EPID Images Research Articles

31. Machine and patient quality controls by portal imaging using independent software

Introduction Because of regulation and by good practice, machine and patient quality controls are implemented in radiotherapy. The objective of this study is to show the feasibility of some of these controls by portal imaging with an independent conversion algorithm of image grey level into absorbed dose in water. Material and methods The irradiations were realized in 6 MV on an accelerator Varian Clinac iX equipped with a MLC 120 leaves and a detector in amorphous silicon (EPID aSi1000). The portal images were converted in dose with the EpiDream algorithm proposed by the company DREAM. After creating a calibration curve, it allows to convert the image into dose matrix without knowledge of irradiation conditions. For machine controls, the tests concerned the constancy measurement of dose rate, beam symmetry, dynamic filter transmission and Dose Leaf Separation (DLS). For the latter, plans simulating gaps on the leaf bench of 0.1, 0.2 and 0.5 mm were created to estimate the capacity of the system to reveal them. The results computed from portal images were compared to the measures usually realized in the department. For pre-treatment IMRT plan controls, doses distributions calculated with EpiDream from portal images were compared to those measured by EDR2 and calculated by TPS using a gamma index 3%–3 mm with a 10% threshold. In order to test the system sensibility, position leaf errors were also voluntarily introduced. Results Quality controls realized with the EPID showed a good agreement with the measurements made periodically in the center. The study of the DLS showed a sensibility of the system close to ionization chamber for the detection of a gap on the leaf bench. The images converted in dose allowed to detect gaps lower than 0.2 mm. The study of IMRT plans showed, on 28 controlled beams, for the EPID-TPS comparison, an average gamma of 0.39 (±0.05) and an average percentage points passing the gamma criterion of 96% (± 3%). Concerning the EPID-EDR2 and EDR2-TPS comparisons, the percentages were respectively 90% (±4%) and 94% (±3%) and the average gamma indices are 0.59 (±0.06) and 0.47 (±0.06). EPID allowed to reveal all of the errors included in the treatment plans by highlighting visually the zones where errors were introduced and by degrading the value of the gamma index. Conclusion The results obtained with EPID images converted in absorbed dose allowed to consider the use of the EPID for the realization of numerous quality controls in radiotherapy. So, the controls could be more quickly realized, in 2D and with a high spatial resolution. In this study, the possibility of realizing the control of the IMRT plans, the DLS, the transmission of the filter, dose rate and symmetry of the beams with the imager portal was demonstrated. A longitudinal follow-up is in progress to confirm these results.

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

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

Pylinac: A toolkit for performing TG-142 QA related tasks on linear accelerator

Introduction In a new radiotherapy center, with multi-modality linear accelerator, the quality control (QC) procedure becomes more complicated and more time consuming. Pylinac is a free software provides TG-142 QC tools. Purpose Is to evaluate the safe use of the software and accuracy in analyzing the QC tests. Materials and methods Pylinac (//github.com/jrkerns/pylinac) contains 8 high-level modules for automatically analyzing images and data generated by linear accelerator. we have tested 4 modules which are star-shot, VMAT, picket fence and CBCT module. It use either scanned film images, EPID or CBCT DICOM images. The software process the images and performs all measurements required by the QC protocol, then the results were compared with results from the standard analysis method Results Current QC procedure is based on visual observation of certain features in acquired images and be able to detect any deviation, such as MLC position or isocenter position. This approach is also a subjective task as it depends on the person who is performing the analysis. Using Pylinac software gives a superior result compared with the routine analysis way as it gives the result in sub millimeter for the star-shot and the picket-fence tests. In the VMAT test the software has the ability to analyze the whole image, rather than performing the test on specific positions on the image. Conclusion The software was able to perform all the tests and detect any deviation with the same accuracy as manual method, also it performs the analysis in a significantly shorter time. Disclosure We have nothing to disclose in relation to the presented work. In a new radiotherapy center, with multi-modality linear accelerator, the quality control (QC) procedure becomes more complicated and more time consuming. Pylinac is a free software provides TG-142 QC tools. Is to evaluate the safe use of the software and accuracy in analyzing the QC tests. Pylinac (//github.com/jrkerns/pylinac) contains 8 high-level modules for automatically analyzing images and data generated by linear accelerator. we have tested 4 modules which are star-shot, VMAT, picket fence and CBCT module. It use either scanned film images, EPID or CBCT DICOM images. The software process the images and performs all measurements required by the QC protocol, then the results were compared with results from the standard analysis method Current QC procedure is based on visual observation of certain features in acquired images and be able to detect any deviation, such as MLC position or isocenter position. This approach is also a subjective task as it depends on the person who is performing the analysis. Using Pylinac software gives a superior result compared with the routine analysis way as it gives the result in sub millimeter for the star-shot and the picket-fence tests. In the VMAT test the software has the ability to analyze the whole image, rather than performing the test on specific positions on the image. The software was able to perform all the tests and detect any deviation with the same accuracy as manual method, also it performs the analysis in a significantly shorter time.

Implementation of an adaptive radiotherapy method using EPID and gamma analysis for head and neck and lung cases

Introduction The challenge of adaptive radiotherapy is to recognize the morphological changes that induce a sufficient dosimetric impact to affect the treatment delivery and require complete replanning. Purpose The objective was to analyze daily EPID images to identify and quantify changes in the anatomy and to detect patients at risk of deviation from their planned treatment. Materials and methods EPID images were acquired at every fraction for 50 patients of each of these 2 anatomical sites, head and neck and lung cases. A gamma analysis was performed relative to the first fraction and multiple parameters were extracted from the daily gamma map. The evolution of these parameters were characterized to identify categories of risk of deviation using a k-means clustering. A quarter of these patients had a complete dose evaluation at the end of the treatment by deforming the CT and original contours onto the last CBCT, and by re-computing the dose on the deformed dataset. Results Categories of risk were identified for each of the anatomical sites. These categories indicated the degree of errors of the daily treatment relative to the first fraction. The combination of gamma parameters and dosimetric analysis established a threshold for which patients were at risk of deviation. From the re-computing plan data, it been confirmed that patients with a strong dosimetric impact were above this threshold. Conclusion Analysis of daily EPID images provides a method to identify patients at risk of deviation from their planned treatment and help for the replanning decision.

Pre-treatment quality assurance of 220 Rapidarc plans analyzed with EpiQA software

Introduction Delivery complexity of volumetric modulated arc-therapy (VMAT) requires a pre-treatment verification process. Electronic portal imaging based on a-Si flat panels are utilized for dosimetric purposes mainly for IMRT pre-treatment verification (QA). Purpose This work is a retrospective analysis of a large dataset of RapidArc routine pre-treatment QA. We report results and analysis of our procedure that combines EPID images and EpiQA commercial software for dose distribution reconstruction. Materials and methods Dose distribution of 220 patients treated with a VMAT technique on Varian DHX-2100 were analyzed. Tumor sites of selected patients were: 70 esophagus, 100 head-neck and 50 lung. The detector reading, for each arc, is converted to a dose map and compared with the TPS (Eclipse) dose distribution, calculated with a virtual water-phantom (0,125 cm calculation grid). Gamma evaluation index was performed to quantitatively compare measured and calculated results using 3%-3 mm, 4%-3 mm and 4%-4 mm agreement criteria. Results Good result were found for all districts. The average values of G 1,5, probably due to the higher modulation. Conclusion EPID images and EpiQA software use for pre-clinical verification show a very good agreement between calculated and measured dose. This QA procedure is easy also in complex treatments such as VMAT. We plan to investigate this study with a calculation grid of 0,25 cm, obtaining a considerable time sparing. Furthermore we would like to compare results obtained with EpiQA software with other systems for pre-treatment QA. Disclosure All authors disclose any financial and personal relationships with other people or organizations that could inappropriately bias this presentation.

Epid-based in vivo dosimetry for complex VMAT treatments dose verification in clinical routine

Introduction Due to the increasing complexity of radiotherapy treatments, comprehensive quality assurance programs are mandatory to check the proper functioning of the whole radiotherapy chain. In vivo dosimetry (IVD), a direct method of measuring radiation doses to cancer patients during treatment, has shown unique features to trace deviations between planned and actually delivered dose distributions. Purpose To assess the usefulness of EPID-based IVD for complex VMAT treatments in clinical routine. Materials and methods 80 patients (30 with head-neck tumor, 30 with pelvic tumors and 20 patients with extracranial metastases) treated with Elekta were enrolled. IVD tests were evaluate by means of (i) R ratio between daily in-vivo isocenter dose and planned dose and (ii) γ -analysis between EPID integral portal images in terms of percentage of points with γ -value smaller than one ( γ %) and mean γ -values ( γ mean), using a global 3%-3 mm criteria. Alert criteria of ±5% for R ratio, γ % γ mean >0.67 were chosen. Results A total of 980 transit EPID images were acquired. The mean R ratio was equal to 1.001 ± 0.015 for all patients with more than 95% of tests were within 5%. For HN, pelvic and SBRT treatments, γ % and γ mean metrics reported 85.9%, 83.1% and 95.4%, and 93.3%, 92.1% and 98.1% of tests within alert criteria, respectively. The systematic use of IVD revealed relevant discrepancies in a few H–N patients due to major anatomical variations and variations due to random anatomical changes in terms of filling of rectum/bladder in pelvic tumors. No discrepancies were detected in SBRT patients. Conclusion IVD was able to detect when delivery was inconsistent with the original plans, allowing physics and medical staff to promptly act in case of major deviations.

Sci‐Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques ‐ 11: Quantification of chest wall motion during deep inspiration breast hold treatments using cine EPID images and a physics based algorithm

Purpose:This work presents an algorithm used to quantify intra‐fraction motion for patients treated using deep inspiration breath hold (DIBH). The algorithm quantifies the position of the chest wall in breast tangent fields using electronic portal images.Methods:The algorithm assumes that image profiles, taken along a direction perpendicular to the medial border of the field, follow a monotonically and smooth decreasing function. This assumption is invalid in the presence of lung and can be used to calculate chest wall position. The algorithm was validated by determining the position of the chest wall for varying field edge positions in portal images of a thoracic phantom. The algorithm was used to quantify intra‐fraction motion in cine images for 7 patients treated with DIBH.Results:Phantom results show that changes in the distance between chest wall and field edge were accurate within 0.1 mm on average. For a fixed field edge, the algorithm calculates the position of the chest wall with a 0.2 mm standard deviation. Intra‐fraction motion for DIBH patients was within 1 mm 91.4% of the time and within 1.5 mm 97.9% of the time. The maximum intra‐fraction motion was 3.0 mm.Conclusions:A physics based algorithm was developed and can be used to quantify the position of chest wall irradiated in tangent portal images with an accuracy of 0.1 mm and precision of 0.6 mm. Intra‐fraction motion for patients treated with DIBH at our clinic is less than 3 mm.

SU‐F‐T‐261: Reconstruction of Initial Photon Fluence Based On EPID Images

Purpose:Verifying an algorithm to reconstruct relative initial photon fluence for clinical use. Clinical EPID and CT images were acquired to reconstruct an external photon radiation treatment field. The reconstructed initial photon fluence could be used to verify the treatment or calculate the applied dose to the patient.Methods:The acquired EPID images were corrected for scatter caused by the patient and the EPID with an iterative reconstruction algorithm. The transmitted photon fluence behind the patient was calculated subsequently. Based on the transmitted fluence the initial photon fluence was calculated using a back‐projection algorithm which takes the patient geometry and its energy dependent linear attenuation into account. This attenuation was gained from the acquired cone‐beam CT or the planning CT by calculating a water‐equivalent radiological thickness for each irradiation direction. To verify the algorithm an inhomogeneous phantom consisting of three inhomogeneities was irradiated by a static 6 MV photon field and compared to a reference flood field image.Results:The mean deviation between the reconstructed relative photon fluence for the inhomogeneous phantom and the flood field EPID image was 3% rising up to 7% for off‐axis fluence. This was probably caused by the used clinical EPID calibration, which flattens the inhomogeneous fluence profile of the beam.Conclusion:In this clinical experiment the algorithm achieved good results in the center of the field while it showed high deviation of the lateral fluence. This could be reduced by optimizing the EPID calibration, considering the off‐axis differential energy response. In further progress this and other aspects of the EPID, eg. field size dependency, CT and dose calibration have to be studied to realize a clinical acceptable accuracy of 2%.

SU‐F‐T‐469: A Clinically Observed Discrepancy Between Image‐Based and Log‐ Based MLC Position

Purpose:To present a clinical case which challenges the base assumption of log‐file based QA, by showing that the actual position of a MLC leaf can suddenly deviate from its programmed and logged position by >1 mm as observed with real‐time imaging.Methods:An EPID‐based exit‐fluence dosimetry system designed to prevent gross delivery errors was used in cine mode to capture portal images during treatment. Visual monitoring identified an anomalous MLC leaf pair gap not otherwise detected by the automatic position verification. The position of the erred leaf was measured on EPID images and log files were analyzed for the treatment in question, the prior day's treatment, and for daily MLC test patterns acquired on those treatment days. Additional standard test patterns were used to quantify the leaf position.Results:Whereas the log file reported no difference between planned and recorded positions, image‐based measurements showed the leaf to be 1.3±0.1 mm medial from the planned position. This offset was confirmed with the test pattern irradiations.Conclusion:It has been clinically observed that log‐file derived leaf positions can differ from their actual positions by >1 mm, and therefore cannot be considered to be the actual leaf positions. This cautions the use of log‐based methods for MLC or patient quality assurance without independent confirmation of log integrity. Frequent verification of MLC positions through independent means is a necessary precondition to trusting log file records. Intra‐treatment EPID imaging provides a method to capture departures from MLC planned positions.Work was supported in part by Varian Medical Systems.

TH‐AB‐201‐10: Portal Dosimetry with Elekta IViewDose:Performance of the Simplified Commissioning Approach Versus Full Commissioning

Purpose:Elekta recently developed a solution for in‐vivo EPID dosimetry (iViewDose, Elekta AB, Stockholm, Sweden) in conjunction with the Netherlands Cancer Institute (NKI). This uses a simplified commissioning approach via Template Commissioning Models (TCMs), consisting of a subset of linac‐independent pre‐defined parameters. This work compares the performance of iViewDose using a TCM commissioning approach with that corresponding to full commissioning. Additionally, the dose reconstruction based on the simplified commissioning approach is validated via independent dose measurements.Methods:Measurements were performed at the NKI on a VersaHD™ (Elekta AB, Stockholm, Sweden). Treatment plans were generated with Pinnacle 9.8 (Philips Medical Systems, Eindhoven, The Netherlands). A farmer chamber dose measurement and two EPID images were used to create a linac‐specific commissioning model based on a TCM. A complete set of commissioning measurements was collected and a full commissioning model was created.The performance of iViewDose based on the two commissioning approaches was compared via a series of set‐to‐work tests in a slab phantom. In these tests, iViewDose reconstructs and compares EPID to TPS dose for square fields, IMRT and VMAT plans via global gamma analysis and isocentre dose difference. A clinical VMAT plan was delivered to a homogeneous Octavius 4D phantom (PTW, Freiburg, Germany). Dose was measured with the Octavius 1500 array and VeriSoft software was used for 3D dose reconstruction. EPID images were acquired. TCM‐based iViewDose and 3D Octavius dose distributions were compared against the TPS.Results:For both the TCM‐based and the full commissioning approaches, the pass rate, mean γ and dose difference were >97%, <0.5 and <2.5%, respectively. Equivalent gamma analysis results were obtained for iViewDose (TCM approach) and Octavius for a VMAT plan.Conclusion:iViewDose produces similar results with the simplified and full commissioning approaches. Good agreement is obtained between iViewDose (simplified approach) and the independent measurement tool.This research is funded by Elekta Limited

SU‐D‐BRA‐05: Time Series Analysis of EPID Images to Identify Patients in Need of Treatment Adaptation

Purpose:to evaluate if time series analysis of portal dose images can be used to identify patients undergoing important anatomical changes.Methods:daily EPID images of every treatment fields were acquired for 48 patients treated for lung cancer. In addition, CBCT were acquired on a regular basis (weekly or biweekly). Gamma analysis was performed relative to the first fraction given that no significant anatomical change was observed on the CBCT of the first fraction compared to the planning CT. Several parameters were extracted from the gamma analysis (e.g. average gamma value, standard deviation, percent above 1). The gamma parameters formed a patient‐specific time series that was analyzed. The first 24 patients were retrospectively evaluated to establish an action threshold that was then applied on the remaining 24 patients. Dosimetric evaluation of patients above that threshold was performed to as assess the level of degradation compared to the initial treatment plan.Results:after performing a clinical retrospective analysis of the first 24 cases, an action threshold on the average gamma value was established at 0.6 and a warning level was set at 0.4. These thresholds were then applied to the remaining 24 cases. Of these, 6 patients (25%) were above the warning level and 4 (17%) were above the action threshold. Dosimetric evaluation was performed on the CBCT for all these 6 patients. Three of these patients had changes above 3% in their PTV coverage, one had changes of about 2% and the remaining two had negligible changes. Patients with the strongest changes all had clear trending in their gamma parameters that could be classified with techniques such as hidden Markov models.Conclusion:by using time series analysis of relative EPID image it was possible to identify a subset of patient most likely to benefit from treatment adaptation.This work was funded in part by Varian Medical Systems

MO-FG-CAMPUS-TeP1-05: Rapid and Efficient 3D Dosimetry for End-To-End Patient-Specific QA of Rotational SBRT Deliveries Using a High-Resolution EPID

Purpose: EPID-based patient-specific quality assurance provides verification of the planning setup and delivery process that phantomless QA and log-file based virtual dosimetry methods cannot achieve. We present a method for EPID-based QA utilizing spatially-variant EPID response kernels that allows for direct calculation of the entrance fluence and 3D phantom dose. Methods: An EPID dosimetry system was utilized for 3D dose reconstruction in a cylindrical phantom for the purposes of end-to-end QA. Monte Carlo (MC) methods were used to generate pixel-specific point-spread functions (PSFs) characterizing the spatially non-uniform EPID portal response in the presence of phantom scatter. The spatially-variant PSFs were decomposed into spatially-invariant basis PSFs with the symmetric central-axis kernel as the primary basis kernel and off-axis representing orthogonal perturbations in pixel-space. This compact and accurate characterization enables the use of a modified Richardson-Lucy deconvolution algorithm to directly reconstruct entrance fluence from EPID images without iterative scatter subtraction. High-resolution phantom dose kernels were cogenerated in MC with the PSFs enabling direct recalculation of the resulting phantom dose by rapid forward convolution once the entrance fluence was calculated. A Delta4 QA phantom was used to validate the dose reconstructed in this approach. Results: The spatially-invariant representation of the EPID response accurately reproduced the entrance fluence with >99.5% fidelity with a simultaneous reduction of >60% in computational overhead. 3D dose for 106 voxels was reconstructed for the entire phantom geometry. A 3D global gamma analysis demonstrated a >95% pass rate at 3%/3mm. Conclusion: Our approach demonstrates the capabilities of an EPID-based end-to-end QA methodology that is more efficient than traditional EPID dosimetry methods. Displacing the point of measurement external to the QA phantom reduces the necessary complexity of the phantom itself while offering a method that is highly scalable and inherently generalizable to rotational and trajectory based deliveries. This research was partially supported by Varian.

MO-FG-202-01: A Fast Yet Sensitive EPID-Based Real-Time Treatment Verification System

Purpose: To create a real-time EPID-based treatment verification system which robustly detects treatment delivery and patient attenuation variations. Methods: Treatment plan DICOM files sent to the record-and-verify system are captured and utilized to predict EPID images for each planned control point using a modified GPU-based digitally reconstructed radiograph algorithm which accounts for the patient attenuation, source energy fluence, source size effects, and MLC attenuation. The DICOM and predicted images are utilized by our C++ treatment verification software which compares EPID acquired 1024×768 resolution frames acquired at ∼8.5hz from Varian Truebeam™ system. To maximize detection sensitivity, image comparisons determine (1) if radiation exists outside of the desired treatment field; (2) if radiation is lacking inside the treatment field; (3) if translations, rotations, and magnifications of the image are within tolerance. Acquisition was tested with known test fields and prior patient fields. Error detection was tested in real-time and utilizing images acquired during treatment with another system. Results: The computational time of the prediction algorithms, for a patient plan with 350 control points and 60×60×42cm^3 CT volume, is 2–3minutes on CPU and <27 seconds on GPU for 1024×768 images. The verification software requires a maximum of ∼9ms and ∼19ms for 512×384 and 1024×768 resolution images, respectively, to perform image analysis and dosimetric validations. Typical variations in geometric parameters between reference and the measured images are 0.32°for gantry rotation, 1.006 for scaling factor, and 0.67mm for translation. For excess out-of-field/missing in-field fluence, with masks extending 1mm (at isocenter) from the detected aperture edge, the average total in-field area missing EPID fluence was 1.5mm2 the out-of-field excess EPID fluence was 8mm^2, both below error tolerances. Conclusion: A real-time verification software, with EPID images prediction algorithm, was developed. The system is capable of performing verifications between frames acquisitions and identifying source(s) of any out-of-tolerance variations. This work was supported in part by Varian Medical Systems.

MO-FG-CAMPUS-TeP1-01: An Efficient Method of 3D Patient Dose Reconstruction Based On EPID Measurements for Pre-Treatment Patient Specific QA

Purpose: To demonstrate an efficient and clinically relevant patient specific QA method by reconstructing 3D patient dose from 2D EPID images for IMRT plans. Also to determine the usefulness of 2D QA metrics when assessing 3D patient dose deviations. Methods: Using the method developed by King et al (Med Phys 39(5),2839–2847), EPID images of IMRT fields were acquired in air and converted to dose at 10 cm depth (SAD setup) in a flat virtual water phantom. Each EPID measured dose map was then divided by the corresponding treatment planning system (TPS) dose map calculated with an identical setup, to derive a 2D “error matrix”. For each field, the error matrix was used to adjust the doses along the respective ray lines in the original patient 3D dose. All field doses were combined to derive a reconstructed 3D patient dose for quantitative analysis. A software tool was developed to efficiently implement the entire process and was tested with a variety of IMRT plans for 2D (virtual flat phantom) and 3D (in-patient) QA analysis. Results: The method was tested on 60 IMRT plans. The mean (± standard deviation) 2D gamma (2%,2mm) pass rate (2D-GPR) was 97.4±3.0% and the mean 2D gamma index (2D-GI) was 0.35±0.06. The 3D PTV mean dose deviation was 1.8±0.8%. The analysis showed very weak correlations between both the 2D-GPR and 2D-GI when compared with PTV mean dose deviations (R2=0.3561 and 0.3632 respectively). Conclusion: Our method efficiently calculates 3D patient dose from 2D EPID images, utilising all of the advantages of an EPID-based dosimetry system. In this study, the 2D QA metrics did not predict the 3D patient dose deviation. This tool allows reporting of the 3D volumetric dose parameters thus providing more clinically relevant patient specific QA.

SU‐D‐202‐05: Evaluation of Four‐Dimensional Dose Reconstruction Under Breathing Irregularity

Purpose:A method of four dimensional dose reconstruction (4D‐DR) using a cine mode of an EPID to determine the delivered 4D dose distribution has been suggested. This method, however, has not been tested under irregular breathing. This study investigated their effects on 4D‐DR.Methods:The 4D‐DR attempts to find the patient's breathing phase associated with each EPID image by comparing it to pre‐generated‐EPID‐quality DRRs of every breathing phases. A lung phantom with a tumor object (3cm diameter cylinder) on a moving platform was used for this test under conditions of amplitude reduction by 1/2 during (1) the entire delivery and (2) the duration when MLC blocked the tumor (hindering the phase determination). The dose delivered to the phantom was inversely reconstructed in the associated phase image of the phantom from each EPID image based on our documented inverse method of the 4D‐DR; the phase‐specific dose was integrated, generating the 4D reconstructed dose. Forward 4D Monte Carlo calculations were used as the 4D dose of a ground truth.Results:The reconstructed dose showed 98.1% gamma pass rate for 3%/3mm under regular breathing (i.e. no amplitude reduction) when compared with the forward 4D dose. When the irregular condition (#1) was adopted for treatment, the pass rate became 94.2%. The irregularity under the MLC block (#2) showed the pass rate of 91.3%. The two conditions did not affect the reconstruction noticeably in DVH plots of the tumor and lung‐tumor.Conclusion:As long as the extent of on‐treatment motion is smaller than that of the planning CT, the 4D‐DR can be done accurately. Otherwise, ontreatment 4D CT images are necessary.

SU‐F‐J‐114: On‐Treatment Imagereconstruction Using Transit Images of Treatment Beams Through Patient and Thosethrough Planning CT Images

Purpose:To reconstruct patient images at the time of radiation delivery using measured transit images of treatment beams through patient and calculated transit images through planning CT images.Methods:We hypothesize that the ratio of the measured transit images to the calculated images may provide changed amounts of the patient image between times of planning CT and treatment. To test, we have devised lung phantoms with a tumor object (3‐cm diameter) placed at iso‐center (simulating planning CT) and off‐center by 1 cm (simulating treatment). CT images of the two phantoms were acquired; the image of the off‐centered phantom, unavailable clinically, represents the reference on‐treatment image in the image quality of planning CT. Cine‐transit images through the two phantoms were also acquired in EPID from a non‐modulated 6 MV beam when the gantry was rotated 360 degrees; the image through the centered phantom simulates calculated image. While the current study is a feasibility study, in reality our computational EPID model can be applicable in providing accurate transit image from MC simulation. Changed MV HU values were reconstructed from the ratio between two EPID projection data, converted to KV HU values, and added to the planning CT, thereby reconstructing the on‐treatment image of the patient limited to the irradiated region of the phantom.Results:The reconstructed image was compared with the reference image. Except for local HU differences&gt;200 as a maximum, excellent agreement was found. The average difference across the entire image was 16.2 HU.Conclusion:We have demonstrated the feasibility of a method of reconstructing on‐treatment images of a patient using EPID image and planning CT images. Further studies will include resolving the local HU differences and investigation on the dosimetry impact of the reconstructed image.

SU‐D‐201‐06: Remote Dosmetric Auditing of VMAT Deliveries for Clinical Trials Using EPID

Purpose:To develop a method for remote dosimetric auditing the delivery of VMAT using EPID which allows for simple, inexpensive and time efficient dosimetric credentialing for clinical trials.Methods:Remote centers are provided with CT datasets and planning guidelines to produce VMAT plans for a head and neck and a post‐prostatectomy treatment. Plans are transferred in the planning system to two virtual water equivalent phantoms, one flat and one cylindrical. Cine images are acquired during VMAT delivery to the EPID in air with gantry angle recorded in image headers. Centers also deliver provided calibration plans to enable EPID signal to dose conversion, determination of the central axis, and correction of EPID sag prior to analysis. EPID images and planned doses are sent to the central site. EPID cine images are converted to dose in the virtual phantoms using an established backprojection method (King et al., Med.Phys. 2012) with EPID backscatter correction. Individual arcs (with gantry angles collapsed to zero) are evaluated at 10 cm depth in the flat phantom using 2D gamma, and total doses are evaluated in the cylindrical phantom using 3D gamma. Results are reported for criteria of 3%,3mm, 3%,2mm and 2%,2mm for all points greater than 10% of global maximum.Results:The pilot study for Varian centers has commenced, and three centers have been audited for head and neck plans and two for post‐prostatectomy plans to date. The mean pass rate for arc‐by‐arc 2D analysis at 3%,3mm is 99.5% and for 3D analysis is 95.8%. A method for Elekta linacs using an inclinometer for gantry angle information is under development.Conclusion:Preliminary results for this new method are promising. The method takes advantage of EPID equipment available at most centers and clinically established software to provide a feasible, low cost solution to credentialing centers for clinical trials.Funding has been provided from Calvary Mater Newcastle Department of Radiation Oncology, TROG Cancer Research and the University of Newcastle. Kimberley Legge is the recipient of an Australian Postgraduate Award. Narges Miri is a recipient of a University of Newcastle postgraduate scholarship.

TU-FG-201-01: 18-Month Clinical Experience of a Linac Daily Quality Assurance (QA) Solution Using Only EPID and OBI

Purpose: To describe the clinical use of a Linear Accelerator (Linac) DailyQA system with only EPID and OBI. To assess the reliability over an 18-month period and improve the robustness of this system based on QA failure analysis. Methods: A DailyQA solution utilizing an in-house designed phantom, combined EPID and OBI image acquisitions, and a web-based data analysis and reporting system was commissioned and used in our clinic to measure geometric, dosimetry and imaging components of a Varian Truebeam Linac. During an 18-month period (335 working days), the Daily QA results, including the output constancy, beam flatness and symmetry, uniformity, TPR20/10, MV and KV imaging quality, were collected and analyzed. For output constancy measurement, an independent monthly QA system with an ionization chamber (IC) and annual/incidental TG51 measurements with ADCL IC were performed and cross-compared to Daily QA system. Thorough analyses were performed on the recorded QA failures to evaluate the machine performance, optimize the data analysis algorithm, adjust the tolerance setting and improve the training procedure to prevent future failures. Results: A clinical workflow including beam delivery, data analysis, QA report generation and physics approval was established and optimized to suit daily clinical operation. The output tests over the 335 working day period cross-correlated with the monthly QA system within 1.3% and TG51 results within 1%. QA passed with one attempt on 236 days out of 335 days. Based on the QA failures analysis, the Gamma criteria is revised from (1%, 1mm) to (2%, 1mm) considering both QA accuracy and efficiency. Data analysis algorithm is improved to handle multiple entries for a repeating test. Conclusion: We described our 18-month clinical experience on a novel DailyQA system using only EPID and OBI. The long term data presented demonstrated the system is suitable and reliable for Linac daily QA.

MO-FG-202-07: Real-Time EPID-Based Detection Metric For VMAT Delivery Errors

Purpose: To create and test an accurate EPID-frame-based VMAT QA metric to detect gross dose errors in real-time and to provide information about the source of error. Methods: A Swiss cheese model was created for an EPID-based real-time QA process. The system compares a treatmentplan- based reference set of EPID images with images acquired over each 2° gantry angle interval. The metric utilizes a sequence of independent consecutively executed error detection Methods: a masking technique that verifies infield radiation delivery and ensures no out-of-field radiation; output normalization checks at two different stages; global image alignment to quantify rotation, scaling and translation; standard gamma evaluation (3%, 3 mm) and pixel intensity deviation checks including and excluding high dose gradient regions. Tolerances for each test were determined. For algorithm testing, twelve different types of errors were selected to modify the original plan. Corresponding predictions for each test case were generated, which included measurement-based noise. Each test case was run multiple times (with different noise per run) to assess the ability to detect introduced errors. Results: Averaged over five test runs, 99.1% of all plan variations that resulted in patient dose errors were detected within 2° and 100% within 4° (∼1% of patient dose delivery). Including cases that led to slightly modified but clinically equivalent plans, 91.5% were detected by the system within 2°. Based on the type of method that detected the error, determination of error sources was achieved. Conclusion: An EPID-based during-treatment error detection system for VMAT deliveries was successfully designed and tested. The system utilizes a sequence of methods to identify and prevent gross treatment delivery errors. The system was inspected for robustness with realistic noise variations, demonstrating that it has the potential to detect a large majority of errors in real-time and indicate the error source. J. V. Siebers receives funding support from Varian Medical Systems.

TH‐CD‐207A‐11: Sensitivity Analysis of Action Limits for Real‐Time EPID‐Based Delivery Verification System Using Artificial Clinical Relevant Error Simulations

Purpose:A real‐time patient treatment delivery verification system using EPID (Watchdog) has been developed as an advanced patient safety tool. In a pilot study data was acquired for 119 prostate and head and neck (HN) IMRT patient deliveries to generate body‐site specific action limits using statistical process control. The purpose of this study is to determine the sensitivity of Watchdog to detect clinically significant errors during treatment delivery.Methods:Watchdog utilizes a physics‐based model to generate a series of predicted transit cine EPID images as a reference data set, and compares these in real‐time to measured transit cine‐EPID images acquired during treatment using chi comparison (4%, 4mm criteria) after the initial 2s of treatment to allow for dose ramp‐up. Four study cases were used; dosimetric (monitor unit) errors in prostate (7 fields) and HN (9 fields) IMRT treatments of (5%, 7%, 10%) and positioning (systematic displacement) errors in the same treatments of (5mm, 7mm, 10mm). These errors were introduced by modifying the patient CT scan and re‐calculating the predicted EPID data set. The error embedded predicted EPID data sets were compared to the measured EPID data acquired during patient treatment. The treatment delivery percentage (measured from 2s) where Watchdog detected the error was determined.Results:Watchdog detected all simulated errors for all fields during delivery. The dosimetric errors were detected at average treatment delivery percentage of (4%, 0%, 0%) and (7%, 0%, 0%) for prostate and HN respectively. For patient positional errors, the average treatment delivery percentage was (52%, 43%, 25%) and (39%, 16%, 6%).Conclusion:These results suggest that Watchdog can detect significant dosimetric and positioning errors in prostate and HN IMRT treatments in real‐time allowing for treatment interruption. Displacements of the patient require longer to detect however incorrect body site or very large geographic misses will be detected rapidly.