Electronic Portal Imaging Research Articles

Electronic portal image enhancement based on nonuniformity correction in wavelet domain.

Electronic portal images are one of the most important tools to verify the ongoing radiotherapy treatment through comparison with a reference image generated during treatment planning. In this procedure, two images are geometrically matched by means of visible bone or other landmarks of interest such as implanted fiducials. However, the intrinsically poor contrast and low spatial resolution of portal images can limit imagequality. In this study, we have provided a multiresolution approach to enhance the quality of portal images acquired from the pelvis treatment fields. The main idea behind this work aims at removing some of the image artifacts that conceal the anatomical information. For this purpose, we have applied the homomorphic filtering on the approximation sub-band of wavelet decomposition to enhance local information. Moreover, in order to sharpen the bone edges, wavelet detail sub-bands were weighted to amplify important image details in the reconstruction of the desired enhanced image. The most appropriate image quality measure was chosen according to the image's characteristics in the spatial domain. By considering the characteristics of portal images as the random and nonperiodic texture, high level of noise, and a nonuniform background, three suitable quality measures of images were assessed: edge content, measure of enhancement, and measure of enhancement byentropy. The higher values of these measures indicate the quality improvement in the processed images through our proposed algorithm. Moreover, the subjective evaluation results indicate that the proposed multiresolution approach significantly enhances the perceived quality of images in comparison with original and the similar approach ( ). Our proposed wavelet-based enhancement algorithm successfully reduced image intensity nonuniformity and enhanced anatomical featured information, which drastically improved the objective metrics values. Subjective evaluation of enhanced image confirmed this qualityimprovement.

PO-1531 Evaluating Electronic Portal Imaging Device for small field measurements

PO-1531 Evaluating Electronic Portal Imaging Device for small field measurements

Determination of the electronic portal imaging device pixel-sensitivity-map for quality assurance applications. Part 2: Photon beam dependence.

PurposeThe EPID PSM is a useful EPID calibration method for QA applications. The dependence of the EPID PSM on the photon beam used to acquire it has been investigated in this study for the four available PSM methods. The aim is to inform upon the viability of applying a single PSM for all available photon beams to simplify PSM implementation and maintenance.MethodsFour methods of PSM determination were each measured once in a single session on a single TrueBeam ® STx linac using 6 MV, 10 MV, 6 MV Flattening‐Filter‐Free (FFF), and 10 MV FFF photon beams. The resultant PSM was assessed for both intra‐ and inter‐method beam dependence via comparison between PSM of the same method compared to the 6 MV PSM and via comparison between PSM of the same beam with the corresponding Monte Carlo PSM. Comparisons were performed via 2D percentage deviation plots with associated histograms, 1D crossplane profiles, and via mean, median, and standard deviation percentage deviation statistics. Generated beam‐response was compared qualitatively via 1D crossplane profile comparison and quantitatively via symmetry assessment with comparison to the IC profiler device.ResultsThe Varian method provided the most consistent PSM with varying photon beam, with median percent deviation from the 6 MV PSM within 0.14% for all other beams. Qualitatively, each method provided similar beam‐response profiles. The measured beam‐response symmetry agreed to within 0.2% between the Calvary Mater Newcastle (CMN) method and IC profiler, but agreement reduced to within 0.9% and 2.2% for the Varian and WashU methods. PSM percent deviation with Monte Carlo PSM was within 0.75% for all methods and beams.ConclusionResults suggest that the PSM may be independent of photon beam to clinically relevant levels. The Varian method of PSM determination introduces the least beam dependence into the measured PSM.

Determination of the electronic portal imaging device pixel-sensitivity-map for quality assurance applications. Part 1: Comparison of methods.

PurposeCalibration of a radiotherapy electronic portal imaging device (EPID) using the pixel‐sensitivity‐map (PSM) in place of the flood field correction improves the utility of the EPID for quality assurance applications. Multiple methods are available for determining the PSM and this study provides an evaluation to inform on which is superior.MethodsThree different empirical methods (“Calvary Mater Newcastle” [CMN], “Varian,” and “WashU”) and a Monte Carlo‐based method of PSM determination were investigated on a single Varian TrueBeam STx linear accelerator (linac) with an aS1200 EPID panel. PSM measurements were performed for each empirical method three successive times using the 6 MV beam. The resulting PSM from each method was compared to the Monte Carlo method as a reference using 2D percentage deviation maps and histograms plus crossplane profiles. The repeatability of generated PSMs was also assessed via 2D standard deviation (SD) maps and histograms. Additionally, the Beam‐Response generated by removal of the PSM from a raw EPID image for each method was visually contrasted. Finally, the practicality of each method was assessed qualitatively and via the measured time required to acquire and export the required images.ResultsThe median pixel‐by‐pixel percentage deviation between each of the empirical PSM methods and the Monte Carlo PSM was ‐0.36%, 0.24%, and 0.74% for the CMN, Varian, and WashU methods, respectively. Ninety‐five percent of pixels were found to be repeatable to within ‐0.21%, 0.08%, 0.19%, and 0.35% (1 SD) for the CMN, Monte Carlo, Varian, and WashU methods, respectively. The WashU method was found to be quickest for data acquisition and export and the CMN the slowest.ConclusionFor the first time four methods of generating the EPID PSM have been compared in detail and strengths and weaknesses of each method have been identified. All methods are considered likely to be clinically acceptable and with similar practical requirements.

Dose estimation in patients treated with radiotherapy for SARS-COVID disease based on EPID measurement

Introduction. COVID radiotherapy requires performance of all radiotherapy (RT) procedures during one site visit due to the infectious nature of the disease. The aim of the study was to develop methods of estimating the delivered dose based on electronic portal image device (EPID) signal during treatment. Material and methods. Electronic portal image device signal was measured as a function of the phantom dose. The dose in 14 COVID patients was estimated for two X6MV beams. Results. The method allows to estimate dose in phantom with uncertainty of 12%. In this case, a systematic error was reported. Therefore, coefficients for clinical data were calculated and used to determine the dose in patients. The mean difference between the dose calculated and the dose measured for the 14 patients was 1%, but the uncertainty of this method was estimated as ±6% Conclusions. The proposed method may be useful in clinical practice as in vivo method. However, due to high uncer­tainty, it should be dedicated to the detection of “big” errors.

Comparing treatment uncertainty for ultra- vs. standard-hypofractionated breast radiation therapy based on in-vivo dosimetry

Background and purposePostoperative ultrahypofractionated radiation therapy (UHFRT) in 5 fractions (fx) for breast cancer patients is as effective and safe as conventionally hypofractionated RT (HFRT) in 15 fx, liberating time for higher-level daily online Image-Guided Radiation Therapy (IGRT) corrections. In this retrospective study, treatment uncertainties occurring in patients treated with 5fx (5fx-group) were evaluated using electronic portal imaging device (EPID)-based in-vivo dosimetry (EIVD) and compared with the results from patients treated with conventionally HFRT (15fx-group) to validate the new technique and to evaluate if the shorter treatment schedule could have a positive effect on the treatment uncertainties.Materials and methodsEPID-based integrated transit dose images were acquired for each treatment fraction in the 5fx-group (203 patients) and on the first 3 days of treatment and weekly thereafter in the 15fx-group (203 patients). A total of 1015 EIVD measurements in the 5fx-group and 1144 in the 15fx-group were acquired. Of the latter group, 755 had been treated with online IGRT correction (i.e., Online-IGRT 15fx-group).ResultsIn the 15fx-group 12.0% of fractions failed (FFs) compared to 3.8% in the 5fx-group and 6.9% in the online-IGRT 15fx-group. Causes for FFs in the 15fx-group compared with the 5fx-group were patient positioning (7.4% vs. 2.2%), technical issues (3.1% vs. 1.2%) and breast swelling (1.4% vs. 0.5%). In the online-IGRT 15fx-group, 2.5% were attributed to patient positioning, 3.8% to technical issues and 0.5% to breast swelling.ConclusionsEIVD demonstrated that UHFRT for breast cancer results in less FFs compared to standard HFRT. A large proportion of this decrease could be explained by using daily online IGRT.

Extending in aqua portal dosimetry with dose inhomogeneity conversion maps for accurate patient dose reconstruction in external beam radiotherapy.

Background and purposeIn aqua dosimetry with electronic portal imaging devices (EPIDs) allows for dosimetric treatment verification in external beam radiotherapy by comparing EPID-reconstructed dose distributions (EPID_IA) with dose distributions calculated with the treatment planning system in water-equivalent geometries. The main drawback of the method is the inability to estimate the dose delivered to the patient. In this study, an extension to the method is presented to allow for patient dose reconstruction in the presence of inhomogeneities.Materials and methodsEPID_IA dose distributions were converted into patient dose distributions (EPID_IA_MC) by applying a 3D dose inhomogeneity conversion, defined as the ratio between patient and water-filled patient dose distributions computed using Monte Carlo calculations. EPID_IA_MC was evaluated against dose distributions calculated with a collapsed cone convolution superposition (CCCS) algorithm and with a GPU‐based Monte Carlo dose calculation platform (GPUMCD) using non-transit EPID measurements of 25 plans. In vivo EPID measurements of 20 plans were also analyzed.ResultsIn the evaluation of EPID_IA_MC, the average γ-mean values (2% local/2mm, 50% isodose volume) were 0.70 ± 0.14 (1SD) and 0.66 ± 0.10 (1SD) against CCCS and GPUMCD, respectively. Percentage differences in median dose to the planning target volume were within 3.9% and 2.7%, respectively. The number of in vivo dosimetric alerts with EPID_IA_MC was comparable to EPID_IA.ConclusionsEPID_IA_MC accommodates accurate patient dose reconstruction for treatment disease sites with significant tissue inhomogeneities within a simple EPID-based direct dose back-projection algorithm, and helps to improve the clinical interpretation of both pre-treatment and in vivo dosimetry results.

Verification of stereotactic radiosurgery plans for multiple brain metastases using a virtual phantom-based procedure.

BackgroundThe purpose of this study was to describe the use of the VIPER software for patient-specific quality assurance (PSQA) of single-isocenter multitarget (SIMT) stereotactic radiosurgery (SRS) plans.Materials and methodsTwenty clinical of intensity-modulated (IMRT) SIMT SRS plans were reviewed. A total of 88 brain metastases were included. Number of lesions per plan and their individual volumes ranged from 2 to 35 and from 0.03 to 32.8 cm3, respectively. Plans were designed with the Eclipse system, and delivered using a Varian CLINAC linac. SRS technique consisted of non-coplanar static-field sliding-window IMRT. Each plan was mapped onto a virtual cylindrical water phantom (VCP) in the Eclipse to calculate a 3D dose distribution (verification plan). The VIPER software reconstructed the 3D dose distribution inside the VCP from the acquired in-air electronic portal image device (EPID) images of the treatment fields. A 3D gamma analysis was used to compare the reconstructed doses to the Eclipse planned doses on the VCP. Gamma passing rates (GPRs) were calculated using 3% global/2 mm criteria and dose thresholds ranged from 10% to 90% of the maximum dose.ResultsThe averages (± 1 SD) of the 3D GPRs over the 20 SRS plans were: 99.9 ± 0.2%, 99.7 ± 0.3%, 99.6 ± 0.5%, 99.3 ± 0.9%,99.1 ± 1.6%, 99.0 ± 1.6%, and 98.5 ± 3.3%, for dose thresholds of 10%, 20%, 30%, 50%, 70%, 80% and 90% respectively.ConclusionsThis work shows the feasibility of the VIPER software for PSQA of SIMT SRS plans, being a reliable alternative to commercially available 2D detector arrays.

Using eclipse scripting to fully automate in-vivo image analysis to improve treatment quality and safety.

PurposeAn automated, in‐vivo system to detect patient anatomy changes and machine output was developed using novel analysis of in‐vivo electronic portal imaging device (EPID) images for every fraction of treatment on a Varian Halcyon. In‐vivo approach identifies errors that go undetected by routine quality assurance (QA) to compliment daily machine performance check (MPC), with minimal physicist workload.MethodsImages for all fractions treated on a Halcyon were automatically downloaded and analyzed at the end of treatment day. For image analysis, compared to first fraction, the mean difference of high‐dose region of interest is calculated. This metric has shown to predict changes in planning treatment volume (PTV) mean dose. Flags are raised for: (Type‐A) treatment fraction whose mean difference exceeds 10%, to protect against large errors, and (Type‐B) patients with three consecutive fractions with mean exceeding ±3%, to protect against systematic trends. If a threshold is exceeded, a physicist is e‐mailed, a report for flagged patients, for investigation. To track machine output changes, for all patients treated on a day, the average and standard deviations are uploaded to a QA portal, along with the reviewed MPC, ensuring comprehensive QA for the Halcyon. To guide clinical implementation, a retrospective study from November 2017 till December 2020 was conducted, which grouped errors by treatment site. This framework has been used prospectively since January 2021.ResultsFrom retrospective data of 1633 patients (35 759 fractions), no Type‐A errors were found and only 45 patients (2.76%) had Type‐B errors. These Type‐B deviations were due to head‐and‐neck weight loss. For 6 months of prospective use (345 patients), 13 patients (3.7%) had Type‐B errors and no Type‐A errors.ConclusionsThis automated system protects against errors that can occur in vivo to provide a more comprehensive QA. This fully automated framework can be implemented in other centers with a Halcyon, requiring a desktop computer and analysis scripts.

Simple Automated Verification of Field Size Indicator for Quality Assurance of Medical Linear Accelerator

Purpose: The purpose of this study is to automate the field size verification to facilitate mechanical check aspect medical linear accelerator (linac) quality assurance in a MATLAB-based algorithm on electronic portal imaging device (EPID) images. Methods: A total of 5 reference datasets (i.e. field sizes of 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, 20 cm × 20 cm, and 25 cm × 25 cm) and 15 test datasets (i.e. reference field sizes plus 1 mm, 3 mm, and 5 mm increments) acquired by 6 MV Elekta Linac were used in this study. The proposed algorithm implemented a full automatic threshold with a value of 230 as a segmentation technique. The automated results were compared with manual results obtained using a ruler. Results: The automated results are comparable to manual results (i.e., the difference of both is within 2% or equal to 3 mm). The range of minimum to maximum difference between automated and manual was 0 - 3 mm and the maximum difference found in the 15.3 cm field size setting. Conclusions: We have successfully developed an automated procedure of field size verification and confirmed that the proposed algorithm provide a fast and accurate results.

Spatial uncertainty of Elekta stereotactic cones in the treatment of multiple brain metastases using multiple cones

PurposeTo evaluate the spatial accuracy of radiation centers (RCs) of Elekta stereotactic cones while changing the cone sizes during intra-fraction treatment of multiple brain metastases (MBM). Materials and methodsIn this study, nine cones of varying sizes (5, 10, 12.5, 15, 17.5, 20, 25, 30, and 35 mm) were used. The ball bearing (BB) was used as the target. The positions of BBs and RCs obtained using electronic portal imaging device (EPID) were transformed to a coordinate system X/Y/Z that was fixed with respect to the treatment room. ResultsThe positions of BBs when using cones sized between 12.5 and 35 mm were relatively stable. The maximum deviations between BBs and RCs were as follows: 1.11 mm in negative X and 1.30 mm in positive X, 0.87 mm in negative Y and 0.69 mm in positive Y, and 0.06 mm in negative Z and 0.58 mm in positive Z. The maximum offset between BBs and RCs gradually increased as the cone size decreased in the X and Y directions. The maximum offsets were 2.30, 1.56, and 0.64 mm in the X, Y, and Z directions, respectively, for the 12.5-mm cone. The maximum distance between the RCs was 0.43 mm when using the G135T270 beam between the 17.5-mm and 12.5-mm cones. ConclusionThe treatment of MBM with Elekta stereotactic cones demonstrated good spatial uncertainty. However, we need to pay particular attention to treatment using two or more cones in clinical practice. More external expansion margins are required in such cases.

Implementation of volumetric-modulated arc therapy for locally advanced breast cancer patients: Dosimetric comparison with deliverability consideration of planning techniques and predictions of patient-specific QA results via supervised machine learning

PurposeThe aim of this study was to implement a clinically deliverable VMAT planning technique dedicated to advanced breast cancer, and to predict failed QA using a machine learning (ML) model to optimize the QA workload. MethodsFor three planning methods (2A: 2-partial arc, 2AS: 2-partial arc with splitting, 4A: 4-partial arc), dosimetric results were compared with patient-specific QA performed with the electronic portal imaging device of the linac. A dataset was built with the pass/fail status of the plans and complexity metrics. It was divided into training and testing sets. An ML metamodel combining predictions from six base classifiers was trained on the training set to predict plans as ‘pass’ or ‘fail’. The predictive performances were evaluated using the unseen data of the testing set. ResultsThe dosimetric comparison highlighted that 4A was the highest dosimetric performant method but also the poorest performant in the QA process. 2AS spared the best heart, but provided the highest dose to the contralateral breast and lowest node coverage. 2A provides a dosimetric compromise between organ at risk sparing and PTV coverage with satisfactory QA results. The metamodel had a median predictive sensitivity of 73% and a median specificity of 91%. ConclusionsThe 2A method was selected to calculate clinically deliverable VMAT plans; however, the 2AS method was maintained when the heart was of particular importance and breath-hold techniques were not applicable. The metamodel provides promising predictive performance, and it is intended to be improved as a larger dataset becomes available.

The Stanford Volumetric Modulated Arc Therapy Total Body Irradiation Technique

PurposeIn this article, we describe the technical aspects of the Stanford volumetric modulated arc therapy (VMAT) total body irradiation (TBI) technique, compare it with other VMAT-TBI techniques, and share our initial experience. Methods and MaterialsFrom September 2019 to August 2021, 35 patients were treated with VMAT-TBI at our institution. Treatment planning was performed using in-house developed automated planning scripts. Organ sparing depended on the regimen: myeloablative (lungs, kidneys, and lenses) and nonmyeloablative with benign disease (lungs, kidneys, lenses, gonads, brain, and thyroid). Quality assurance was performed using electronic portal imaging device portal dosimetry and Mobius3D. Robustness was evaluated for the first 10 patients by performing local and global isocenter shifts of 5 mm. Treatment was delivered using image-guided radiation therapy for every isocenter and every fraction. In vivo measurements were performed on the match line between the VMAT and anterior-posterior/posterior-anterior fields and on the testes for the first fraction. ResultsThe lungs, lungs – 1 cm, and kidneys Dmean were consistently spared to 57.6% ± 4.4%, 40.7% ± 5.5%, and 70.0% ± 9.9% of the prescription dose, respectively. Gonadal sparing (Dmean = 0.69 ± 0.13 Gy) was performed for all patients with benign disease. The average planning target volume (PTV) maximum dose to 1 cubic centimeter (D1cc) was 120.7% ± 6.4% for all patients. The average Gamma passing rate for the VMAT plans was 98.1% ± 1.6% (criterion of 3%/2 mm). Minimal differences were observed between Mobius3D- and Eclipse AAA-calculated PTV Dmean (0.0% ± 0.3%) and lungs Dmean (–2.5% ± 1.2%). Robustness evaluation showed that the PTV Dmax and lungs Dmean were insensitive to small positioning deviations between the VMAT isocenters (1.1% ± 2.4% and 1.2% ± 1.0%, respectively). The average match-line dose measurement indicated patient setup was reproducible (96.1% ± 4.5% relative to prescription dose). Treatment time, including patient setup and beam-on, was 47.5 ± 9.5 min. ConclusionsThe Stanford VMAT-TBI technique, from simulation to treatment delivery, was presented and compared with other VMAT-TBI techniques. Together with publicly shared autoplanning scripts, our technique may provide the gateway for wider adaptation of this technology and the possibility of multi-institutional studies in the cooperative group setting.

A transit portal dosimetry method for respiratory gating quality assurance with a dynamic 3D printed tumor phantom.

BackgroundsRespiratory gating is one of the motion management techniques that is used to deliver radiation dose to a tumor at a specific position under free breathing. However, due to the dynamic feedback process of this approach, regular equipment quality assurance (QA) and patient‐specific QA checks need to be performed. This work proposes a new QA methodology using electronic portal imaging detector (EPID) to determine the target localization accuracy of phase gating.MethodsQA tools comprising 3D printed spherical tumor phantoms, programmable stages, and an EPID detector are characterized and assembled. Algorithms for predicting portal dose (PD) through moving phantoms are developed and verified using gamma analysis for two spherical tumor phantoms (2 cm and 4 cm), two different 6 MV volumetric modulated arc therapy plans, and two different gating windows (30%–70% and 40%–60%). Comparison between the two gating windows is then performed using the Wilcoxon signed‐rank test. An optimizer routine, which is used to determine the optimal window, based on maximal gamma passing rate (GPR), was applied to an actual breathing curve and breathing plan. This was done to ascertain if our method yielded a similar result with the actual gating window.ResultsHigh GPRs of more than 97% and 91% were observed when comparing the predicted PD with the measured PD in moving phantom at 2 mm/2% and 1 mm/1% levels, respectively. Analysis of gamma heatmaps shows an excellent agreement with the tumor phantom.The GPR of 40%–60% PD was significantly lower than that of the 30%–70% PD at the 1 mm/1% level (p = 0.0064). At the 2 mm/2% level, no significant differences were observed. The optimizer routine could accurately predict the center of the gating window to within a 10% range.ConclusionWe have successfully performed and verified a new method for QA with the use of a moving phantom with EPID for phase gating with real‐time position management.

A feasibility study for in vivo treatment verification of IMRT using Monte Carlo dose calculation and deep learning-based modelling of EPID detector response

BackgroundThis paper describes the development of a predicted electronic portal imaging device (EPID) transmission image (TI) using Monte Carlo (MC) and deep learning (DL). The measured and predicted TI were compared for two-dimensional in vivo radiotherapy treatment verification.MethodsThe plan CT was pre-processed and combined with solid water and then imported into PRIMO. The MC method was used to calculate the dose distribution of the combined CT. The U-net neural network-based deep learning model was trained to predict EPID TI based on the dose distribution of solid water calculated by PRIMO. The predicted TI was compared with the measured TI for two-dimensional in vivo treatment verification.ResultsThe EPID TI of 1500 IMRT fields were acquired, among which 1200, 150, and 150 fields were used as the training set, the validation set, and the test set, respectively. A comparison of the predicted and measured TI was carried out using global gamma analyses of 3%/3 mm and 2%/2 mm (5% threshold) to validate the model's accuracy. The gamma pass rates were greater than 96.7% and 92.3%, and the mean gamma values were 0.21 and 0.32, respectively.ConclusionsOur method facilitates the modelling process more easily and increases the calculation accuracy when using the MC algorithm to simulate the EPID response, and has potential to be used for in vivo treatment verification in the clinic.

Assessment of using electronic portal imaging device for analysing bolus material utilised in radiation therapy

Abstract In this study, the feasibility of using electronic portal imaging device (EPID) as an analysing equipment for bolus material was evaluated in terms of its dosimetric parameters. Seven superflab bolus material samples, 4 samples of 0.5 cm (S1–S4) and 3 samples of 1 cm thickness (S5–S7), were analysed and compared with tissue equivalent water phantom, which was selected for reference material. Gamma analysis method was used to evaluate the dose distribution of the boluses. In addition, 487,204 point-dose values of each bolus were compared with the point-doses of corresponding reference material by using Spearman’s correlation coefficient analysis. The passing rates varied from 58.3 to 100% for 0.5 cm thick samples, and on the other hand, the passing rates of all the 1 cm thick boluses were 100%. All the correlation coefficient values were above 0.975. The correlation was statistically significant for all the samples (p < 0.001). The correlation coefficient of S4 bolus sample was the highest among the 0.5 cm thick bolus samples with a result of 0.989. Likewise, S5 and S6 bolus samples were the highest among the 1 cm thick ones with a result of 0.995. The results indicated that the material planned to be used as bolus can be evaluated with EPID in daily use.

Assessing the accuracy of electronic portal imaging device (EPID)-based dosimetry: I. Quantities influencing long-term stability.

The aim of this study is to reduce the uncertainty associated with determining dose-to-water using an amorphous silicon electronic portal imaging detector (EPID) under reference conditions by identifying and accounting for operational and environmental factors influencing the long-term stability of EPID response. Measurements of the EPID relative response, corrected for variations in linear accelerator (linac) output, were performed regularly over a period of 12 months. For every acquired image set, measurements of detector supply voltages, internal operating temperature, and ambient environmental conditions were obtained. Pearson r correlation coefficients were then calculated for each pair of variables, a subset of which were fitted using multiple linear regression to develop a predictive model of EPID response. Principal component analysis was performed on the dataset to reveal the internal structure of the data in a way that best accounts for the observed variations. The +5.5V power supply voltage, internal operating temperature, and the accumulated dose absorbed in EPID were identified as having the greatest influence on the long-term stability of EPID response. By correcting for the combined effect of these variables, the mean difference in linac output as measured by the EPID relative to a reference class chamber improved from -0.46% to 0.23% over the period of the study. This work suggests that the stability of an EPID over a period of a year can be improved by a factor of two by monitoring and accounting for the effects of variations in power supply voltage, internal temperature of the detector, and accumulated absorbed dose.

Assessing the accuracy of electronic portal imaging device (EPID)‐based dosimetry: II. Evaluation of a dosimetric uncertainty budget and development of a new film‐in‐EPID absorbed dose calibration methodology

The aim of this study is to reduce the uncertainty associated with determining dose-to-water using an amorphous silicon electronic portal imaging detector (EPID) under reference conditions by developing a direct calibration formalism based on radiochromic film measurements made within the EPID panel and detailed Monte Carlo simulations. To our knowledge, this is the first EPID-based dosimetry study reporting an uncertain budget METHODS: Pixel sensitivity and relative off-axis response were mapped by simultaneously irradiating film contained within the imager panel and acquiring an EPID image set. The detector panel was disassembled for the purpose of modeling the EPID in detail using the EGSnrc DOSXYZnrc usercode, which was in turn used to calculate dose-to-film in the EPID and dose-to-water in water conversion factors RESULTS: A direct comparison of the two correction methodologies investigated in this work, the previously established empirical method and the proposed simultaneous measurement approach involving in-EPID film dosimetry, produced an agreement with an RMS deviation of 1.4% overall. A combined standard relative uncertainty of 3.3% (k=1) was estimated for the determination of absorbed dose to water at the position of the EPID using the proposed calibration methodology CONCLUSIONS: This work describes a direct method of calibrating EPID response in terms of absorbed dose to water requiring fewer measurements than other empirical approaches, and without 2D spatial interpolation of correction factors.

A Large Area Pixelated Silicon Array Detector for Independent Transit In Vivo Dosimetry

A large area pixelated silicon array detector named “MP987” has been developed for in vivo dosimetry. The detector was developed to overcome the non-water equivalent response of EPID (Electronic Portal Imaging Device) dosimetry systems, due to the shortfalls of the extensive corrections required. The detector, readout system and software have all been custom designed to be operated independently from the linac with the array secured directly above the EPID, to be used in combination with the 6 MV imaging system. Dosimetry characterisation measurements of percentage depth dose (PDD), dose rate dependence, radiation damage, output factors (OF), profile measurements, linearity and uniformity were performed. Additionally, the first pre-clinical tests with this novel detector of a transit dosimetry characterization and a collapsed IMRT (intensity-modulated radiation therapy) study are presented. Both PDD and OF measurements had a percentage difference of less than 2.5% to the reference detector. A maximum change in sensitivity of 4.3 ± 0.3% was observed after 30 kGy of gamma accumulated dose. Transit dosimetry measurements through a homogeneous Solid Water phantom had a measured dose within error of the TPS calculations, for field sizes between 3 × 3 cm2 and 10 × 10 cm2. A four-fraction collapsed IMRT plan on a lung phantom had absolute dose pass fractions between the MP987 and TPS (treatment planning system) from 94.2% to 97.4%, with a 5%/5 mm criteria. The ability to accurately measure dose at a transit level, without the need for correction factors derived from extensive commissioning data collection procedures, makes the MP987 a viable alternative to the EPID for in vivo dosimetry. This MP987 is this first of its kind to be successfully developed specifically for a dual detector application.

Initial Clinical Experience with Electronic Portal Imaging Device-Based In Vivo Dosimetry for Radiotherapy in Gynecological Cancers

Initial Clinical Experience with Electronic Portal Imaging Device-Based In Vivo Dosimetry for Radiotherapy in Gynecological Cancers