Specific QA Research Articles

25 - Practical implementation of a patient specific QA on Cyberknife using Gafchromic® EBT3 film: One-year results analysis

25 - Practical implementation of a patient specific QA on Cyberknife using Gafchromic® EBT3 film: One-year results analysis

Characterization and validation of a Monte Carlo code for independent dose calculation in proton therapy treatments with pencil beam scanning

We propose a method of creating and validating a Monte Carlo (MC) model of a proton Pencil Beam Scanning (PBS) machine using only commissioning measurements and avoiding the nozzle modeling. Measurements with a scintillating screen coupled with a CCD camera, ionization chamber and a Faraday Cup were used to model the beam in TOPAS without using any machine parameter information but the virtual source distance from the isocenter. Then the model was validated on simple Spread Out Bragg Peaks (SOBP) delivered in water phantom and with six realistic clinical plans (many involving 3 or more fields) on an anthropomorphic phantom. In particular the behavior of the moveable Range Shifter (RS) feature was investigated and its modeling has been proposed. The gamma analysis (3%,3 mm) was used to compare MC, TPS (XiO-ELEKTA) and measured 2D dose distributions (using radiochromic film). The MC modeling proposed here shows good results in the validation phase, both for simple irradiation geometry (SOBP in water) and for modulated treatment fields (on anthropomorphic phantoms). In particular head lesions were investigated and both MC and TPS data were compared with measurements. Treatment plans with no RS always showed a very good agreement with both of them (-Passing Rate (PR) > 95%). Treatment plans in which the RS was needed were also tested and validated. For these treatment plans MC results showed better agreement with measurements (-PR > 93%) than the one coming from TPS (-PR < 88%). This work shows how to simplify the MC modeling of a PBS machine for proton therapy treatments without accounting for any hardware components and proposes a more reliable RS modeling than the one implemented in our TPS. The validation process has shown how this code is a valid candidate for a completely independent treatment plan dose calculation algorithm. This makes the code an important future tool for the patient specific QA verification process.

Patient specific IMRT quality assurance with film, ionization chamber and detector arrays: Our institutional experience

Abstract Purpose This study was conducted to review patient specific IMRT QA delivered at our institution using Varian LINACs and TomoTherapy Hi-Art system and categorized according to the anatomical site, type of treatment machine, and treatment planning systems (TPS). Material and methods Three thousand and three hundred seven patient data were evaluated for a time ranging from 2006 to 2011; these data were gauged using several methods used in the QA process. Ion chambers and film were used in 1738 patient plan QA in the earlier years followed by ion chamber arrays in 1569 patient plan QA in the latter years. Patients were grouped according to several parameters including TPS, site of treatment, and type of treatment machine in comparing the measured versus computed dose differences. From 2006 through early 2009, 736 TomoTherapy plans, 651 Pinnacle3 plans, and 351 Corvus plans were evaluated using ion chambers and films. The pass criterion at the institution at the time of these measurements was 3% dose difference and 3 mm distance to agreement. For the years ranging from 2009 to 2011, 1569 patient IMRT QAs were performed and evaluated on the institution's pass criteria of 90% γ value on Varian linacs with Millennium 80, 120 and High-Definition 120 multileaf collimators. Results Average point dose difference between measured and calculated plans for Pinnacle3, Hi-ART TomoTherapy, and Corvus TPS were 0.1205%, −0.0042%, and −0.0178%. Among the QA plans measured using a 2D array, average gamma values for brain, head and neck, thorax, abdomen, and pelvis were 97.2%, 95.7%, 96.2%, 97.0%, and 96.2%, respectively. Average gamma values based on 80, 120, HD 120 and TomoTherapy MLC configurations were 96.5%, 96.2%, 96.3%, and 97%, respectively. A 2-tailed paired Student's T-test did not reveal the presence of statistically significant differences based on either TPS, anatomical sites, number of beams or arcs, number of control points, or the MLC configuration (p-value>0.05). Conclusions Among the TPSs, Tomoplan and Corvus had the best agreement with the point dose measurement. Based on anatomical location of treatment site, head and neck cancers had the lowest gamma value for the patients treated and brain sites had the highest gamma value. However, the values are not significantly different. TomoTherapy machines continue to have the best overall gamma values as compared to CLINAC machines.

SU‐E‐T‐116: Analysis of Patient Specific VMAT QA Passing Rates with Delta4 for Matched Machines

Purpose:To test whether unified vendor specified beam conformance for matched machines implies volumetric modulated arc radiotherapy(VMAT) delivery consistency.Methods:Twenty‐two identical patient QA plans, eleven 6MV and eleven 15MV, were delivered to the Delta⁴(Scandidos, Uppsala, Sweden) on two Varian TrueBEAM matched machines. Sixteen patient QA plans, nine 6 MV and seven 10 MV, were delivered to Delta⁴ on two Elekta Agility matched machines. The percent dose deviation(%DDev), distance‐to‐agreement(DTA), and the gamma analysis(γ) were collected for all plans and the differences in measurements were tabulated between matched machines. A paired t‐test analysis of the data with an alpha of 0.05 determines statistical significance. Power(P) was calculated to detect a difference of 5%; all data except Elekta %DDev sets were strong with above a 0.85 power.Results:The average differences for Varian machines (%DDev, DTA, and γ) are 6.4%, 1.6% and 2.7% for 6MV, respectively, and 8.0%, 0.6%, and 2.5% for 15MV. The average differences for matched Elekta machines (%DDev, DTA, and γ) are 10.2%, 0.6% and 0.9% for 6 MV, respectively, and 7.0%, 1.9%, and 2.8% for 10MV.A paired t‐test shows for Varian the %DDev difference is significant for 6MV and 15MV(p‐value6MV=0.019, P6MV=0.96; p‐value15MV=0.0003, P15MV=0.86). Differences in DTA are insignificant for both 6MV and 15MV(p‐value6MV=0.063, P6MV=1; p‐value15MV=0.907, P15MV=1). Varian differences in gamma are significant for both energies(p‐value6MV=0.025, P6MV=0.99; p‐value15MV=0.013, P15MV=1). A paired t‐test shows for Elekta the difference in %DDev is significant for 6MV but not 10MV(p‐value6MV=0.00065, P6MV=0.68; p‐value10MV=0.262, P10MV=0.39). Differences in DTA are statistically insignificant(p‐value6MV=0.803, P6MV = 1; p‐value10MV=0.269, P10MV=1). Elekta differences in gamma are significant for 10MV only(p‐value6MV=0.094, P6MV=1; p‐value10MV=0.011, P10MV=1).Conclusion:These results show vendor specified beam conformance across machines does not ensure equivalent patient specific QA pass rates. Gamma differences are statistically significant in three of the four comparisons for two pairs of vendor matched machines.

SU‐E‐T‐637: Proton Aperture Quality Assurance Using Computed Tomography

Purpose:To develop an automated algorithm for aperture quality assurance for double scattering proton radiotherapy.Methods:Anterior‐Posterior scout images were acquired on a CT scanner for five brass apertures (2 large, 3 small) with 120 kVp and 10mA. A semi‐automated algorithm was developed for the QA process. The input includes a user selecting the scout image and the associated patient plan, field and aperture size. The program automatically thresholds the scout image. Next, a connectivity algorithm is used to determine the points associated with the central shape to obtain the contour. To compare with the plan contour, the program converts the detected and plan contours into polar coordinates, interpolates the data to a 1 degree spaced grid, and determines the differences in radial distance at each grid point.Results:The mean and maximum difference and the percentage of points with distance differences less than 1.25 mm (due to the divergent cut of the aperture) between the detected and plan field aperture contours were obtained. A repeatability coefficient was derived based on repeated scanning and processing of three of the apertures. The mean difference for five apertures was 0.44 +/−0.08 mm. The maximum difference in distance was 1.2 +/−0.23 mm for all apertures. The Repeatability Coefficient was +/−0.038 mm, indicating the technique is highly repeatable. The mean percent of points with distance less than 1.25 mm was 97.41 +/−0.35%.Conclusion:Automated CT scout image based proton aperture QA is feasible, saves time, and provides a quantitative metric for proton patient specific aperture QA.

WE-AB-BRB-09: Real Time In Vivo Scintillating Fiber Array Detector for Medical LINACS

Purpose: An in vivo transmission scintillation fiber detector was developed to monitor patient treatment in real time for the enhancement of patient safety and treatment accuracy. The detector system is capable of monitoring each pulse from a medical LINAC during treatment to determine the dose delivered as treatment progresses. Methods: The detector system consists of 60 parallel scintillating fibers coupled to fast data processing optoelectronics that can monitor the beam fluence in real time. Each 2.5mm2 square fiber is aligned with an MLC leaf pair and is long enough to capture a 40cm field. The fibers are embedded within a water equivalent polymer substrate that is secured in the LINAC accessory tray. The fibers are coupled to high speed photosensors and front end amplifiers that filter noise and pass each pulse to a high speed analog-to-digital converter. The system components are capable of detecting pulse repetition times shorter than what is delivered by a medical LINAC to ensure true real time data acquisition. Results: The system was able to capture and record the signal from each linac pulse and display the information in real time with no pulse pile up. It was found that the fiber array attenuates 2.65% of the beam which can easily be compensated for in treatment planning. The fibers responded linearly with dose, are independent of clinical beam energies, and are independent of dose rate. Calibration of the system was performed as a function of beam energy, beam size, dose rate, and monitor units to optimize beam fluence error detection. Conclusion: The detector system presented provides true real time in vivo beam monitoring to enhance patient safety and treatment delivery accuracy. Furthermore, the detector can be used for current patient specific QA.

SU‐E‐T‐460: Impact of the LINAC Repetition Rate On a High‐Resolution Liquid Ionization Chamber Array for Patient‐Specific QA

Purpose:The purpose of this study is to investigate the LINAC repetition‐rate (dose‐rate) dependence of OCTAVIUS 1000SRS liquid ionization chamber (LIC) array for patient specific QA of SRT plans delivered with flattening‐filter‐free (FFF) beams.Methods:1) The repetition‐rate dependence of 1000SRS was measured in a phantom constructed with 5‐cm solid water above and below the array for build‐up and backscatter. A 0.3cc calibrated ion chamber was also placed along the central axis 2.3cm below the center chamber of the array for normalizing LINAC output fluctuation. The signals from the center chamber of the array under different repetition rates in the range of 400–2400 MU/min for 6xFFF and 10xFFF beams on a Varian TrueBeamSTx LINAC, normalized by the independent chamber readings, were analyzed for the array response dependence on repetition rates. 2) Twelve Step‐and‐shoot IMRS QA plans (6xFFF and 10xFFF) were delivered to the array under different repetition rates for analysis and comparison. 3) The absolute doses measured by the center chamber were compared to measurements using an independent ionization chamber with the identical setup, taken as the gold standard. 4) The correction factors based on the actual delivery repetition rate were applied to the measurements, and the results were compared again to the gold standard.Results:1) The 1000SRS array exhibited repetition‐rate dependence for FFF beams, up to 5% for 6xFFF and 10% for 10xFFF; 2) The array showed clinically‐acceptable repetition‐rate dependence for regular flattened beams; 3) This repetition‐rate dependence significantly affected the measurement accuracy, thereby affecting IMRS QA results; 4) By applying an empirical repetition‐rate correction, the corrected measurements agreed better with the gold standard ion chamber measurements.Conclusion:OCTAVIUS 1000SRS LIC array exhibited considerable repetition‐rate dependence for FFF beams, which will affect the accuracy of the absolute QA measurements, especially for IMRS plans with the step‐and‐shoot technique.

SU‐E‐T‐480: IMRT/VMAT QA Employing a High Resolution Measurement Array: Initial Experience and Limitations

Purpose:Patient specific QA for IMRT/VMAT is time‐consuming. A high resolution ionization chamber array was employed which rotates synchronously with the gantry. Commissioning, initial experience and limitations of this measurement technique will be discussed.Methods:A high resolution ionization chamber array, Octavius1500 (PTW‐Freiburg, Germany), was employed with the Octavius4D rotating cylindrical phantom. Measured plans are reconstructed into a 3D dose distribution using measured depth dose curves (Verisoft 6.0 PTW‐Freiburg). Plans of varying complexity were selected from our patient collective treated at a TrueBeam 1.5 linac (Varian Medical Systems) with HD120 MLC (VMS). Cross‐calibration was performed using a thimble chamber (30016, PTW‐Freiburg) placed within the cylindrical phantom (SSD 840mm, SCD 1000mm). All dose distributions were recalculated using a 1.5mm dose grid (Eclipse, AAA10.0.28/Acuros10.0.28, VMS) using a virtual homogeneous cylindrical phantom provided by the array manufacturer (density 1.016 gcm‐3). All analyses were performed using Verisoft and compared to routine QA analysis (Octavius729 with octagonal phantom) results.Results:Depth dose curves from 1cm × 1cm to 26cm × 26cm were measured for 3D dose calculation commissioning in Verisoft 6.0. Excellent agreement was observed between TPS PDD and PDD derived from the Octavius1500 array measurement. For highly modulated plans the overall 3D gamma (2.0mm, 3.0%) ranged between 91.3% to 96.9%. The Octavius729 (2D) array results ranged from 97.9 to 100.0% for PTVs.Conclusion:The Octavius1500 and 4D cylindrical phantom provides an integrated QA solution. Complex VMAT plans, comprising multiple PTVs, can be verified with a single (rotating) measurement of all arcs. Hitherto two measurements (horizontal and vertical) had to be performed per PTV. Overall the results are good. However, some refinement may need to be made to the reconstruction. Further work needs to be done to determine appropriate action levels for routine analysis.

SU‐E‐T‐417: Experimental Verification of Robustness Optimized Intensity Modulated Proton Therapy Plans for Craniospinal Irradiation

Purpose:The robustness‐optimized intensity modulated proton therapy (RO‐IMPT) plans takes into account the setup errors to eliminate the hot/cold spots caused by setup errors at the junction for craniospinal (CSI) irradiation fields. The RO‐IMPT requires a large area for the junction of two fields. This work is to verify the treatment planning system calculated dose with introduction of setup errors by two‐dimensional (2D) dose measurement. Two fields of total lengths of 45 cm with junctions of 21 cm were measured.Methods:2D dose distributions in the coronal plane were measured by a commercial 2D ion‐chamber‐array detector in a cubic solid water phantom. A total spine field consisting of an upper and a lower spine field was measured at the depths of 5.4 and 6.9 cm. The active area for the measurement was 24×24 cm22. The junction and the area outside the junction of the spine fields were measured separately to complete the verification for the total field. Six different amounts of set‐up errors along the superior‐inferior direction up to 1.4 cm were intentionally applied. The measured dose with and without set‐up errors were compared with the calculated dose with and without set‐up errors using Gamma‐index analysis. 3%/3mm and 2%/2mm dose/distance agreement criteria were used. The point dose at the center of the junction was also compared.Results:For all fields with various introduced setup errors, 100 % of pixels passed the 3%/3mm criteria and on average 96% of pixels passed 2%/2mm criteria. Differences in the measured and calculated doses were within 1.1%.Conclusion:The efficacy of the robustness for the CSI RO‐IMPT plans were verified by measurement. The measurement method can be used for patient treatment field specific QA for CSI RO‐IMPT plans in our institution in the near future.

SU‐E‐T‐345: Effect of DLG and MLC Transmission Value Set in the Treatment Planning System (TPS) On Dosimetric Accuracy of True Beam Hypofractionated SRT/SBRT and 2Gy/fx Prostate Rapid Arc Plans

Purpose:Due to limited commissioning time, we previously only released our True beam non‐FFF mode for prostate treatment. Clinical demand now pushes us to release the non‐FFF mode for SRT/SBRT treatment. When re‐planning on True beam previously treated SRT/SBRT cases on iX machine we found the patient specific QA pass rate was worse than iX's, though the 2Gy/fx prostate Result had been as good. We hypothesize that in TPS the True beam DLG and MLC transmission values, of those measured during commissioning could not yet provide accurate SRS/SBRT dosimetry. Hence this work is to investigate how the TPS DLG and transmission value affects Rapid Arc plans’ dosimetric accuracy.Methods:We increased DLG and transmission value of True beam in TPS such that their percentage differences against the measured matched those of iX's. We re‐calculated 2 SRT, 1 SBRT and 2 prostate plans, performed patient specific QA on these new plans and compared the results to the previous.Results:With DLG and transmission value set respectively 40 and 8% higher than the measured, the patient specific QA pass rate (at 3%/3mm) improved from 95.0 to 97.6% vs previous iX's 97.8% in the case of SRT. In the case of SBRT, the pass rate improved from 75.2 to 93.9% vs previous iX's 92.5%. In the case of prostate, the pass rate improved from 99.3 to 100%. The maximum dose difference in plans before and after adjusting DLG and transmission was approximately 1% of the prescription dose among all plans.Conclusion:The impact of adjusting DLG and transmission value on dosimetry might be the same among all Rapid Arc plans regardless hypofractionated or not. The large variation observed in patient specific QA pass rate might be due to the data analysis method in the QA software being more sensitive to hypofractionated plans.

SU‐E‐T‐601: Patient Specific QA Check for Radiotherapy with Carbon Beam at SAGA HIMAT

Purpose:To verify a patient specific QA for a prescribed dose per monitor unit (MU) and range check of carbon ion radiotherapy with a beam wobbling method at SAGA HIMAT.Methods:The dose distribution in a water phantom was recalculated with a treatment plan made by XiO‐N (Mitsubishi Electric Corporation, Tokyo). The depth dose distributions at central axis were compared with measurements using pinpoint ionization chamber (PTW31014). Measured data were analyzed for dose and distance difference with criteria of 3% and 2mm. To check for the MU and range, analysis data were used at center and distal end of a spread out bragg peak (SOBP) depth, respectively.Results:Treatment plan of 1400 beams in 500 patients were used for analysis. The pass rate of the MU and range was about 98% and 74%, respectively. The rate of both was 73%. The plan data passed only the MU check were verified depth dose profile. The data passed only the range and failed of both were compared depth dose distributions at off axis. If it was failed, we discussed with physicians.Conclusion:Most of patient specific QA check for a prescribed MU and range check at SAGA HIMAT were passed.

SU‐E‐T‐150: Brachytherapy QA Employing a High Resolution Liquid Filled Ionisation Chamber Array: Initial Experience and Limitations

Purpose:Verifying a complex 3D brachytherapy dose distribution by measurement is non‐trivial. Ideally a photon detector array should be independent of energy and angle, have high spatial resolution and be robust for routine clinical use.Methods:An iridium‐192 source was used. A PMMA jig was constructed comprising an outer slab and a central insert with eight milled channels for 1.33mm (outer diameter) steel needles, see figure. All calculations were performed using an empty CT study reconstructing eight virtual needles (QA‐CT), using the v2 source model (Elekta AG, Sweden). A high resolution liquid filled ionisation chamber array SRS1000, together with Verisoft software v6.0 (PTW Freiburg, Germany), was used to perform measurements of plans of increasing complexity to evaluate its suitability for device‐ and patient‐specific QA. The dimension of backscatter material was investigated. The patient plan dwell time distribution was entered manually into the QA‐CT and the dose distribution was calculated.Results:Our measurements indicate that the array is independent of energy and angle. The resulting measured dose values are linearly interpolated to 2025 values. Shifts of 1mm of the entire needle are readily detectable. Individual dwell position shifts (2.5mm) are also readily measurable. Moreover a dwell time increase of 1 second both in the edge and central region are detectable.Conclusion:The high resolution SRS1000 array is a powerful instrument for brachytherapy QA enabling 977 simultaneous measurements to be performed. Our measurements suggest 60mm of RW3 backscatter material upstream and downstream are sufficient. Local percentage difference analysis is useful for device based QA, normalized relative percentage difference is arguably better for patient specific QA. Automated transfer of patient plan dwell time distribution to the QA plan is required to enable a comprehensive patient QA study to be performed. Moreover the described measurement technique can be used for commissioning new brachytherapy applicators.

SU‐E‐T‐117: Analysis of the ArcCHECK Dosimetry Gamma Failure Using the 3DVH System

Purpose:To evaluate gamma analysis failure for the VMAT patient specific QA using ArcCHECK cylindrical phantom. The 3DVH system(Sun Nuclear, FL) was used to analyze the dose difference statistic between measured dose and treatment planning system calculated dose.Methods:Four case of gamma analysis failure were selected retrospectively. Our institution gamma analysis indexes were absolute dose, 3%/3mm and 90%pass rate in the ArcCHECK dosimetry. The collapsed cone convolution superposition (CCCS) dose calculation algorithm for VMAT was used. Dose delivery was performed with Elekta Agility. The A1SL(standard imaging, WI) and cavity plug were used for point dose measurement. Delivery QA plans and images were used for 3DVH Reference data instead of patient plan and image. The measured data of ‘.txt’ file was used for comparison at diodes to acquire a global dose level. The,.acml’ file was used for AC‐PDP and to calculated point dose.Results:The global dose of 3DVH was calculated as 1.10 Gy, 1.13, 1.01 and 0.2 Gy respectively. The global dose of 0.2 Gy case was induced by distance discrepancy. The TPS calculated point dose of was 2.33 Gy to 2.77 Gy and 3DVH calculated dose was 2.33 Gy to 2.68 Gy. The maximum dose differences were −2.83% and −3.1% for TPS vs. measured dose and TPS vs. 3DVH calculated respectively in the same case. The difference between measured and 3DVH was 0.1% in that case. The 3DVH gamma pass rate was 98% to 99.7%.Conclusion:We found the TPS calculation error by 3DVH calculation using ArcCHECK measured dose. It seemed that our CCCS algorithm RTP system over estimated at the central region and underestimated scattering at the peripheral diode detector point. The relative gamma analysis and point dose measurement would be recommended for VMAT DQA in the gamma failure case of ArcCHECK dosimetry.

SU‐E‐T‐593: Outcomes and Toxicities From a Clinical Trial of APBI Using MERT+IMRT with the Same XMLC

Purpose:We present the results from a clinical trial of accelerated partial breast irradiation (APBI), using mixed modulated photon and electron beams (MERT+IMRT) with the same photon multileaf collimator (xMLC).Methods:Seven patients were enrolled in the first year of the APBI clinical trial. Patients were selected following the conditions included in the NSABP B‐39/RTOG 0413 protocol. The targets and clinically relevant normal structures were contoured on the CT images following this protocol for APBI‐EBRT. All treatments were delivered using combined modulated electron and photon beams by means of the same xMLC installed in a SIEMENS Primus linac, with a reduced SSD equal to 60 cm for electron beams. The plans were performed with a treatment planning system based on full Monte Carlo simulations, called CARMEN, developed by our group. Simultaneously, an alternative IMRT plan was calculated with the commercial TPS PINNACLE v8.0m (Philips), and both plans were compared. An ad‐hoc breast phantom with semi‐spherical geometry called NAOMI was designed for a specific QA protocol. Patients received a total dose of 38.5 Gy, delivered in 10 fractions over 5 consecutive days, with a twice‐a‐day hypofractionated schema.Follow‐up visits during 2.5 years on average were repeated at 1 month post‐treatment, every 3 months for the first year, and every 6 months for the second year. Toxicity was scored according to National Cancer Institute Common Terminology Criteria for Adverse Events (CTCAE version 3.0).Results:This APBI technique achieved high loco‐regional control rates and showed low acute toxicity (grade 1 of CTCAE) and no toxicities from first month onwards. Photographic assessment of cosmesis showed skin excellent results.Conclusion:The clinical results achieved with MERT+IMRT by using the same xMLC are comparable or even better than those obtained with other APBI techniques, thanks to a software solution without any additional equipment or specific device.

MO‐E‐BRB‐04: Real‐Time Exit‐Fluence Delivery Validation

Recent high profile reports of technical failures and human errors causing severe radiation‐ induced injuries and deaths come in support of the sustained efforts to ensure patient safety in the delivery of radiation treatments. In addition, highly conformal radiation therapies and escalated fraction doses mandate increased and sustained accuracy of the entire radiotherapy process. Consequently, and as a Result of AAPM and ASTRO led efforts patient specific quality assurance for specialized radiation treatments such as IMRT, SRS/SBRT and Arc Therapy had become a three‐tier process: Pre‐treatment, during treatment, and post treatment patient specific QA. Traditional patient QA consists of pre‐treatment data transfer integrity dosimetric verifications and during‐treatment geometric verifications. However, as treatment adaptation becomes closer to deployment in the clinics, during treatment validation via exit detectors had become a realistic QA option, permitting plan assessment in near real time. Post‐treatment, machine logs allow comparisons of a range of mechanical parameters. A combination of these techniques could be used in evaluating inter‐fraction, and intra‐fraction delivery over a long time period such as an year, to evaluate the significant errors per site, per treatment technique. This type of data mining over longer periods of time provides the potential to recognize suboptimal radiation treatments, while allowing to identify systematic, possibly significant errors. This would allow creation of a data base of realized errors, small and large in dosimetry that could be for process or equipment improvement. This educational symposium will describe and review patient QA techniques, results, and strategies for patient specific quality assurance.Learning Objectives: review the goals of pre‐treatment QA for various specialized procedures review methods and means for pre‐treatment QA, limitations and tolerances review the scenarios where Varian/Tomo Log files could be used. review error detection via log file analysis Learn how exit detector signal recorded in log file could be used to improve patient safety understand real‐time EPID‐based transmission dosimetry methods understand the goals of real‐time delivery monitoring CMS Innovation grant. Rapid response supportive and palliative care program for patients with advanced cancer, starts July, 2012; Accuray Inc. Exit detector based QA software evaluation, Sept, 2012‐Sept, 2013; ROI GRant from ASTRO “Safety and Quality: IMRT treatment delivery accuracy”

SU‐E‐T‐773: Use of a Commercial TPS for Deriving Exit Dose Distribution for Patient Specific QA with An EPID

Purpose:A method to derive exit dose distributions with a commercial treatment planning system for comparison to Epid measured doses.Methods:In contrast to published methods, we present an approach that uses a treatment planning system (Raystation, Raysearch) to calculate exit dose patterns based on a modified patient CT dataset. The EPID is assumed as water equivalent and therefore is represented as water filled ring in the images. This is done by: ‐Export a renamed copy of the patient CT and plan data set. —Manipulate the CT‐data to a field of view of 1000 mm. — Insert the water filled cylinder ring, centered to the target point. — Manipulate plan file with the new target and dose prescription point. — Import and recalculate dose on the manipulated data set. — Export and extract exit field dose matrix from the cylinder (optional entrance dose). — Calibrate matrix to size and rebin data from the cylinder to EPID equivalent data. — Correct Epid measurement for scatter, position and field size. ‐Evaluate and compare data in Verisoft (PTW). Data manipulation and extraction is done by a simple tool (IDL, Exelis).Results:This method was tested on a Siemens Artiste 6MV for field sizes up to 27×27cm2 limited by used geometry and EPID size. First phantom measurements show good results for fields up to 20×20cm (pass rate &gt; 95% for 3%, 3mm Gamma index) while larger fields have higher discrepancies towards the field edges. This might Result from off axis softening of the beam and the higher sensitivity of the detector to beam scatter.Conclusion:This method might simplify the use of exit dosimetry with the EPID for patient specific QA as it uses the dose calculation of a commercial treatment planning system. The concept was proven by phantom data sets, giving acceptable results.

SU‐E‐T‐571: Newly Emerging Integrated Transmission Detector Systems Provide Online Quality Assurance of External Beam Radiation Therapy

Purpose:Two newly emerging transmission detectors positioned upstream from the patient have been evaluated for online quality assurance of external beam radiotherapy. The prototype for the Integral Quality Monitor (IQM), developed by iRT Systems GmbH (Koblenz, Germany) is a large‐area ion chamber mounted on the linac accessory tray to monitor photon fluence, energy, beam shape, and gantry position during treatment. The ion chamber utilizes a thickness gradient which records variable response dependent on beam position. The prototype of Delta4 Discover™, developed by ScandiDos (Uppsala, Sweden) is a linac accessory tray mounted 4040 diode array that measures photon fluence during patient treatment. Both systems are employable for patient specific QA prior to treatment delivery.Methods:Our institution evaluated the reproducibility of measurements using various beam types, including VMAT treatment plans with both the IQM ion chamber and the Delta4 Discover diode array. Additionally, the IQM's effect on photon fluence, dose response, simulated beam error detection, and the accuracy of the integrated barometer, thermometer, and inclinometer were characterized. The evaluated photon beam errors are based on the annual tolerances specified in AAPM TG‐142.Results:Repeated VMAT treatments were measured with 0.16% reproducibility by the IQM and 0.55% reproducibility by the Delta4 Discover. The IQM attenuated 6, 10, and 15 MV photon beams by 5.43±0.02%, 4.60±0.02%, and 4.21±0.03% respectively. Photon beam profiles were affected &lt;1.5% in the non‐penumbra regions. The IQM's ion chamber's dose response was linear and the thermometer, barometer, and inclinometer agreed with other calibrated devices. The device detected variations in monitor units delivered (1%), field position (3mm), single MLC leaf positions (13mm), and photon energy.Conclusion:We have characterized two new transmissions detector systems designed to provide in‐vivo like measurements upstream from the patient. Both systems demonstrate substantial utility for online treatment verification and QA of photon external beam radiotherapy.

Retrospective review of locally set tolerances for VMAT prostate patient specific QA using the COMPASS® system

PurposeThis study retrospectively reviewed locally set pass rates/tolerances for COMPASS® pre-treatment quality assurance results for RapidArc prostate plans to determine if these are appropriate. This was performed via quantifying the agreement between treatment planning system calculations and measurements based on absolute dose comparisons (3% tolerance for all dose points) and global gamma index assessment (3%/3 mm criterion for 97% of points). MethodSeventy-three prostate one-arc RapidArc plans, delivered by four dosimetrically matched linacs, were measured using the MatriXX Evolution two-dimensional array and analysed using COMPASS® (v.3, IBA Dosimetry). For the planning target volumes (PTV) considered, the D99%, D50%, D1% and DMean differences were analysed. The percentage volume with gamma greater than 1, average gamma and DMean difference were investigated for all structures. Nine plans were also assessed across the linac fleet to investigate potential linac dependence of results. Results and ConclusionsRegarding PTV DMean differences, all plans fell within the 3% tolerance and mostly within 2%, although there was a relatively small systematic difference. The absolute percentage differences of average and median doses suggested a weak linac dependence of the results which was found to be clinically insignificant. New stricter tolerances were established both for dose comparisons and gamma evaluation. Correlation between the gamma pass rates and the differences in the D99%, D50% and D1% was found to be moderate suggesting that gamma analysis in isolation has questionable clinical meaning and should only be used to indicate outliers for further analysis.

Institutional Patient-specific IMRT QA Does Not Predict Unacceptable Plan Delivery

To determine whether in-house patient-specific intensity modulated radiation therapy quality assurance (IMRT QA) results predict Imaging and Radiation Oncology Core (IROC)-Houston phantom results. IROC Houston's IMRT head and neck phantoms have been irradiated by numerous institutions as part of clinical trial credentialing. We retrospectively compared these phantom results with those of in-house IMRT QA (following the institution's clinical process) for 855 irradiations performed between 2003 and 2013. The sensitivity and specificity of IMRT QA to detect unacceptable or acceptable plans were determined relative to the IROC Houston phantom results. Additional analyses evaluated specific IMRT QA dosimeters and analysis methods. IMRT QA universally showed poor sensitivity relative to the head and neck phantom, that is, poor ability to predict a failing IROC Houston phantom result. Depending on how the IMRT QA results were interpreted, overall sensitivity ranged from 2% to 18%. For different IMRT QA methods, sensitivity ranged from 3% to 54%. Although the observed sensitivity was particularly poor at clinical thresholds (eg 3% dose difference or 90% of pixels passing gamma), receiver operator characteristic analysis indicated that no threshold showed good sensitivity and specificity for the devices evaluated. IMRT QA is not a reasonable replacement for a credentialing phantom. Moreover, the particularly poor agreement between IMRT QA and the IROC Houston phantoms highlights surprising inconsistency in the QA process.

A study on the correlation between plan complexity and gamma index analysis in patient specific quality assurance of volumetric modulated arc therapy

AimTo evaluate the new Octavius 4D system for patient specific quality assurance and to study the correlation between plan complexity and gamma index analysis in patient specific quality assurance of VMAT using the Octavius 4D system. BackgroundMcNiven (2010) proposed a study to evaluate the utility of a complexity metric, the Modulation Complexity Score, to evaluate the relationship of the metric with deliverability in IMRT. Materials and methodsEvaluation of the Octavius 4D system was carried out by gamma evaluation of user defined MLC created patterns and AAPM TG 119 benchmark plans. The relationship between plan complexity expressed as Modulation Complexity Score (MCS) and the gamma index analysis was established by a planar and volumetric gamma analysis of 106 clinically approved VMAT patient plans of different sites. ResultsAverage volumetric 3D global gamma evaluation (3mm/3%) results for the evaluation plans was 97.41% for 6MV X-rays and 98.30% for 15MV X-rays. Average MCS values for the head and neck, pelvic and thoracic plans were 0.2224, 0.3615 and 0.1874. Average volumetric 3D global gamma analysis (3mm/3%) results for the head and neck, pelvic and thoracic VMAT plans were 95.45%, 97.51% and 96.98%, respectively. Out of 90 correlation analyses between the MCS and gamma passing rate, only 3 had the r value greater than 0.5. ConclusionsThe Octavius 4D system is a suitable device for patient specific pretreatment QA. Global and local gamma analysis results showed a weak correlation with the MCS.