors occurred with amplitude variation or baseline drift, indicating limited robustness of 4DCT-based ITV.

Evaluating the plan quality of a general head-and-neck knowledge-based planning model versus separate unilateral/bilateral models.

The implementation of knowledge-based planning (KBP) continues to grow in radiotherapy clinics. KBP guides radiation treatment design by generating clinically acceptable plans in a timely and resource-efficient manner. The role of multiple KBP models tailored for variations within a disease site remains undefined in part because of the substantial effort and number of training cases required to create a high-quality KBP model. In this study, our aim was to explore whether site-specific KBP models lead to clinically meaningful differences in plan quality for head-and-neck (HN) patients when compared to a general model. One KBP model was created from prior volumetric-modulated arc therapy (VMAT) cases that treated unilateral HN lymph nodes while another model was created from VMAT cases that treated bilateral HN nodes. Thirty cases from each model (60 cases total) were randomly selected to create a third, general model. These models were applied to 60 HN test cases - 30 unilateral and 30 bilateral - to generate 180 VMAT plans in Eclipse. Clinically relevant dose metrics were compared between models. Paired-sample t-tests were used for statistical analysis, with the threshold for statistical significance set a priori at 0.007, taking into consideration multiple hypothesis testing to avoid type I error. For unilateral test cases, the unilateral model-generated plans had significantly lower spinal cord maximum doses (12.1 Gy vs 19.3 Gy, p < 0.001) and oral cavity mean doses (20.8 Gy vs 23.0 Gy, p < 0.001), compared with the bilateral model-generated plans. The unilateral and general models generated comparable plans for unilateral HN test cases. For bilateral test cases, the bilateral model created plans had significantly lower brainstem maximum doses (10.8 Gy vs 12.2 Gy, p < 0.001) and parotid mean doses (24.0 Gy vs 25.5 Gy, p < 0.001) when compared to the unilateral model. Right parotid mean doses were lower for bilateral model plans compared to general model plans (23.8 Gy vs 24.4 Gy). The general model created plans with significantly lower brainstem maximum doses (10.3 Gy vs 10.8 Gy) and oral cavity mean doses (35.3 Gy vs 36.7 Gy) when compared with bilateral model-generated plans. The general model outperformed the bilateral model in several dose metrics but they were not deemed clinically significant. For both case sets, the unilateral and general model created plans had higher monitor units when compared to the bilateral model, likely due to more stringent constraint settings. All other dose metrics were comparable. This study demonstrates that a balanced general HN model created using carefully curated treatment plans can produce high quality plans comparable to dedicated unilateral and bilateral models.

IMRT QA and gamma comparisons: The impact of detector geometry, spatial sampling, and delivery technique on gamma comparison sensitivity.

To separately quantify sensitivity differences in patient-specific quality assurance comparisons analyzed with the gamma comparison for different measurement geometries, spatial samplings, and delivery techniques [intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT)]. Error-free calculations for 20 IMRT and 20 VMAT cases were compared to calculations with known induced errors of varying magnitudes, using gamma comparisons. Five error types (MU scaling, three different MLC errors, and collimator errors) were induced in plan calculations on three different detector geometries - ArcCHECK, MapCHECK, and Delta 4. To study detector geometry sensitivity effects alone, gamma comparisons were made with 1mm error-free calculations compared to 1mm error-induced calculations for each device. Effects of spatial sampling were studied by making the same gamma comparisons, but down-sampling the error-induced calculations to the real spatial sampling of each device. Additionally, 1mm vs 1mm comparisons between the IMRT and VMAT cases were compared to investigate sensitivity differences between IMRT and VMAT using IMRT and VMAT cohorts with similar ranges of plan complexity and average aperture size. For each case, induced error type, and device, five different gamma criteria were studied to ensure sensitivity differences between devices, spatial sampling scenarios, and delivery technique were not gamma criterion specific, resulting in over 36,000 gamma comparisons. For IMRT cases, Delta4 and MapCHECK devices had similar error sensitivities for lagging leaf, bank shift, and MU errors, while the ArcCHECK had considerably lower sensitivity than the planar-type devices. For collimator errors and perturbational leaf errors the ArcCHECK had higher error sensitivity than planar-type devices. This behavior was independent of gamma parameters (percent dose difference, distance-to-agreement, and low dose threshold), though use of local normalization resulted in error sensitivites that were markedly similar between all three devices. Differences between detector geometries were less pronounced for VMAT deliveries. Error sensitivity for a given gamma criterion when comparing IMRT and VMAT deliveries on the same devices showed that VMAT plans were more sensitive to some specific error types and less sensitive to others, when compared to IMRT plans. For the ArcCHECK device, the sensitivity of IMRT and VMAT cases was quite similar, whereas this was not the case for the planar-type devices. When comparing error sensitivity between 1mm vs 1mm calculations and 1mm vs the real spatial sampling for each device, results showed that increased spatial sampling did not systematically increase error sensitivity. Noticeable differences in error sensitivity were observed for different detector geometries, but differences were dependent on induced error type, and a particular device geometry did not offer universal improvements in error sensitivity across studied error types. This study demonstrates that the sensitivity of the gamma comparison does not largely hinge on detector spatial sampling. VMAT deliveries were generally less sensitive to errors when compared to IMRT plans for the planar-type devices, while similar sensitivities were observed between delivery techniques for the ArcCHECK device. Results of this work suggest that a universal gamma criterion is inappropriate for IMRT QA and that the percent pixels passing is an insufficient metric for evaluating quality assurance checks in the clinic.

A Novel Machine Learning Model for Dose Prediction in Prostate Volumetric Modulated Arc Therapy Using Output Initialization and Optimization Priorities.

Treatment planning for prostate volumetric modulated arc therapy (VMAT) can take 5–30 min per plan to optimize and calculate, limiting the number of plan options that can be explored before the final plan decision. Inspired by the speed and accuracy of modern machine learning models, such as residual networks, we hypothesized that it was possible to use a machine learning model to bypass the time-intensive dose optimization and dose calculation steps, arriving directly at an estimate of the resulting dose distribution for use in multi-criteria optimization (MCO). In this study, we present a novel machine learning model for predicting the dose distribution for a given patient with a given set of optimization priorities. Our model innovates upon the existing machine learning techniques by utilizing optimization priorities and our understanding of dose map shapes to initialize the dose distribution before dose refinement via a voxel-wise residual network. Each block of the residual network individually updates the initialized dose map before passing to the next block. Our model also utilizes contiguous and atrous patch sampling to effectively increase the receptive fields of each layer in the residual network, decreasing its number of layers, increasing model prediction and training speed, and discouraging overfitting without compromising on the accuracy. For analysis, 100 prostate VMAT cases were used to train and test the model. The model was evaluated by the training and testing errors produced by 50 iterations of 10-fold cross-validation, with 100 cases randomly shuffled into the subsets at each iteration. The error of the model is modest for this data, with average dose map root-mean-square errors (RMSEs) of 2.38 ± 0.47% of prescription dose overall patients and all optimization priority combinations in the patient testing sets. The model was also evaluated at iteratively smaller training set sizes, suggesting that the model requires between 60 and 90 patients for optimal performance. This model may be used for quickly estimating the Pareto set of feasible dose objectives, which may directly accelerate the treatment planning process and indirectly improve final plan quality by allowing more time for plan refinement.

Feasibility and potential advantages using VMAT in SRS metastasis treatments.

Utilization of stereotactic radiosurgery (SRS) for brain metastases (BM) has become the technique of choice as opposed to whole brain radiation therapy (WBRT). The aim of this work is to evaluate the feasibility and potential benefits in terms of normal tissue (NT) and dose escalation of volumetric modulated arc therapy (VMAT) in SRS metastasis treatment. A VMAT optimization procedure has therefore been developed for internal dose scaling which minimizes planner dependence. Five patient-plans incorporating treatment with frame-based SRS with dynamic conformal arc technique (DA) were re-planned for VMAT. The lesions selected were between 4-6 cm3. The same geometry used in the DA plans was maintained for the VMAT cases. A VMAT planning procedure was performed attempting to scale the dose in inner auxiliary volumes, and to explore the potential for dose scaling with this technique. Comparison of dose-volume histogram (DVH) parameters were obtained. VMAT allows a superior NT sparing plus conformity and dose scaling using the auxiliary volumes. The VMAT results were significantly superior in NT sparing, improving both the V10 and V12 values in all cases, with a 2-3 cm3 saving. In addition, VMAT improves the dose coverage D95 by about 0.5 Gy. The objective of dose escalation was achieved with VMAT with an increment of the Dmean and the Dmedian of about 2 Gy. This work shows a benefit of VMAT in SRS treatment with significant NT sparing. A VMAT optimization procedure, based on auxiliary inner volumes, has been developed, enabling internal dose escalation.

An Investigation of Multileaf Collimator Performance Dependence on Gantry Angle Using Machine Log Files.

Background:Quality assurance of linear accelerators (linacs) is an important part of ensuring accurate radiotherapy treatment deliveries. The aim of this study is to investigate the role of gravity on the positional accuracy of multileaf collimator (MLC) leaves during complex radiotherapy treatments on linacs. This investigation is based on the analysis of the machine log files from five different linacs in multiple centers.Materials and Methods:Three main categories of deliveries were considered: Picket fence, volumetric modulated arc therapy (VMAT) (both delivering with continuous gantry rotation), and sliding gap tests delivered at cardinal gantry angles, to determine the error of the MLC in relation to the gantry angle.Results:Analysis of picket fence tests revealed a dependence of the error upon the gantry angle. For the majority of deliveries, the MLC showed greater error at gantry angles 270 and 90. The errors computed for the cardinal angles for sliding gap tests were all statistically different with greatest error arising at gantry angle 270 and least error at gantry 90. For picket fence, sliding gap, and VMAT cases, MLC errors were dependent on the gantry angle.Conclusions:The errors in leaf positioning were found to be dependent on the gantry angle. For sliding gap tests, the error was greater at gantry angle 270° and 90° and less when the leaf motion was perpendicular to the force of gravity.

Comparison of Rapid to Standard Volumetric Modulated Arc Therapy for Palliative Radiotherapy in Lung Cancer Patients.

Patients with incurable lung cancer often present with debilitating symptoms that require urgent palliative radiotherapy. Volumetric modulated arc therapy (VMAT) provides several dosimetric advantages compared to basic non-conformal techniques, but involves complex planning resulting in a slower turn-around time for treatment. A simplified planning technique known as ‘rapid VMAT’ was developed with an aim to deliver palliative treatment to patients within 48 hours. The purpose of this study was to prospectively compare the dosimetric quality of rapid VMAT plans to standard VMAT plans. Fourteen consecutive rapid VMAT cases were re-planned de novo as per standard VMAT planning guidelines. Planning target volume (PTV) and organs at risk (OARs) were then compared. PTV coverage and dose to OARs including the spinal canal, lung, heart, and esophagus were similar between rapid and standard VMAT. Each plan was ready for treatment within 48 hours of the CT simulation. This study describes an expedited process for which palliative radiotherapy can be delivered to lung tumors with a similar robust quality that is provided for curative intent VMAT radiotherapy plans.

Radiosurgery treatment planning using conformal arc informed volumetric modulated arc therapy

Linac based radiosurgery to multiple metastases is commonly planned with volumetric modulated arc therapy (VMAT) as it effectively achieves high conformality to complex target arrangements. However, as the number of targets increases, VMAT can struggle to block between targets, which can lead to highly modulated and/or nonconformal multi-leaf collimator (MLC) trajectories that unnecessarily irradiation of healthy tissue. In this study we introduce, describe, and evaluate a treatment planning technique called Conformal Arc Informed VMAT (CAVMAT), which aims to reduce the dose to healthy tissue while generating highly conformal treatment plans. CAVMAT is a hybrid technique which combines the conformal MLC trajectories of dynamic conformal arcs with the MLC modulation and versatility of inverse optimization. CAVMAT has 3 main steps. First, targets are assigned to subgroups to maximize MLC blocking between targets. Second, arc weights are optimized to achieve the desired target dose, while minimizing MU variation between arcs. Third, the optimized conformal arc plan serves as the starting point for limited inverse optimization to improve dose conformity to each target. Twenty multifocal VMAT cases were replanned with CAVMAT with 20Gy applied to each target. The total volume receiving 2.5Gy[cm3], 6Gy[cm3], 12Gy[cm3], and 16Gy[cm3], conformity index, treatment delivery time, and the total MU were used to compare the VMAT and CAVMAT plans. In addition, CAVMAT was compared to a broad range of planning strategies from various institutions (108 linear accelerator based plans, 14 plans using other modalities) for a 5-target case utilized in a recent plan challenge. For the linear accelerator-based plans, a plan complexity metric based on aperture opening area and perimeter, total monitor units (MU), and MU for a given aperture opening was utilized in the plan challenge scoring algorithm to compare the submitted plans to CAVMAT. After re-planning the 20 VMAT cases, CAVMAT reduced the average V2.5Gy[cm3] by 25.25 ± 19.23%, V6Gy[cm3] by 13.68 ± 18.97%, V12Gy[cm3] by 11.40 ± 19.44%, and V16Gy[cm3] by 6.38 ± 19.11%. CAVMAT improved conformity by 3.81 ± 7.57%, while maintaining comparable target dose. MU for the CAVMAT plans increased by 24.35 ± 24.66%, leading to an increased treatment time of 2 minutes. For the plan challenge case, CAVMAT was 1 of 12 linac based plans that met all plan challenge scoring criteria. Compared to the average submitted VMAT plan, CAVMAT increased the V10%Gy[%] of healthy tissue (Brain-PTV) by roughly 3.42%, but in doing so was able to reduce the V25%Gy[%] by roughly 3.73%, while also reducing V50%Gy[%], V75%Gy[%], and V100%Gy[%]. The CAVMAT technique successfully eliminated insufficient MLC blocking between targets prior to the inverse optimization, leading to less complex treatment plans and improved tissue sparing. Tissue sparing, improved conformity, and decreased plan complexity at the cost of slight increase in treatment delivery time indicates CAVMAT to be a promising method to treat brain metastases.

Plan-Class Specific Reference Quality Assurance for Volumetric Modulated Arc Therapy

Background: There have been much efforts to develop the proper and realistic machine Quality Assurance (QA) reflecting on real Volumetric Modulated Arc Therapy (VMAT) plan. In this work we propose and test a special VMAT plan of plan-class specific (pcsr) QA, as a machine QA so that it might be a good solution to supplement weak point of present machine QA to make it more realistic for VMAT treatment. Materials and Methods: We divided human body into 5 treatment sites: brain, head and neck, chest, abdomen, and pelvis. One plan for each treatment site was selected from real VMAT cases and contours were mapped into the computational human phantom where the same plan as real VMAT plan was created and called plan-class specific reference (pcsr) QA plan. We delivered this pcsr QA plan on a daily basis over the full research period and tracked how much MLC movement and dosimetric error occurred in regular delivery. Several real patients under treatments were also tracked to test the usefulness of pcsr QA through comparisons between them. We used dynalog file viewer (DFV) and Dynalog file to analyze position and speed of individual MLC leaf. The gamma pass rate from portal dosimetry for different gamma criteria was analyzed to evaluate analyze dosimetric accuracy. Results and Discussion: The maxRMS of MLC position error for all plans were all within the tolerance limit of < 0.35 cm and the positional variation of maxPEs for both pcsr and real plans were observed very stable over the research session. Daily variations of maxRMS of MLC speed error and gamma pass rate for real VMAT plans were observed very comparable to those in their pcsr plans in good acceptable fluctuation. Conclusion: We believe that the newly proposed pcsr QA would be useful and helpful to predict the mid-term quality of real VMAT treatment delivery.

A Collimator Setting Optimization Algorithm for Dual-Arc Volumetric Modulated Arc Therapy in Pancreas Stereotactic Body Radiation Therapy.

Purpose:To optimize collimator setting to improve dosimetric quality of pancreas volumetric modulated arc therapy plan for stereotactic body radiation therapy.Materials and Methods:Fifty-five volumetric modulated arc therapy cases in stereotactic body radiation therapy of pancreas were retrospectively included in this study with internal review board approval. Different from the routine practice of initializing collimator settings with a template, the proposed algorithm simultaneously optimizes the collimator angles and jaw positions that are customized to the patient geometry. Specifically, this algorithm includes 2 key steps: (1) an iterative optimization algorithm via simulated annealing that generates a set of potential collimator settings from 39 cases with pancreas stereotactic body radiation therapy, and (2) a multi-leaf collimator modulation scoring system that makes the final decision of the optimal collimator settings (collimator angles and jaw positions) based on organs at risk sparing criteria. For validation, the other 16 cases with pancreas stereotactic body radiation therapy were analyzed. Two plans were generated for each validation case, with one plan optimized using the proposed algorithm (Planopt) and the other plan with the template setting (Planconv). Each plan was optimized with 2 full arcs and the same set of constraints for the same case. Dosimetric results were analyzed and compared, including target dose coverage, conformity, organs at risk maximum dose, and modulation complexity score. All results were tested by Wilcoxon signed rank tests, and the statistical significance level was set to .05.Results:Both plan groups had comparable target dose coverage and mean doses of all organs at risk. However, organs at risk (stomach, duodenum, large/small bowel) maximum dose sparing (D0.1 cc and D0.03 cc) was improved in Planopt compared to Planconv. Planopt also showed lower modulation complexity score, which suggests better capability of handling complex shape and sparing organs at risk .Conclusions:The proposed collimator settings optimization algorithm successfully improved dosimetric performance for dual-arc pancreas volumetric modulated arc therapy plans in stereotactic body radiation therapy of pancreas. This algorithm has the capability of immediate clinical application.

A Practical Method to Optimize Quality Assurance Results of Arc Therapy Plans in Beam Modeling.

Dosimetric accuracy of a volumetric modulated arc therapy (VMAT) plan is directly related to the beam model, particularly with multileaf collimator characterization. Inappropriate dosimetric leaf gap (DLG) value can lead to a suboptimal beam model, with significant failure in patient-specific quality assurance (PSQA) of VMAT plans. This study addressed the systematic issue of beam modeling and developed a practical method to determine the optimal DLG value for a beam model. Several complex VMAT plans were selected for the quality assurance analysis using the variable DLG values. The results of three-dimensional (3D) Gamma analysis as a function of the DLG at 3%/3 mm, 2%/2 mm, and 1%/1 mm criteria were fitted by a polynomial curve. The DLG value corresponding to the maximum Gamma passing rate for each polynomial fitting function was derived, and the average was calculated to be the optimal DLG value for each model. The 3D Gamma analysis was repeated with the optimal DLG value to verify the dosimetric accuracy of each VMAT case by PSQA. Gamma passing rates are seen to vary considerably with the DLG values and different analysis criteria (3%/3 mm, 2%/2 mm, and 1%/1 mm) for each case. The optimal DLG derived for each model was 1.16 mm and 1.10 mm, much larger than the measured value (about 0.3 mm). The beam models with the optimal DLG was able to produce an average Gamma passing rate of 97.1% (range, 94.6%– 99.1%) at 3%/3 mm and 93.5% (range, 89.0%– 96.5%) at 2%/2 mm for one beam model, and 97.1% (range, 94.8%– 99.1%) at 3%/3 mm, and 93.3% (range, 88.8%– 96.7%) at 2%/2 mm for another. The overall accuracy of dose calculation for VMAT plans should be optimized with a compromise of varied modulation complexities in a beam model. We have developed a practical method to derive the optimal DLG value for each beam model based on the Gamma passing criterion. This technique should be applicable in general for all beam energies and patient cases.

Monaco and film dosimetry of 3D CRT, IMRT and VMAT cases in a realistic pelvic prosthetic phantom

The dosimetry of patients with metallic hip implants during irradiation of pelvic lesions is challenging due to dose distortions caused by implants. This work presents a dosimetric comparison of various multi-field photon-beam dose distributions in the presence of unilateral hip titanium prosthesis (UHTiP) embedded in a unique pelvic phantom made out of water-equivalent nylon slices. The impact of the UHTiP on the accuracy of dose calculations from a Monaco TPS (treatment planning system) using the X-ray voxel Monte Carlo (XVMC) algorithm was benchmarked against measured dose data using Gafchromic EBT3 film. Multi-field beam arrangements including a 4-field box, 5-field 3DCRT (three-dimensional conformal radiation therapy), 6-field IMRT (intensity modulated radiation therapy) and a single-arc VMAT (volumetric modulated arc therapy) plan were set up for 6MV and 15MV beams. These plans were generated for the pelvic phantom that contains the prosthesis with film inserted. Compared to Monaco TPS dose calculations, film measurements showed enhanced dose in the prosthesis which was not predicted by Monaco due to its limitation in relative density assignment. The enhanced prosthesis dose increased with increase in beam energy and decreased with the complexity of the treatment plans, with VMAT giving the least escalated dose. The dose increased between 5% and 19% for 6MV and between 6% and 21% for 15MV. A gamma index analysis showed that 70–92% of dose points (excluding the prosthesis) were within 3% discrepancy. Increasing the number of treatment fields increases target dose coverage and improves the agreement between film and Monaco. When the relative electron density (RED) in the prosthesis was varied between 3.72 and 15 the dose discrepancy between film and Monaco increased from 30% to 57% for 6MV and from 30% to 50% for 15MV. The study indicates that beam weights for fields that pass through the prosthesis should be minimised and its RED must be correct for accurate dose calculation on Monaco.

Gafchromic EBT-XD film: Dosimetry characterization in high-dose, volumetric-modulated arc therapy.

Radiochromic films are important tools for assessing complex dose distributions. Gafchromic EBT‐XD films have been designed for optimal performance in the 40–4,000 cGy dose range. We investigated the dosimetric characteristics of these films, including their dose‐response, postexposure density growth, and dependence on scanner orientation, beam energy, and dose rate with applications to high‐dose volumetric‐modulated arc therapy (VMAT) verification. A 10 MV beam from a TrueBeam STx linear accelerator was used to irradiate the films with doses in the 0–4,000 cGy range. Postexposure coloration was analyzed at postirradiation times ranging from several minutes to 48 h. The films were also irradiated with 6 MV (dose rate (DR): 600 MU/min), 6 MV flattening filter‐free (FFF) (DR: 1,400 MU/ min), and 10 MV FFF (DR: 2,400 MU/min) beams to determine the energy and dose‐rate dependence. For clinical examinations, we compared the dose distribution measured with EBT‐XD films and calculated by the planning system for four VMAT cases. The red channel of the EBT‐XD film exhibited a wider dynamic range than the green and blue channels. Scanner orientation yielded a variation of ∼3% in the net optical density (OD). The difference between the film front and back scan orientations was negligible, with variation of ∼1.3% in the net OD. The net OD increased sharply within the first 6 hrs after irradiation and gradually afterwards. No significant difference was observed for the beam energy and dose rate, with a variation of ∼1.5% in the net OD. The gamma passing rates (at 3%, 3 mm) between the film‐ measured and treatment planning system (TPS)‐calculated dose distributions under a high dose VMAT plan in the absolute dose mode were more than 98.9%.PACS number(s): 87.56 Fc

Application programming in C# environment with recorded user software interactions and its application in autopilot of VMAT/IMRT treatment planning.

An autopilot scheme of volumetric‐modulated arc therapy (VMAT)/intensity‐modulated radiation therapy (IMRT) planning with the guidance of prior knowledge is established with recorded interactions between a planner and a commercial treatment planning system (TPS). Microsoft (MS) Visual Studio Coded UI is applied to record some common planner‐TPS interactions as subroutines. The TPS used in this study is a Windows‐based Eclipse system. The interactions of our application program with Eclipse TPS are realized through a series of subroutines obtained by prerecording the mouse clicks or keyboard strokes of a planner in operating the TPS. A strategy to autopilot Eclipse VMAT/IMRT plan selection process is developed as a specific example of the proposed “scripting” method. The autopiloted planning is navigated by a decision function constructed with a reference plan that has the same prescription and similar anatomy with the case at hand. The calculation proceeds by alternating between the Eclipse optimization and the outer‐loop optimization independent of the Eclipse. In the C# program, the dosimetric characteristics of a reference treatment plan are used to assess and modify the Eclipse planning parameters and to guide the search for a clinically sensible treatment plan. The approach is applied to plan a head and neck (HN) VMAT case and a prostate IMRT case. Our study demonstrated the feasibility of application programming method in C# environment with recorded interactions of planner‐TPS. The process mimics a planner's planning process and automatically provides clinically sensible treatment plans that would otherwise require a large amount of manual trial and error of a planner. The proposed technique enables us to harness a commercial TPS by application programming via the use of recorded human computer interactions and provides an effective tool to greatly facilitate the treatment planning process.PACS number(s): 87.55.D‐, 87.55.kd, 87.55.de

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

Evaluation of dosimetric effect caused by slowing with multi-leaf collimator (MLC) leaves for volumetric modulated arc therapy (VMAT)

BackgroundThis study is to report 1) the sensitivity of intensity modulated radiation therapy (IMRT) QA method for clinical volumetric modulated arc therapy (VMAT) plans with multi-leaf collimator (MLC) leaf errors that will not trigger MLC interlock during beam delivery; 2) the effect of non-beam-hold MLC leaf errors on the quality of VMAT plan dose delivery.Materials and methods.Eleven VMAT plans were selected and modified using an in-house developed software. For each control point of a VMAT arc, MLC leaves with the highest speed (1.87-1.95 cm/s) were set to move at the maximal allowable speed (2.3 cm/s), which resulted in a leaf position difference of less than 2 mm. The modified plans were considered as ‘standard’ plans, and the original plans were treated as the ‘slowing MLC’ plans for simulating ‘standard’ plans with leaves moving at relatively lower speed. The measurement of each ‘slowing MLC’ plan using MapCHECK®2 was compared with calculated planar dose of the ‘standard’ plan with respect to absolute dose Van Dyk distance-to-agreement (DTA) comparisons using 3%/3 mm and 2%/2 mm criteria.ResultsAll ‘slowing MLC’ plans passed the 90% pass rate threshold using 3%/3 mm criteria while one brain and three anal VMAT cases were below 90% with 2%/2 mm criteria. For ten out of eleven cases, DVH comparisons between ‘standard’ and ‘slowing MLC’ plans demonstrated minimal dosimetric changes in targets and organs-at-risk.ConclusionsFor highly modulated VMAT plans, pass rate threshold (90%) using 3%/3mm criteria is not sensitive in detecting MLC leaf errors that will not trigger the MLC leaf interlock. However, the consequential effects of non-beam hold MLC errors on target and OAR doses are negligible, which supports the reliability of current patient-specific IMRT quality assurance (QA) method for VMAT plans.

Dosimetric impact of dental metallic crown on intensity-modulated radiotherapy and volumetric-modulated arc therapy for head and neck cancer.

Metal dental restoration materials cause dose enhancement upstream and dose disturbance downstream of the high‐density inhomogeneous regions in which these materials are used. In this study, we evaluated the impact of a dental metallic crown (DMC) on intensity‐modulated radiotherapy (IMRT) and volumetric‐modulated arc therapy (VMAT) for head and neck cancer. Additionally, the possibility of sparing the oral mucosa from dose enhancement using an individual intraoral mouthpiece was evaluated. An experimental oral phantom was designed to verify the dosimetric impact of a DMC. We evaluated the effect on single beam, parallel opposing beam, arc beam, IMRT, and VMAT treatment plans. To evaluate the utility of a 3‐mm‐thick intraoral mouthpiece, the doses across the mouthpiece were measured. For single beam irradiation, the measured doses at the entrance and exit planes of the DMC were 51% higher and 21% lower than the calculated dose by the treatment planning system, respectively. The maximum dose enhancements were 22% and 46% for parallel opposing beams and the 90° arc rotation beam, respectively. For IMRT and VMAT, the measured doses adjacent to the DMC were 12.2%±6.3% (mean±1.96 SD) and 12.7%±2.5% higher than the calculated doses, respectively. With regard to the performance of the intraoral mouthpiece for the IMRT and VMAT cases, the disagreement between measured and calculated doses at the outermost surface of the mouthpieces were −2.0%, and 2.0%, respectively. Dose enhancements caused by DMC‐mediated radiation scattering occurred during IMRT and VMAT. Because it is difficult to accurately estimate the dose perturbations, careful consideration is necessary when planning head and neck cancer treatments in patients with DMCs. To spare the oral mucosa from dose enhancement, the use of an individual intraoral mouthpiece should be considered.PACS numbers: 87.55.km, 87.55.N‐, 87.55.Qr

Dosimetric Impact of the Jaw-Tracking Technique in Volumetric Modulated Arc Therapy

Objective: The jaw-tracking technique has been developed to reduce the radiation beam transmission in the regions blocked by multileaf collimator (MLC). The aim of this study is the dosimetric evaluation of the jaw-tracking technique in Volumetric Modulated Arc Therapy (VMAT). Material and Methods: 31 VMAT cases treated with the jaw-tracking technique were employed and re-planned with fixed jaw to analyze the dosimetric influences of the planning target volume (PTV) and organs at risk (OARs) volume. The treatment sites were liver, lung, and pancreas. All plans were optimized and calculated under jawtracking and fixed jaw conditions to cover the prescription dose to 95% of PTV (D95%) using the treatment planning system. The dosimetric verification of the treatment plans, the uniformity of the target dose distributions, the partial volume doses in OARs, and the low-dose volume were evaluated to verify the dosimetric impact of the jaw-tracking. Results: The jaw-tracking technique appeared to be able to provide some clinical advantages compared to the fixed-jaw technique. The dose uniformities in targets were similar between in jaw-tracking technique and in fixed jaw. It appeared that the jaw-tracking technique could significantly reduce the partial volume dose of OARs, for the kidney (p=0.008) and duodenum (p=0.028) in liver cancer cases, for the esophagus (p=0.015) in the lung cancer cases, and for the normal liver (p=0.005) and kidney (p=0.005) in the pancreatic cancer cases. The low-dose volumes with the jaw tracking technique were calculated to be smaller than those with fixed jaw setup when the effective maximum field dimension were 1.5 cm larger than the equivalent spherical diameter of the PTVs. Conclusions: The partial volume dose of OARs and the low-dose volume could be significantly reduced by application of the jaw-tracking technique without any adverse effect of the dosimetric parameter for targets in VMAT.

Dosimetric differences in flattened and flattening filter-free beam treatment plans.

This study investigated the dosimetric differences in treatment plans from flattened and flattening filter-free (FFF) beams from the TrueBeam System. A total of 104 treatment plans with static (sliding window) intensity-modulated radiotherapy beams and volumetric-modulated arc therapy (VMAT) beams were generated for 15 patients involving three cancer sites. In general, the FFF beam provides similar target coverage as the flattened beam with improved dose sparing to organ-at-risk (OAR). Among all three cancer sites, the head and neck showed more important differences between the flattened beam and FFF beam. The maximum reduction of the FFF beam in the mean dose reached up to 2.82 Gy for larynx in head and neck case. Compared to the 6 MV flattened beam, the 10 MV FFF beam provided improved dose sparing to certain OARs, especially for VMAT cases. Thus, 10 MV FFF beam could be used to improve the treatment plan.