Complex Volumetric Modulated Arc Therapy Research Articles

Clinical experiment on quality control comparison of complex treatment plans of the VMAT technique using a diode-based cylindrical phantom (ArcCHECK) and an amorphous silicon-based planar detector (A-Si1000)

BackgroundWith increasing application of Volumetric Modulated Arc Therapy (VMAT) and the complexity of treatment plans, necessity of patient-specific quality assurance procedures is raised. Therefore, this study aimed to perform VMAT plan control using the ArcCHECK® (Sun Nuclear) and the EPID (Electronic Portal Imaging Device) A-Si1000 and make a comparison. MethodsThe study was carried out at AL KAWTAR Oncology Center (Fez-Meknes/Morocco) with the TrueBeamTM accelerator and EclipseTM Treatment Planning System (TPS). GeoGebra 6.0 and Origin 2019 software were applied to statistical analysis and to reconstruct the dose distribution maps for ArcCHECK® Phantom, respectively. Gamma index test with the 3%/3 and 2%/2 mm criteria applied to compare two pretreatment quality control instrumentation. ResultsAlthough the EPID A-Si1000 and the ArcCHECK® phantom both were capable of performing VMAT plan control, results of gamma index analysis revealed some differences in the quality of the reviewed plans with the two devices. Conclusionbased on the results, a single device to implement patient QA for complex VMAT plans may not be reliable. Therefore, to ensure patient protection against machine or human errors, applying the EPID A-Si1000 and the ArcCHECK® phantom both are proposed to evaluate the quality of complex VMAT plans for different cancer types.

A comparative analysis of Acuros XB and the analytical anisotropic algorithm for volumetric modulation arc therapy

BackgroundThis study aimed to verify the dosimetric impact of Acuros XB (AXB) (AXB, Varian Medical Systems Palo Alto CA, USA), a two model-based algorithm, in comparison with Anisotropic Analytical Algorithm (AAA ) calculations for prostate, head and neck and lung cancer treatment by volumetric modulated arc therapy (VMAT ), without primary modification to AA. At present, the well-known and validated AA algorithm is clinically used in our department for VMAT treatments of different pathologies. AXB could replace it without extra measurements. The treatment result and accuracy of the dose delivered depend on the dose calculation algorithm.Materials and methodNinety-five complex VMAT plans for different pathologies were generated using the Eclipse version 15.0.4 treatment planning system (TPS). The dose distributions were calculated using AA and AXB (dose-to-water, AXBw and dose-to-medium, AXBm), with the same plan parameters for all VMAT plans. The dosimetric parameters were calculated for each planning target volume (PTV) and involved organs at risk (OA R). The patient specific quality assurance of all VMAT plans has been verified by Octavius®-4D phantom for different algorithms.ResultsThe relative differences among AA, AXBw and AXBm, with respect to prostate, head and neck were less than 1% for PTV D95%. However, PTV D95% calculated by AA tended to be overestimated, with a relative dose difference of 3.23% in the case of lung treatment. The absolute mean values of the relative differences were 1.1 ± 1.2% and 2.0 ± 1.2%, when comparing between AXBw and AA, AXBm and AA, respectively. The gamma pass rate was observed to exceed 97.4% and 99.4% for the measured and calculated doses in most cases of the volumetric 3D analysis for AA and AXBm, respectively.ConclusionThis study suggests that the dose calculated to medium using AXBm algorithm is better than AAA and it could be used clinically. Switching the dose calculation algorithm from AA to AXB does not require extra measurements.

DVH analysis using a transmission detector and model-based dose verification system as a comprehensive pretreatment QA tool for VMAT plans: Clinical experience and results.

PurposeDose volume histogram (DVH)‐based analysis is utilized as a pretreatment quality assurance tool to determine clinical relevance from measured dose which is difficult in conventional gamma‐based analysis. In this study, we report our clinical experience with an ionization‐based transmission detector and model‐based verification system, using DVH analysis, as a comprehensive pretreatment QA tool for complex volumetric modulated arc therapy plans.Methods and MaterialsSeventy‐three subsequent treatment plans categorized into four clinical sites (Head and Neck, Thorax, Abdomen, and Pelvis) were evaluated. The average dose (Dmean) and dose received by 1% (D1) of the planning target volumes (PTVs) and organs at risks (OARs) calculated using the treatment planning system (TPS) were compared to a computed (model‐based) and reconstructed dose, from the measured fluence, using DVH analysis. The correlation between gamma (3% 3 mm) and DVH‐based analysis for targets was evaluated. Furthermore, confidence and action limits for detector and verification systems were established.ResultsLinear regression confirmed an excellent correlation between TPS planned and computed dose using a model‐based verification system (r 2 = 1). The average percentage difference between TPS calculated and reconstructed dose for PTVs achieved using DVH analysis for each site is as follows: Head and Neck — 0.57 ± 2.8% (Dmean) and 2.6 ± 2.7% (D1), Abdomen — 0.19 ± 2.8% and 1.64 ± 2.2%, Thorax — 0.24 ± 2.1% and 3.12 ± 2.8%, Pelvis 0.37 ± 2.4% and 1.16 ± 2.3%, respectively. The average percentage of passed gamma values achieved was above 95% for all cases. However, no correlation was observed between gamma passing rates and DVH difference (%) for PTVs (r 2 = 0.11). The results demonstrate a confidence limit of 5% (Dmean and D1) for PTVs using DVH analysis for both computed and reconstructed dose distribution.ConclusionDVH analysis of treatment plan using a model‐based verification system and transmission detector provided useful information on clinical relevance for all cases and could be used as a comprehensive pretreatment patient‐specific QA tool.

Distributive quality assurance and delivery of stereotactic ablative radiotherapy treatments amongst beam matched linear accelerators: A feasibility study.

PurposeBeam matching occurs on all linacs to some degree and when two are more are matched to each other, patients are able to be transferred between machines. Quality assurance of plans can also be performed “distributively” on any of the matched linacs. The degree to which machines are matched and how this translates to like delivery of plans has been the focus of a number of studies. This concept has not yet been explored for stereotactic techniques which require a higher degree of accuracy. This study proposes beam matching criteria which allows for the distributive delivery and quality assurance of stereotactic body radiotherapy (SBRT) plans.MethodTwo clinically relevant and complex volumetric modulated arc therapy (VMAT) SBRT spine and lung plans were chosen as benchmarking cases. These were delivered on nine previously beam matched linacs with quality assurance performed through ArcCheck and film exposure in the sagittal plane. Measured doses were compared to their treatment planning system predictions through gamma analysis at a range of criteria.ResultsDespite differences in beam match parameters and variations in small fields, all nine linacs produced accurate deliveries with a tight deviation in the population sample. Pass rates were well above suggested tolerances at the recommended gamma criterion. Film was able to detect dose errors to a greater degree than ArcCheck.ConclusionDistributive quality assurance and delivery of stereotactic ablative radiotherapy treatments amongst beam matched linacs is certainly feasible provided the linacs are matched to a strict protocol like that suggested in this study and regular quality assurance is performed on the matched fleet. Distributive quality assurance and delivery of SBRT provides the possibility of efficiency gains for physicists as well as treatment staff.

199. Evaluation of beam’s attenuation using the Delta4 Discover transmission detector in various treatment application

Purpose Delta4 Discover (ScandiDos, Uppsala, Sweden) is a new device developed in order to monitor online complex radiation techniques such as volumetric-modulated arc therapy (VMAT). It can be inserted between the radiation source and the patient so it can potentially impact the beam quality that the patient receives. The aim of this work is to investigate, in terms of beam’s attenuation, the feasibility of using Discover transmission array [1] . Methods 9 VMAT treatment plans, generated using Pinnacle3’s SmartArc algorithm v.9.10 (Philips Fitchburg, USA), for various treatment sites, were delivered using Delta4 Phantom+ (ScandiDos, Uppsala, Sweden). The response, with and without the Discover mounted on the linac was evaluated. All plans were delivered on a Varian TrueBeam STx equipped with HD MLC. For phantom verification, plans were recalculated replacing the patient planning CT scan with a phantom CT scan. Dose distributions of each plan delivered with and without Discover were verified using the Delta4 Phantom + and compared to calculated dose. Results Analyzing dose difference profiles, the Relative Dose Difference, between the measured dose relative to both configurations, showed that the absorption of beam radiation observed was on average about 1%. About Median Dose Deviation between the calculated and measured dose for both configurations we found that the all Median Dose Deviation shift values are ranging from 0.2% to 1.3%. Conclusions The thin transmission detector Delta4 Discover shows good potential for real time quality assurance. The minimal attenuation observed potentially allows for a real time verification and independent monitoring of complex VMAT deliveries during patient treatment.

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.

Application of dual-energy CT to suppression of metal artefact caused by pedicle screw fixation in radiotherapy: a feasibility study using original phantom

The objective of the present study was the determination of the potential dosimetric benefits of using metal-artefact-suppressed dual-energy computed tomography (DECT) images for cases involving pedicle screw implants in spinal sites. A heterogeneous spinal phantom was designed for the investigation of the dosimetric effect of the pedicle-screw-related artefacts. The dosimetric comparisons were first performed using a conventional two-directional opposed (AP-PA) plan, and then a volumetric modulated arc therapy (VMAT) plan, which are both used for the treatment of spinal metastases in our institution. The results of Acuros® XB dose-to-medium (Dm) and dose-to-water (Dw) calculations using different imaging options were compared with experimental measurements including the chamber and film dosimetries in the spinal phantom. A dual-energy composition image with a weight factor of −0.2 and a dual-energy monochromatic image (DEMI) with an energy level of 180 keV were found to have superior abilities for artefact suppression. The Dm calculations revealed greater dosimetric effects of the pedicle screw-related artefacts compared to the Dw calculations. The results of conventional single-energy computed tomography showed that, although the pedicle screws were made from low-Z titanium alloy, the metal artefacts still have dosimetric effects, namely, an average (maximum) Dm error of 4.4% (5.6%) inside the spinal cord for a complex VMAT treatment plan. Our findings indicate that metal-artefact suppression using the proposed DECT (DEMI) approach is promising for improving the dosimetric accuracy near the implants and inside the spinal cord (average (maximum) Dm error of 1.1% (2.0%)).

SU-F-P-64: The Impact of Plan Complexity Parameters On the Plan Quality and Deliverability of Volumetric Modulated Arc Therapy with Canonical Correlation Analysis

Purpose: To evaluate the impact of complexity indices on the plan quality and deliverability of volumetric modulated arc therapy (VMAT), and to determine the most significant parameters in the generation of an ideal VMAT plan. Methods: A multi-dimensional exploratory statistical method, canonical correlation analysis (CCA) was adopted to study the correlations between VMAT parameters of complexity, quality and deliverability, as well as their contribution weights with 32 two-arc VMAT nasopharyngeal cancer (NPC) patients and 31 one-arc VMAT prostate cancer patients. Results: The MU per arc (MU/Arc) and MU per control point (MU/CP) of NPC were 337.8±25.2 and 3.7±0.3, respectively, which were significantly lower than those of prostate cancer patients (MU/Arc : 506.9±95.4, MU/CP : 5.6±1.1). The plan complexity indices indicated that two-arc VMAT plans were more complex than one-arc VMAT plans. Plan quality comparison confirmed that one-arc VMAT plans had a high quality than two-arc VMAT plans. CCA results implied that plan complexity parameters were highly correlated with plan quality with the first two canonical correlations of 0.96, 0.88 (both p<0.001) and significantly correlated with deliverability with the first canonical correlation of 0.79 (p<0.001), plan quality and deliverability was also correlated with the first canonical correlation of 0.71 (p=0.02). Complexity parameters of MU/CP, segment area (SA) per CP, percent of MU/CP less 3 and planning target volume (PTV) were weighted heavily in correlation with plan quality and deliveability . Similar results obtained from individual NPC and prostate CCA analysis. Conclusion: Relationship between complexity, quality, and deliverability parameters were investigated with CCA. MU, SA related parameters and PTV volume were found to have strong effect on the plan quality and deliverability. The presented correlation among different quantified parameters could be used to improve the plan quality and the efficiency of the radiotherapy process when creating a complex VMAT plan.

MO-FG-CAMPUS-TeP2-02: First Experiences and Perspectives in Using Direct Multicriteria Optimization (MCO) On Volumetric-Modulated Arc Therapy (VMAT) for Head and Neck Cancer

Purpose: To report the first experiences and perspectives in using direct multicriteria optimization (MCO) on volumetric-modulated arc therapy (VMAT) for head and neck (H&N) cancer. Methods: Ten prior patients with tumors in representative H&N regions were selected to evaluate direct MCO-VMAT in RayStation v5.0 beta. The patients were previously treated by intensity-modulated radiation therapy (IMRT) with MCO on an Elekta linear accelerator with Agility multileaf collimator. To avoid radiating eyes and shoulders, MCO-VMAT required one to three partial-arc groups, with each group consisting of single or dual arcs. All MCO-VMAT plans were approved by a radiation oncologist. The MCO-VMAT and MCO-IMRT plans were compared using V100, D5, homogeneity index (HI) and conformity index (CI) for planning target volume (PTV), Dmean and D50 for six parallel organs and Dmax for five serial organs. Patient-specific quality assurance (QA) was performed using ArcCHECK for MCO-VMAT and Matrixx for MCO-IMRT with results analyzed using gamma criteria of 3%/3mm. Results: MCO-VMAT provided better V100 (+0.8%) lower D5(− 0.3 Gy), lower HI (−0.27) and comparable CI (+0.05). MCO-VMAT decreased Dmean and D50 for multiple parallel organs in seven of the ten patients. On average the reduction ranged from 2.1 (larynx) to 7.6 Gy (esophagus). For the nasal cavity and nasopharynx plans significant reduction in Dmax was observed for optics (up to 11 Gy) brainstem (6.4 Gy), cord (2.1 Gy) and mandible (6.7 Gy). All MCO-VMAT and -IMRT plans passed clinical QA. MCO-VMAT required slightly longer planning time due to the more complex VMAT optimization. The net beam-on time for the MCO-VMAT plans ranged from 80 to 242 seconds, up to 9 minutes shorter than MCO-IMRT. Conclusion: With similar target coverage, reduced organ dose, comparable planning time, and significantly faster treatment, MCO-VMAT is very likely to become the modality of choice in RayStation v5.0 for H&N cancer.

Pancreatic cancer planning: Complex conformal vs modulated therapies

To compare the roles of intensity-modulated radiation therapy (IMRT) and volumetric- modulated arc therapy (VMAT) therapy as compared to simple and complex 3-dimensional chemoradiotherpy (3DCRT) planning for resectable and borderline resectable pancreatic cancer. In all, 12 patients who received postoperative radiotherapy (8) or neoadjuvant concurrent chemoradiotherapy (4) were evaluated retrospectively. Radiotherapy planning was performed for 4 treatment techniques: simple 4-field box, complex 5-field 3DCRT, 5 to 6-field IMRT, and single-arc VMAT. All volumes were approved by a single observer in accordance with Radiation Therapy Oncology Group (RTOG) Pancreas Contouring Atlas. Plans included tumor/tumor bed and regional lymph nodes to 45Gy; with tumor/tumor bed boosted to 50.4Gy, at least 95% of planning target volume (PTV) received the prescription dose. Dose-volume histograms (DVH) for multiple end points, treatment planning, and delivery time were assessed. Complex 3DCRT, IMRT, and VMAT plans significantly (p < 0.05) decreased mean kidney dose, mean liver dose, liver (V30, V35), stomach (D10%), stomach (V45), mean right kidney dose, and right kidney (V15) as compared with the simple 4-field plans that are most commonly reported in the literature. IMRT plans resulted in decreased mean liver dose, liver (V35), and left kidney (V15, V18, V20). VMAT plans decreased small bowel (D10%, D15%), small bowel (V35, V45), stomach (D10%, D15%), stomach (V35, V45), mean liver dose, liver (V35), left kidney (V15, V18, V20), and right kidney (V18, V20). VMAT plans significantly decreased small bowel (D10%, D15%), left kidney (V20), and stomach (V45) as compared with IMRT plans. Treatment planning and delivery times were most efficient for simple 4-field box and VMAT. Excluding patient setup and imaging, average treatment delivery was within 10minutes for simple and complex 3DCRT, IMRT, and VMAT treatments. This article shows significant improvements in 3D plan performance with complex planning over the more frequently compared 3- or 4-field simple 3D planning techniques. VMAT plans continue to demonstrate potential for the most organ sparing. However, further studies are required to identify if dosimetric benefits associated with inverse optimized planning can be translated into clinical benefits and if these treatment techniques are value-added therapies for this group of patients with cancer.

Toward optimal organ at risk sparing in complex volumetric modulated arc therapy: An exponential trade‐off with target volume dose homogeneity

Conventional radiotherapy typically aims for homogenous dose in the planning target volume (PTV) while sparing organs at risk (OAR). The authors quantified and characterized the trade-off between PTV dose inhomogeneity (IH) and OAR sparing in complex head and neck volumetric modulated arc therapy plans. Thirteen simultaneous integrated boost plans were created per patient, for ten patients. PTV boost(B)/elective(E) optimization priorities were systematically increased. IHB and IHE, defined as (100% - V95%) + V107%, were evaluated against the average of the mean dose to the combined composite swallowing and combined salivary organs (D-OAR(comp)). To investigate the influence of OAR size and position with respect to PTVB/E, OAR dose was evaluated against a modified Euclidean distance (DMB/DME) between OAR and PTV. Although the achievable D-OAR(comp) for a given level of PTV IH differed between patients, excellent logarithmic fits described the D-OAR(comp)/IHB and IHE relationship in all patients (mean R(2) of 0.98 and 0.97, respectively). Allowing an increase in average IHB and IHE over a clinically acceptable range, e.g., from 0.4% ± 0.5% to 2.0% ± 2.0% and 6.9% ± 2.8% to 14.8% ± 2.7%, respectively, corresponded to a decrease in average dose to the composite salivary and swallowing structures from 30.3 ± 6.5 to 23.6 ± 4.7 Gy and 32.5 ± 8.3 to 26.8 ± 9.3 Gy. The increase in PTVE IH was mainly accounted for by an increase in V107, by on average 5.9%, rather than a reduction in V95, which was on average only 2%. A linear correlation was found between the OAR dose to composite swallowing structures and contralateral parotid and submandibular gland, with DME (R(2) = 0.83, 0.88, 0.95). Only mean ipsilateral parotid dose correlated with DMB (R(2) = 0.87). OAR sparing is highly dependent on the permitted PTVB/E IH. PTVE IH substantially influences OAR doses. These results are relevant for clinical practice and for future automated treatment-planning strategies.

A comprehensive investigation of the accuracy and reproducibility of a multitarget single isocenter VMAT radiosurgery technique

Recent trends in stereotactic radiosurgery use multifocal volumetric modulated arc therapy (VMAT) plans to simultaneously treat several distinct targets. Conventional verification often involves low resolution measurements in a single plane, a cylinder, or intersecting planes of diodes or ion chambers. This work presents an investigation into the consistency and reproducibility of this new treatment technique using a comprehensive commissioned high-resolution 3D dosimetry system (PRESAGE(®)∕Optical-CT). A complex VMAT plan consisting of a single isocenter but five separate targets was created in Eclipse for a head phantom containing a cylindrical PRESAGE(®) dosimetry insert of 11 cm diameter and height. The plan contained five VMAT arcs delivering target doses from 12 to 20 Gy. The treatment was delivered to four dosimeters positioned in the head phantom and repeated four times, yielding four separate 3D dosimetry verifications. Each delivery was completely independent and was given after image guided radiation therapy (IGRT) positioning using Brainlab ExacTrac and cone beam computed tomography. A final delivery was given to a modified insert containing a pin-point ion chamber enabling calibration of PRESAGE(®) 3D data to dose. Dosimetric data were read out in an optical-CT scanner. Consistency and reproducibility of the treatment technique (including IGRT setup) was investigated by comparing the dose distributions in the four inserts, and with the predicted treatment planning system distribution. Dose distributions from the four dosimeters were registered and analyzed to determine the mean and standard deviation at all points throughout the dosimeters. A dose standard deviation of <3% was found from dosimeter to dosimeter. Global 3D gamma maps show that the predicted and measured dose matched well [3D gamma passing rate was 98.0% (3%, 2 mm)]. The deliveries of the irradiation were found to be consistent and matched the treatment plan, demonstrating high accuracy and reproducibility of both the treatment machine and the IGRT procedure. The complexity of the treatment (multiple arcs) and dosimetry (multiple strong gradients) pose a substantial challenge for comprehensive verification. 3D dosimetry can be uniquely effective in this scenario.

MO‐D‐108‐01: Cloud‐Based Monte Carlo Patient‐Specific Quality Assurance (QA) Method for Volumetric‐Modulated Arc Therapy (VMAT): Clinical and Educational Impacts

Purpose: Patient‐specific QA for VMAT is labor‐intensive,especially in cases which tend to be associated with severe heterogeneities or small aperture beams. We developed a robust cloud‐based Monte Carlo method as an independent QA tool or backup for commercial dosimeters. It is also educationally innovative because it helps physicists to comprehend the underlying physics of complex VMAT procedures. Methods: The workflow of the cloud‐based voxel Monte Carlo method(cVMC++) developed here is the following: (1) After a VMAT plan is approved by the physician, a dose verification plan is created and delivered to the phantom using the Varian Trilogy™ or TrueBeam™ system. (2) Actual delivery parameters (i.e., dose fraction, gantry angle, and MLC leaf position) are extracted from dynamic log or trajectory files. (3) Based on the delivery parameters, a new phase space is constructed for each control point. The 3D dose distribution in the phantom containing the detector array (e.g., Delta 4 from ScandiDos) are then recomputed using cVMC++. (4) Comparison and Gamma index analyses are then conducted to evaluate the agreement between measured, recomputed, and planned dose distributions. To enhance the educational function, we are developing visualization and animation modules to display the intermediate processes such as leaf movement, dose distribution and DVH accumulation with each control point. The software has been wrapped as a web‐based service so that it can readily be used by physicists via a browser. To test the robustness of this system, we examined several representative VMAT treatments. Results: For all cases, agreement between measured, recomputed and planned doses was examined with the gamma criterion of &gt;90% of the points satisfying &lt;3% of relative dose difference and &lt;3 mm of distance to agreement. Conclusion: The proposed method offers a robust patient specific QA tool for VMAT treatments and provides a valuable educational tool for physicists. NIH (1R01 CA133474) and Department of Radiation Oncology of Stanford University Seed Grant

Measurement comparison and Monte Carlo analysis for volumetric‐modulated arc therapy (VMAT) delivery verification using the ArcCHECK dosimetry system

The objective of this study is to validate the capabilities of a cylindrical diode array system for volumetric‐modulated arc therapy (VMAT) treatment quality assurance (QA). The VMAT plans were generated by the Eclipse treatment planning system (TPS) with the analytical anisotropic algorithm (AAA) for dose calculation. An in‐house Monte Carlo (MC) code was utilized as a validation tool for the TPS calculations and the ArcCHECK measurements. The megavoltage computed tomography (MVCT) of the ArcCHECK system was adopted for the geometry reconstruction in the TPS and for MC simulations. A 10×10 cm2 open field validation was performed for both the 6 and 10 MV photon beams to validate the absolute dose calibration of the ArcCHECK system and also the TPS dose calculations for this system. The impact of the angular dependency on noncoplanar deliveries was investigated with a series of 10×10 cm2 fields delivered with couch rotation 0° to 40°. The sensitivity of detecting the translational (1 to 10 mm) and the rotational (1° to 3°) misalignments was tested with a breast VMAT case. Ten VMAT plans (six prostate, H&N, pelvis, liver, and breast) were investigated to evaluate the agreement of the target dose and the peripheral dose among ArcCHECK measurements, and TPS and MC dose calculations. A customized acrylic plug holding an ion chamber was used to measure the dose at the center of the ArcCHECK phantom. Both the entrance and the exit doses measured by the ArcCHECK system with and without the plug agreed with the MC simulation to 1.0%. The TPS dose calculation with a 2.5 mm grid overestimated the exit dose by up to 7.2% when the plug was removed. The agreement between the MC and TPS calculations for the ArcCHECK without the plug improved significantly when a 1 mm dose calculation grid was used in the TPS. The noncoplanar delivery test demonstrated that the angular dependency has limited impact on the gamma passing rate (<1.2% drop) for the 2%–3% dose and 2 mm–3 mm DTA criteria. A 1° rotational misalignment introduces 11.3% (3%/3 mm) to 21.3% (1%/1 mm) and 0.2% (3%/3 mm) to 0.8% (1%/1 mm) Gamma passing rate drop for ArcCHECK system and MatriXX system, respectively. Both systems have comparable sensitivity to the AP misalignments. However, a 2 mm RL misalignment introduces gamma passing rate drop ranging from 0.9% (3%/3 mm) to 4.0% (1%/1 mm) and 5.0% (3%/3 mm) to 12.0% (1%/1 mm) for ArcCHECK and MatriXX measurements, respectively. For VMAT plan QA, the gamma analysis passing rates ranged from 96.1% (H&N case) to 99.9% (prostate case), when using the 3%/3 mm DTA criteria for the peripheral dose validation between the TPS and ArcCHCEK measurements. The peripheral dose validation between the MC simulation and ArcCHECK measurements showed at least 97.9% gamma passing rates. The central dose validation also showed an agreement within 2.2% between TPS/MC calculations and ArcCHECK measurements. The worst discrepancy was found in the H&N case, which is the most complex VMAT case. The ArcCHECK system is suitable for VMAT QA evaluation based on the sensitivity to detecting misalignments, the clinical impact of the angular dependency, and the correlation between the dose agreements in the peripheral region and the central region. This work also demonstrated the importance of carrying out a thorough validation of both the TPS and the dosimetry system prior to utilizing it for QA, and the value of having an independent dose calculation tool, such as the MC method, in clinical practice.PACS number: 87.55.Qr

A real time dose monitoring and dose reconstruction tool for patient specific VMAT QA and delivery

To develop a real time dose monitoring and dose reconstruction tool to identify and quantify sources of errors during patient specific volumetric modulated arc therapy (VMAT) delivery and quality assurance. The authors develop a VMAT delivery monitor tool called linac data monitor that connects to the linac in clinical mode and records, displays, and compares real time machine parameters with the planned parameters. A new measure, called integral error, keeps a running total of leaf overshoot and undershoot errors in each leaf pair, multiplied by leaf width, and the amount of time during which the error exists in monitor unit delivery. Another tool reconstructs Pinnacle(3)™ format delivered plan based on the saved machine logfile and recalculates actual delivered dose in patient anatomy. Delivery characteristics of various standard fractionation and stereotactic body radiation therapy (SBRT) VMAT plans delivered on Elekta Axesse and Synergy linacs were quantified. The MLC and gantry errors for all the treatment sites were 0.00 ± 0.59 mm and 0.05 ± 0.31°, indicating a good MLC gain calibration. Standard fractionation plans had a larger gantry error than SBRT plans due to frequent dose rate changes. On average, the MLC errors were negligible but larger errors of up to 6 mm and 2.5° were seen when dose rate varied frequently. Large gantry errors occurred during the acceleration and deceleration process, and correlated well with MLC errors (r = 0.858, p = 0.0004). PTV mean, minimum, and maximum dose discrepancies were 0.87 ± 0.21%, 0.99 ± 0.59%, and 1.18 ± 0.52%, respectively. The organs at risk (OAR) doses were within 2.5%, except some OARs that showed up to 5.6% discrepancy in maximum dose. Real time displayed normalized total positive integral error (normalized to the total monitor units) correlated linearly with MLC (r = 0.9279, p < 0.001) and gantry errors (r = 0.742, p = 0.005). There is a strong correlation between total integral error and PTV mean (r = 0.683, p = 0.015), minimum (r = 0.6147, p = 0.033), and maximum dose (r = 0.6038, p = 0.0376). Errors may exist during complex VMAT planning and delivery. Linac data monitor is capable of detecting and quantifying mechanical and dosimetric errors at various stages of planning and delivery.

SU‐E‐T‐355: Volumetric Modulated Arc Therapy Treatment Quality Assurance Using an Ion Chamber Array and a Software‐Modified Phantom

Purpose: To systematically test an algorithm to correct for the directional response of a 2D ion chamber array (PTW Seven29TM) and implement techniques to use it in quality assurance (QA) of volumetric modulated arc therapy (VMAT). Methods: The 27×27 ion chamber array was placed in sagittal orientation between two slabs of 10‐cm thick Plastic WaterTM to measure the directional response. Photon beams from anterior hemisphere of 10×10cm2 field size and energies of 6, 10 and 15MV with flattening filter, and 6 and 10MV without flattening filter from Varian TrueBeamTM were used with gantry angles ranging from 90° to 270°(IEC). An artificial cylindrical half‐shell of 1‐cm thickness was inserted into the backside 10‐cm slab in the treatment planning system and assigned the CT number of 2000 Hounsfield units to compensate for the Seven29TM directional response. The method was tested using half‐arc plans with 5×5cm2 and 10×10cm2 field size in sagittal orientation and full‐arc VMAT patient treatment plans in both sagittal and coronal orientations. Results: In the absence of the attenuation shell, measurements of the Seven29TM in sagittal orientation indicate a relatively flat response for gantry angles 90° to 15°, decreasing to 97.2% at 0°, and an approximately uniform under response of ∼89% for the beams from the backside quadrant. Using the correction method resulted in agreement within 2.8% for each beam angle and 1.9% for half‐arc deliveries. Results of a typical VMAT QA plan gave gamma test (3 mm, 3%) passing rates of 95.3% and 99.7% in sagittal and coronal orientations, respectively. Conclusions: The combination of Seven29TM and software‐modified Plastic WaterTM phantom proved to be a fast and reliable tool for QA of VMAT. Through this characterization of its directional response, accurate orthogonal 2D dose maps can be generated for independent verification of the treatment delivery from complex VMAT plans.

SU‐FF‐T‐141: Initial Clinical Experience with Elekta VMAT

Purpose: Elekta recently introduced a new delivery control system called PreciseBeam® VMAT that provides dynamic delivery capabilities including volumetric modulated arc therapy (VMAT). VMAT is a rotational approach to the delivery of IMRT that can be delivered on a conventional linear accelerator. In this study, we report on our initial clinical experience with Elekta VMAT. Method and Materials: The PreciseBeam® VMAT control system has been installed on an Elekta Synergy and an Elekta Precise linear accelerator in our clinic. We have tested the delivery system in conjunction with three inverse planning solutions: (1) an in‐house arc sequencing algorithm that translates optimized fluence maps into deliverable VMAT plans, (2) the Ergo++ planning system that utilizes anatomy based inverse planning, and (3) the SmartArc planning module in the Pinnacle3 treatment planning system. The optimized treatment plans have been verified using phantom irradiations with the IBA MatriXX 2D ion chamber array embedded in a MULTcube phantom. Results: Using our arc‐sequencing algorithm for VMAT planning, we treated our first Elekta VMAT patient under an institutional review board (IRB) protocol in July 2008. In January 2009, we treated our first Elekta VMAT case planned using Ergo++. Our initial focus was on more simple concave targets. More recently, we have opened up VMAT for all clinical sites. Conclusion: Elekta's Precise Beam® VMAT control system can safely and efficiently delivery highly complex VMAT treatment plans. For simple targets such as prostate, pancreatic, and lung cases, single arc VMAT plans are sufficient and be typically delivered in less than 3 minutes. For more complex cases such as head‐and‐neck, the use of multiple arcs provides a dosimetric benefit and plans typically take between 4 and 6 minutes to deliver.Research sponsored in part through a grant from Elekta.

TH-D-AUD B-03: Initial Experience with the Delivery of Volumetric Modulated Arc Therapy

Purpose: Recently, there has been a renewed interest in the delivery of arcbased IMRT using conventional linear accelerators. Elekta and Varian have developed linear acceleratorcontrol systems that are capable of delivering rotational IMRT by combining gantry rotation, dynamic MLC leaf motion, and a variable dose rate. Elekta's new Precise Beam Infinity™ control system has been installed in our clinic and acceptance testing and plan verifications have been performed. In this study, we will report on our initial experiences with volumetric modulated arc therapy (VMAT) delivered using an Elekta Precise linear accelerator.Method and Materials: VMAT is a radiotherapydelivery technique that combines the dosimetric advantages of rotational delivery with the dose painting capabilities of IMRT. We have developed an arc sequencing algorithm that translates optimized fluence maps into deliverable VMAT treatment plans. In this investigation, one head‐and‐neck and five prostate plans have been delivered in an effort to quantify the efficiency and accuracy of the VMAT delivery system. Results: A complex head‐and‐neck plan involving two targets and a simultaneous boost was delivered in 5 minutes 29 seconds. For this three‐arc head‐and‐neck plan, the point dose agreed within 1.9%. Additional measurements for 5 prostate cases demonstrated an average delivery time of 3 minutes 28 seconds with all ion chamber measurements agreeing within 3%. Film measurements demonstrated close agreement between the predicted and measured isodose curves. Measurements have also been performed to quantify efficiency of single‐arc versus multi‐arc VMAT deliveries.Conclusion: Elekta's Precise Beam Infinity control system can safely and efficiently delivery highly complex VMAT treatment plans. In our initial investigation, all plans delivered in less than 5.5 minutes. Numerous additional plan verifications will be performed as we move towards an anticipated clinical implementation of VMAT in May 2008. Research sponsored in part through a grant from Elekta.