Volumetric Modulated Arc Therapy Patient Research Articles

Design, Fabrication, and Validation of a Polymethyl Methacrylate Head Phantom for Dosimetric Verification of Cranial Radiotherapy Treatment Plans.

Purpose:The present study aims to design and fabricate a novel, versatile, and cost-effective Polymethyl Methacrylate (PMMA) head phantom for the dosimetric pretreatment verification of radiotherapy (RT) treatment plans.Materials and Methods:The head phantom designing involves slice-wise modeling of an adult head using PMMA. The phantom has provisions to hold detectors such as ionization chambers of different sizes, Gafchromic films, gel dosimeter, and optically stimulated luminescence dosimeter. For the point dose verification purpose, 15 volumetric modulated arc therapy patient plans were selected, and doses were measured using a CC13 ionization chamber. The percentage gamma passing rate was calculated for acceptance criteria 3%/3 mm and 2%/2 mm using OmniPro I’mRT film QA software, and Gafchromic EBT3 films were used for 2D planar dose verification.Results:Treatment planning system calculated, and the measured point doses showed a percentage deviation ranged from 0.26 to 1.92. The planar dose fluence measurements, for set acceptance criteria of 3%/3 mm and 2%/2 mm, percentages of points having gamma value <1 were in the range of 99.17 ± 0.25 to 99.88 ± 0.15 and 93.16 ± 0.38 to 98.89 ± 0.23, respectively. Measured dose verification indices were within the acceptable limit.Conclusions:The dosimetric study reveals that head phantom can be used for routine pretreatment verification for the cranial RT, especially for stereotactic radiosurgery/RT as a part of patient-specific quality assurance. The presently fabricated and validated phantom is novel, versatile, and cost-effective, and many institutes can afford it.

One-year Follow-up of Volumetric-modulated Arc Therapy for Treating Chest Wall and Regional Nodes in Locally Advanced Breast Cancer

We evaluated dosimetric, toxicity and early clinical outcomes of volumetric-modulated arc therapy (VMAT) treating chest wall and regional nodes in locally advanced breast cancer. Between July 2016 and January 2018, treatment outcomes of 125 patients with stage II ∼III breast cancer treated with postoperative radiotherapy including chest wall and regional nodes using VMAT were analyzed. All patients underwent mastectomy. We gave radiation to the chest wall + infraclavicular region, supraclavicular area and part of the axillary bed for patients with axillary nodes 1-3 positive. For patients with axillary nodes ≥ 4 positive, internal mammary nodes were included additionally. Doses to chest wall and regional nodes were 50Gy, delivered in 25 fractions over 5 weeks. The median follow-up was 21 months, range 12–30 months. VMAT showed good homogeneity and improved conformity. The acute skin toxicity by the end of treatment was Grade 0 in 15/125 (12%) patients, Grade 1 in 88 /125(70%), Grade 2 in 22 /125 (18%), and Grade 3 in 0. No Grade 4 toxicities were observed. Sixty-five of 125(52%) underwent Grade 1 acute esophagitis. The incidence of lung toxicity of grade 2 or more was 2/125(1.6%), and cardiac toxicity was not observed. No patient developed local recurrence. VMAT appears safe for the treatment of locally advanced breast cancer. Long-term follow-up is needed to assess the benefit and harm of VMAT and careful patient selection is necessary in choosing VMAT as RT technique.

9 MLC error detection with EPIBeam for VMAT patient quality assurance

Introduction In recent years, the part of Volumetric Modulated Arc Therapy (VMAT) in external radiotherapy treatments has become more and more important. In fact, this technique allows to deliver the right dose to the tumor region while sparing surrounding organs. To get this complexity, the correct functioning of the Multi-Leaf Collimator (MLC) is particularly critical. Controls are recommended to ensure that the accelerator is able to deliver correctly each treatment plan. Electronic Portal Imaging Devices (EPIDs) may be useful to perform some of patient specific Quality Assurance (QA) [1] . This study proposes to evaluate if, associated with the software EPIBeam (Dosisoft®), they are able to detect MLC errors which could have a significant clinical impact. Methods Ten VMAT plans QA for patients treated for prostate cancer using a Synergy (Elekta®) accelerator were performed using two methods. The first method uses ArcCheck (SunNuclear®) and the second uses the EPID IView GT (Elekta®) associated with the EPIBeam software. Gamma analyses were used using global 3%/ 3 mm criterion. In addition, for each plan, MLC errors were introduced. All the leaves of the same bank were shift by 0.5, 1, 2 and 3 mm in the opening direction. The clinical impact of these errors has been evaluated in the MONACO planning software (Elekta®). These modified plans were checked in their turn. A control is considered satisfactory if more than 95% of the points have a gamma index greater than 1. Results For MLC error free treatment plans, EPIBeam achieves better gamma analysis results: mean = 99.8% [Min: 98.7%/ Max = 100%] versus 99% [Min: 98.1%/ Max: 99.7%] with ArcCheck. For errors less than or equal to 1 mm, all controls were within the tolerances for both modalities. For 2 mm errors, the number of processing plan within tolerances is 1/10 for ArcCheck versus 4/10 with EPIBeam. For 3 mm errors, no plan is within tolerances for the ArcCHECK while a plan passes for EPIbeam. It should be noted that the 2 and 3 mm errors induces significate changes on the dose distribution inside the patient. For example, an increase of the maximum rectal wall dose of 2.9% for 2 mm and 4.9% for 3 mm are observed. Conclusions With the 3%/3 mm criterion for gamma index analysis, less MLC errors were detected with EPIBEAM as with ArcCheck. However, by working on the analysis criteria, the error detection capability may be improved. It should be also interesting to combine EPIBeam with other controls such as log file analysis to have better detection efficiency while keeping time savings [2] .

Physical dosimetry of volumetric modulated arc therapy (VMAT) using EPID and 2D array for quality assurance

OutlineTo address the correspondence of measured and predicted doses for different malignant tumours utilizing various gamma criteria and QA for confirmation of VMAT with an EPID and 2D array detector. Methods24 patients with different malignant tumors were treated by VMAT techniques on Varian IX linear accelerator with 6 MV photon beams. Eclipse treatment planning system (TPS) is used to plan Patient’s charts. Gamma Index (GI) variation was compared to the procedure of pre-treatment verification in VMAT plans. ResultsThe gamma criteria (DD/DTA) of dose difference and distance to agreement for (3%/3 mm), mean ± SD are γ≤1% = 99.42% ± 0.67%, γmax = 2.11 ± 0.56 and γavg = 0.19 ± 0.05 by EPID, and γ%≤1 = 99.36% ± 0.53%, γmax = 1.65 ± 0.45 and γavg = 0.22 ± 0.05 by using 2D array detector. ConclusionsSpecific QA of VMAT patient (using EPID or 2D array) display great possibility to spare time and to verify individual IMRT fields. 3%/3 mm is the most appropriate of gamma criteria (DD/DTA) for VMAT plans quality assurance. Control charts are a beneficial method for verification assessment for patient specific quality control.

Commissioning and validation of COMPASS system for VMAT patient specific quality assurance

Pre-treatment patient specific quality assurance (QA) of advanced treatment techniques such as volumetric modulated arc therapy (VMAT) is one of important QA in radiotherapy. The fast and reliable dosimetric device is required. The objective of this study is to commission and validate the performance of COMPASS system for dose verification of VMAT technique. The COMPASS system is composed of an array of ionization detectors (MatriXX) mounted to the gantry using a custom holder and software for the analysis and visualization of QA results. We validated the COMPASS software for basic and advanced clinical application. For the basic clinical study, the simple open field in various field sizes were validated in homogeneous phantom. And the advanced clinical application, the fifteen prostate and fifteen nasopharyngeal cancers VMAT plans were chosen to study. The treatment plans were measured by the MatriXX. The doses and dose-volume histograms (DVHs) reconstructed from the fluence measurements were compared to the TPS calculated plans. And also, the doses and DVHs computed using collapsed cone convolution (CCC) Algorithm were compared with Eclipse TPS calculated plans using Analytical Anisotropic Algorithm (AAA) that according to dose specified in ICRU 83 for PTV.

Analyzing the performance of ArcCHECK diode array detector for VMAT plan

AimThe aim of this study is to evaluate performance of ArcCHECK diode array detector for the volumetric modulated arc therapy (VMAT) patient specific quality assurance (QA). VMAT patient specific QA results were correlated with ion chamber measurement. Dose response of the ArcCHECK detector was studied. BackgroundVMAT delivery technique improves the dose distribution. It is complex in nature and requires proper QA before its clinical implementation. ArcCHECK is a novel three dimensional dosimetry system. Materials and methodsTwelve retrospective VMAT plans were calculated on ArcCHECK phantom. Point dose and dose map were measured simultaneously with ion chamber (IC-15) and ArcCHECK diode array detector, respectively. These measurements were compared with their respective TPS calculated values. ResultsThe ion chamber measurements are in good agreement with TPS calculated doses. Mean difference between them is 0.50% with standard deviation of 0.51%. Concordance correlation coefficient (CCC) obtained for ion chamber measurements is 0.9996. These results demonstrate a strong correlation between the absolute dose predicted by our TPS and the measured dose. The CCC between ArcCHECK doses and TPS predictions on the CAX was found to be 0.9978. In gamma analysis of dose map, the mean passing rate was 98.53% for 3% dose difference and 3mm distance to agreement. ConclusionsThe VMAT patient specific QA with an ion chamber and ArcCHECK phantom are consistent with the TPS calculated dose. Statistically good agreement was observed between ArcCHECK measured and TPS calculated. Hence, it can be used for routine VMAT QA.

In-house spread sheet based monitor unit verification program for volumetric modulated arc therapy

Independent monitor unit verification calculation (MUVC) has been recommended by several authors for intensity modulated radiotherapy (IMRT) as a patient specific quality assurance tool. Aim of the present work is to develop an in-house excel spread sheet based MUVC program for volumetric modulated arc therapy (VMAT) using Clarkson's integration technique. Total scatter factor (Sc,p) and tissue maximum ratio (TMR) for circular fields obtained from Treatment planning system (TPS) were used for the calculation. Multileaf collimator (MLC) interleaf leakage, MLC round edge transmission and tongue and groove effect were accounted. MUVC calculation was performed for 58 patients both for patient anatomy and for homogenous cylindrical phantom. Radiological path lengths were used as water equivalent depths (WED) for calculations using patient anatomy. Monitor unit (MU) discrepancies between −2.60% and 0.28% with mean deviation of −0.92% ± 0.75% were obtained for homogenous cylindrical phantom calculations. MUVC for patient anatomy resulted in large variations between −19.02% and 0.67% for 14 plans where isocenter was at a region below −350 HU. But For 44 plans where the isocenter was at a region above −350 HU, variations between −3.44% and 0.48% were obtained with mean deviation of −1.73% ± 1.12%. For VMAT patient specific quality assurance, the independent MUVC algorithm can be used as an easy and quick auxiliary to measurement based verification for plans with isocenter at a region above −350 HU.

Sensitivity of volumetric modulated arc therapy patient specific QA results to multileaf collimator errors and correlation to dose volume histogram based metrics

This study investigates the impact of systematic multileaf collimator (MLC) positional errors on gamma analysis results used for quality assurance (QA) of Rapidarc treatments. In addition, this study evaluates the relationship of these gamma analysis results and clinical dose volume histogram metrics (DVH) for Rapidarc treatment plans. Five prostate plans were modified by the introduction of systematic MLC errors. The MLC shifts to each individual active leaf introduced were 0.25, 0.5, 0.75, and 1 mm. All QA verification plans were delivered and estimated 3D patient dose or high density phantom dose were obtained based on the ArcCHECK measurement files. QA gamma analysis of 3%/3 mm and 2%/2 mm were implemented and relationships to dose differences in DVH metrics encountered due to MLC errors were determined. Tolerances of 3% and 5% for DVH metric were implemented to determine the sensitivity of gamma analysis to MLC errors. A calculation of sensitivity was determined from the number of incidences of false negative and false positive cases in gamma analysis results. The sensitivity of global gamma analysis for criteria of 3%/3 mm was 0.78 and for 2%/2 mm was 0.82. A number of instances occurred for an acceptable VMAT QA gamma index which did not indicate a DVH metric dose error greater than 5%. The correlation between global gamma analysis using criteria 3%/3 mm and DVH metric dose error were all <0.8 indicating less than a strong correlation. There is a greater sensitivity for detection of dosimetric errors occurring in a Rapidarc plan using gamma criteria of 2%/2 mm than 3%/3 mm. However, there is lack of consistently strong correlation between global gamma indexes and clinical DVH metrics for PTV and bladder and rectum for Rapidarc plans. It is recommended that the sole use of gamma index for Rapidarc QA plan evaluation could be insufficient and a methodology for evaluation of delivered dose to patient is required.

The dosimetric impact of dental implants on head-and-neck volumetric modulated arc therapy

This work aims to investigate the dosimetric impact of dental implants on volumetric modulated arc therapy (VMAT) for head-and-neck patients and to evaluate the effectiveness of using the material's electron-density ratio for the correction. An in-house Monte Carlo (MC) code was utilized for the dose calculation to account for the scattering and attenuation caused by the high-Z implant material. Three different dental implant materials were studied in this work: titanium, Degubond®4 and gold. The dose perturbations caused by the dental implant materials were first investigated in a water phantom with a 1 cm3 insert. The per cent depth dose distributions of a 3 × 3 cm2 photon field were compared with the insert material as water and the three selected dental implant materials. To evaluate the impact of the dental implant on VMAT patient dose calculation, four head-and-neck cases were selected. For each case, the VMAT plan was designed based on the artifact-corrected patient geometry using a treatment planning system (TPS) that was typically utilized for routine patient treatment. The plans were re-calculated using the MC code for five situations: uncorrected geometry, artifact-corrected geometry and artifact-corrected geometry with one of the three different implant materials. The isodose distributions and the dose–volume histograms were cross-compared with each other. To evaluate the effectiveness of using the material's electron-density ratio for dental implant correction, the implant region was set as water with the material's electron-density ratio and the calculated dose was compared with the MC simulation with the real material. The main effect of the dental implant was the severe attenuation in the downstream. The 1 cm3 dental implant can lower the downstream dose by 10% (Ti) to 51% (Au) for a 3 × 3 cm2 field. The TPS failed to account for the dose perturbation if the dental implant material was not precisely defined. For the VMAT patient dose calculation, the presence of dental implants degrades the PTV coverage significantly. With the material's electron-density ratio applied, the dose calculation accuracy in the water phantom and the VMAT patient was improved to a clinically acceptable level. The effects of the dental implant material can be clinically significant and its impact varies with the density of the dental implant material. We demonstrated that it was effective to use the material's electron-density ratio to account for the dosimetric impact of the dental implant.

Development of a Geant4 based Monte Carlo Algorithm to evaluate the MONACO VMAT treatment accuracy

A method to evaluate the dosimetric accuracy of volumetric modulated arc therapy (VMAT) treatment plans, generated with the MONACOTM (version 3.0) treatment planning system in realistic CT-data with an independent Geant4 based dose calculation algorithm is presented.Therefore a model of an Elekta Synergy linear accelerator treatment head with an MLCi2 multileaf collimator was implemented in Geant4. The time dependent linear accelerator components were modeled by importing either logfiles of an actual plan delivery or a DICOM-RT plan sequence. Absolute dose calibration, depending on a reference measurement, was applied. The MONACO as well as the Geant4 treatment head model was commissioned with lateral profiles and depth dose curves of square fields in water and with film measurements in inhomogeneous phantoms. A VMAT treatment plan for a patient with a thoracic tumor and a VMAT treatment plan of a patient, who received treatment in the thoracic spine region including metallic implants, were used for evaluation.MONACO, as well as Geant4, depth dose curves and lateral profiles of square fields had a mean local gamma (2%, 2mm) tolerance criteria agreement of more than 95% for all fields. Film measurements in inhomogeneous phantoms with a global gamma of (3%, 3mm) showed a pass rate above 95% in all voxels receiving more than 25% of the maximum dose. A dose-volume-histogram comparison of the VMAT patient treatment plans showed mean deviations between Geant4 and MONACO of -0.2% (first patient) and 2.0% (second patient) for the PTVs and (0.5±1.0)% and (1.4±1.1)% for the organs at risk in relation to the prescription dose.The presented method can be used to validate VMAT dose distributions generated by a large number of small segments in regions with high electron density gradients. The MONACO dose distributions showed good agreement with Geant4 and film measurements within the simulation and measurement errors.

SU‐E‐T‐349: Variation in VMAT QA Pass Rate Caused by Detector Setup Errors

Purpose: To test the sensitivity of pass rates of Gamma Index to the detector setup errors in six degrees of freedom. Methods: VMAT (Volumetric Modulated Arc Therapy) treatment technique is used in our clinic on an Elekta Beam Modulator with a HexaPOD couch. The HexaPOD system is a robotic patient positioning platform with six degrees of freedom. ScandiDos Delta4 system is used for our VMAT patient QAs. Delta4 is a pseudo 3D diode detector array which is capable of calculating 3D Gamma Index. Seven prostate and four H&amp;N VMAT treatment plans were delivered. The measurements were compared with the Pinnacle TPS calculations and the pass rates of Gamma Index were calculated. Then the HexPod couch was shifted or rotated either by 1 mm or 1 degree in any of the six degrees of freedom to simulate the setup errors, and the pass rates were re‐measured and re‐calculated. The criterion used for Gamma Index calculation was 3% and 3mm. A detector point is considered pass if the calculated Gamma Index is smaller than 1. Pass rates of Gamma Index with different detector setup errors in the six degrees of freedom were compared. Results: Regarding the rotational setup error of different directions, the changes of pass rates of Gamma Index are significantly different (p &lt; 0.001). While one degree change in Roll angle causes the least change of the pass rates of Gamma Index, one degree change in Pitch angle causes the biggest change. Regarding the translational setup error of different directions, the changes of pass rates of Gamma Index are insignificant (p &gt; 0.1). Conclusions: Although 1 mm or 1 degree machine‐type tolerance was recommended by AAPM TG reports, they don't translate to the same amount of changes in pass rates of Gamma Index for VMAT QA.

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‐E‐T‐445: Use of MAPPHAN for Patient Specific VMAT Plan Verification

Purpose: Volumetric modulated arc therapy (VMAT) has been introduced in many clinics. The need for patient specific quality assurance tests for VMAT was reflected in a large number of publications on this subject. Many centers have used the Sun Nuclear Mapcheck diode array for IMRT patient specific QA, and thus the retrofitted MAPPHAN/Mapcheck may appear an economical and trustworthy solution for VMAT. Implementing this phantom was not straightforward. Following manufacturerˈs guidelines for comparing measured and calculated dose distributions, the pass rates were considerably lower than with other volumetric dosimeters. We present a simple method to improve these results to clinically acceptable values. Methods: We commissioned VMAT for two Elekta Infinity accelerators with the Pinnacle3(v9) treatment planning system. For MAPPHAN dose calculation, we used a CT scan of the phantom directly or assigned a density of 1.05 g/cm3 to the whole phantom, as recommended by the manufacturer and tested these on 5 prostate plans. To improve plan/measurement agreement we created an artificial CT scan using two density configurations as possible estimates of the actual phantom. To minimize couch effects, we placed the phantom on 7cm of Styrofoam. Validity of the plans was also verified with the Delta4 phantom (Scandidos) which gave 100% agreement under TG1 19 criteria. Results: Using MAPPHAN as suggested by the manufacturer gave suboptimal results. A literature search revealed that this problem was also identified by others (1). Using an artificial CT representation and raising the phantom improves agreement with plans from (93+/−6)% to (98+/−3)% (TG1 19), approaching Delta4 results Conclusions: Problems with using the MAPPHAN phantom can stem from insufficient knowledge of the phantom composition and treatment couch attenuation. A simple method was developed to improve the results of VMAT patient specific QA.

SU-E-T-70: Comparison of Two 3D Gamma Index Calculation Schemes

Purpose: To compare two 3D Gamma Index calculation schemes for VMAT plans. Methods: VMAT (Volumetric Modulated Arc Therapy) treatment technique was commissioned for our clinic on an Elekta Synergy machine. We selected to use ScandiDos Delta4TM system for our VMAT patient QAs. Delta4 is a pseudo 3D diode detector array which calculates Gamma index from 3D dose distributions. Delta4 offers two ways to calculate Gamma Index: global Gamma Index and local Gamma Index. The global gamma calculates the dose difference relative to the normalization dose, which can be iso‐center dose, prescribed or maximum dose. The local gamma calculates the dose difference relative to the current measurement point under examination. Thirty two test plans for the treatments of different sites including prostates, HN, lung, abdomen, pancreas node, anus, brain post fossa, medium sternum and liver were selected for this study. Pass rates from the two Gamma Index calculation schemes with 3% and 3mm criterion were compared. Results: The pass rate from global Gamma Index calculation is high than those from the global Gamma Index (p<0.001). Correlation coefficient of the two pass rates is 0.78. The differences of pass rates range from 0% to 9.5%. The most dramatic differences happen for high modulated (more complex) plans and where there are some low dose plateau regions. Conclusions: Global Gamma Index doesn't describe the details of high modulated plans with some low dose regions. Pass rate of local Gamma Index should be examined in addition to global Gamma Index. Pass rates of local Gamma Index for different structures are desired.

Dosimetric Validation for Patient-specific Quality Assurance using the Delta4 4 Phantom with Volumetric Modulated Arc Therapy

Purpose/Objective(s)To dosimetrically quantify the accuracy of the Delta4 (Scandidos, Sweden) phantom for patient-specific quality assurance of volumetric modulated arc therapy (VMAT) patient plans using SmartArc Pinnacle3 treatment planning system (Philips Medical, Fitchburg WI).Materials/MethodsNine (n = 9) SmartArc patient plans were mapped onto the Delta4 phantom and the plan was exported via DICOM RT to the Delta4 software. Treatment plans consisted of 1-3 arcs with the majority using 2 arcs. The Delta4 phantom was calibrated using a Varian 2100 linear accelerator (Varian Medical, Palo Alto, CA). Treatment plans were delivered using either a Varian 2100 with Millennium 120 MLC or NovalisTX. Measurements were compared to calculated doses using the gamma index and profile comparison. Measurement points evaluated were truncated to those within 20% to 500% of the maximum measured dose. The gamma index evaluation criteria varied from 1 mm/1% to 5 mm/5% to evaluate the accuracy of the radiation delivery.ResultsGood agreement between planned and detector measured dose was observed. For a DTA/dose difference tolerance of 3 mm/3% for the gamma index, the percentage of points satisfying this tolerance were 96,1% (90.5%-99.9%). The percentage of points satisfying the gamma index for the selected tolerance criteria decreased when the tolerance was lowered. With a 2 mm/2%, only three patients' plans satisfied the 90% threshold. For 1 mm/1%, no plans satisfied the threshold.ConclusionsThis preliminary study indicates that Delta4 is clinically useful and show good agreement between measured and calculated doses for VMAT patient plans using SmartArc. Purpose/Objective(s)To dosimetrically quantify the accuracy of the Delta4 (Scandidos, Sweden) phantom for patient-specific quality assurance of volumetric modulated arc therapy (VMAT) patient plans using SmartArc Pinnacle3 treatment planning system (Philips Medical, Fitchburg WI). To dosimetrically quantify the accuracy of the Delta4 (Scandidos, Sweden) phantom for patient-specific quality assurance of volumetric modulated arc therapy (VMAT) patient plans using SmartArc Pinnacle3 treatment planning system (Philips Medical, Fitchburg WI). Materials/MethodsNine (n = 9) SmartArc patient plans were mapped onto the Delta4 phantom and the plan was exported via DICOM RT to the Delta4 software. Treatment plans consisted of 1-3 arcs with the majority using 2 arcs. The Delta4 phantom was calibrated using a Varian 2100 linear accelerator (Varian Medical, Palo Alto, CA). Treatment plans were delivered using either a Varian 2100 with Millennium 120 MLC or NovalisTX. Measurements were compared to calculated doses using the gamma index and profile comparison. Measurement points evaluated were truncated to those within 20% to 500% of the maximum measured dose. The gamma index evaluation criteria varied from 1 mm/1% to 5 mm/5% to evaluate the accuracy of the radiation delivery. Nine (n = 9) SmartArc patient plans were mapped onto the Delta4 phantom and the plan was exported via DICOM RT to the Delta4 software. Treatment plans consisted of 1-3 arcs with the majority using 2 arcs. The Delta4 phantom was calibrated using a Varian 2100 linear accelerator (Varian Medical, Palo Alto, CA). Treatment plans were delivered using either a Varian 2100 with Millennium 120 MLC or NovalisTX. Measurements were compared to calculated doses using the gamma index and profile comparison. Measurement points evaluated were truncated to those within 20% to 500% of the maximum measured dose. The gamma index evaluation criteria varied from 1 mm/1% to 5 mm/5% to evaluate the accuracy of the radiation delivery. ResultsGood agreement between planned and detector measured dose was observed. For a DTA/dose difference tolerance of 3 mm/3% for the gamma index, the percentage of points satisfying this tolerance were 96,1% (90.5%-99.9%). The percentage of points satisfying the gamma index for the selected tolerance criteria decreased when the tolerance was lowered. With a 2 mm/2%, only three patients' plans satisfied the 90% threshold. For 1 mm/1%, no plans satisfied the threshold. Good agreement between planned and detector measured dose was observed. For a DTA/dose difference tolerance of 3 mm/3% for the gamma index, the percentage of points satisfying this tolerance were 96,1% (90.5%-99.9%). The percentage of points satisfying the gamma index for the selected tolerance criteria decreased when the tolerance was lowered. With a 2 mm/2%, only three patients' plans satisfied the 90% threshold. For 1 mm/1%, no plans satisfied the threshold. ConclusionsThis preliminary study indicates that Delta4 is clinically useful and show good agreement between measured and calculated doses for VMAT patient plans using SmartArc. This preliminary study indicates that Delta4 is clinically useful and show good agreement between measured and calculated doses for VMAT patient plans using SmartArc.

SU-GG-T-241: Patient Specific QA Using EBT2 Films for VMAT

Purpose: To report on EBT2 film based patient specific QA scheme for evaluation of the in‐house volumetric modulated arc therapy (VMAT) planning algorithm applied to the new rotational delivery technique. Methods: Planning CTimages of selected prostate patients were transferred to the in‐house developed treatment planning (TP) system. Deliverable VMAT plans were optimized by a dose‐volume‐histogram based objective function and a conjugate gradient search. Two approaches of VMAT patient specific QA using EBT2 films at 100 cm SAD with 15MV photons on Varian Trilogy™ were developed: 1) through a phantom mounted on an isocentric mounting fixture, and 2) through a rectangular phantom (25×25×20 cm3) positioned on the treatment couch. Calibration films were irradiated at dmax (3 cm) for doses up to 10 Gy. All films were scanned using an Epson 10000XL flatbed scanner with 72 dpi (0.035 mm) pixel resolution. Pixel values were converted to doses using the established calibration curve. The 2D dose distributions were generated from the film data analysis. In‐house software was utilized to compare the measured doses from films with the associated TP data for each methodology. Results and Discussion: Both testing techniques reported less than 2% discrepancy of the absolute doses at the isocenter. First approach showed better passing rates than the second method. In both cases that the measured planar dose distributions achieved more than 90% passing rates with a set of 3% /3 mm dose and DTA criteria. EBT2 film dosimetry in phantom was in good agreement with TP results, and all film results in our study have created an acceptable passing rate with fine spatial resolution compared to other detector‐arrays QA methods. Conclusions: The patient specific EBT2 film QA for VMAT prostate patients showed extremely satisfactory results. EBT2 film analysis with either setup approach here was useful for VMAT patient QA.

SU-GG-T-230: Performance Monitoring of VMAT Delivery

Purpose: To develop a system to measure the per-segment dose of volumetric modulated arc therapy (VMAT) beams, for delivery system and patient plan quality assurance (QA). Methods and Materials: VMAT delivery validation requires measuring the dose as a function of gantry angle in order to identify gantry angle dependent errors. A dose measurement system has been developed which uses custom software and hardware, and a commercially available inclinometer, to sample the dose detected by an ionization chamber located inside of a phantom as a function of gantry angle, enabling per-segment evaluation of VMAT delivery. For routine QA of a VMAT delivery system, fields were created in which a 2 × 2 cm MLC aperture is continuously centered on an ionization chamber positioned at 8.0 cm distance from the isocenter. The MLC leaf trajectory follows a sinusoidal path over a 360 ° gantry arc. Fields were generated to deliver both a constant MU and a constant dose per segment. The measured dose is compared with the calculated values for each beam segment. Gantry angle errors were introduced to evaluate the sensitivity of the system to potential errors. The delivery system dosimetry was assessed at various dose rates and gantry speeds. Prostate and head and neck VMAT patient plans have been evaluated similarly by comparing measured and planned per-segment dose. Ion chambers were placed both at isocenter and off-axis to provide gantry angle error sensitivity. Plans were generated on Pinnacle Ver. 8.9 treatment planning system; delivered on an Elekta Synergy accelerator. Results: The dosimetry system was able to accurately measure the per-segment dose delivery of VMAT plans. Using the fields created for routine delivery system QA, gantry angle errors of 1° were detectable. Conclusions: A method has been developed to effectively monitor the dose delivery performance of a VMAT delivery system.

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.

SU‐FF‐T‐310: Evaluation of Angular Response of 2D Diode Array Detectors with Buildup Phantom

Purpose: To evaluate the angular response of a 2D diode array with a buildup phantom using simulated volumetric modulated arc therapy (VMAT) beams. Method and Materials: A 2D diode array (MapCHECK™) with a 5‐cm thick buildup phantom (MapPHAN™) was scanned in horizontal and vertical orientations on a Philips Brilliance Big Bore CT scanner. Four treatment plans were generated for the phantom in 4 settings at 2 orientations with 10×10 cm2 open fields or different MLC‐shaped fields. All plans consist of eighteen equally spaced 6MV photon beams. Inhomogeneity corrections were applied to all four treatment plans. Dose deliveries were carried out on a Varian Cl‐iX machine while MapCHECK™ with MapPHAN™ combination was used for measuring the delivered doses with reproduced scanning parameters. The measured doses were compared with the plans using an in‐house evaluation software. A measured arc calibration factor from the setup was determined and applied to the final measurements. Results: The measured isodoses of all four plans show agreement with the plans. A gamma index with a 3% (dose difference, DD) and 3mm (distance‐to‐agreement, DTA) criteria was also used to evaluate comparisons of measured and TP data. Among the four plans in this study, the plan of horizontal position with different MLC‐shaped fields had the highest passing rate and the plan of vertical position with open fields was the worst. This is probably because the calibration is at horizontal position and different MLC shapes could even out some uncertainties due to the limited number of detectors. Conclusions: In this independent study, the angular response of MapCHECK™ with MapPHAN™ for rotational dosimetry has been evaluated in both coronal and sagittal directions. Results from our unique setting demonstrate that with the use of a properly calibrated 2D diode array and a buildup phantom VMAT patient QA is feasible and accurate.