Varian Electronic Portal Imaging Device Research Articles

Investigating the Electronic Portal Imaging Device for Small Radiation Field Measurements.

Purpose:With the advent of state-of-the-art treatment technologies, the use of small fields has increased, and dosimetry in small fields is highly challenging. In this study, the potential use of Varian electronic portal imaging device (EPID) for small field measurements was explored for 6 and 15 MV photon beams.Materials and Methods:The output factors and profiles were measured for a range of jaw-collimated square field sizes starting from 0.8 cm × 0.8 cm to 10 cm × 10 cm using EPID. For evaluation purpose, reference data were acquired using Exradin A16 microionization chamber (0.007 cc) for output factors and stereotactic field diode for profile measurements in a radiation field analyzer.Results:The output factors of EPID were in agreement with the reference data for field sizes down to 2 cm × 2 cm and for 2 cm × 2 cm; the difference in output factors was +2.06% for 6 MV and +1.56% for 15 MV. For the lowest field size studied (0.8 cm × 0.8 cm), the differences were maximum; +16% for 6 MV and +23% for 15 MV photon beam. EPID profiles of both energies were closely matching with reference profiles for field sizes down to 2 cm × 2 cm; however, penumbra and measured field size of EPID profiles were slightly lower compared to its counterpart.Conclusions:EPID is a viable option for profile and output factor measurements for field sizes down to 2 cm × 2 cm in the absence of appropriate small field dosimeters.

26. Evaluation of the EpiDream method for quality assurance (QA) of electron beams using a Varian electronic portal imaging device (EPID)

Introduction The aim of the study is to evaluate the use of EPID for electron beams QA defined by the 07/27/2007 French decision with a new approach in absorbed dose to water reconstruction: the EpiDream method. Methods The study was performed on two Varian LINACs, equipped with EPID (either with the AS500 or AS1000) for electron beams of 6, 9, 12, 15 and 18 MeV. Each EPID was calibrated with EpiDream application in order to obtain from the Grey Level (GL) image a dose matrix computed in water at the reference depth(zref) of the energy studied. For each energy, GL integrated images were corrected of dark field and acquired using AM Maintenance system(Varian) at the detector source distance (DSD) = 100 cm and DSD = (100 + zref) cm for different parameters : field sizes(6 × 6–20 × 20 cm2, delivered MU (3-300 MU), dose rate(100–300 MU/min). For each irradiation condition, a gamma index evaluation was performed between dose matrix computed by Eclipse TPS algorithm (eMC v13.6, Varian) and dose matrix computed by EpiDream method (DM EpiDream). For a 15 × 15 cm2 field size, a comparison was performed between radial and transverse dose profiles measured with a 0.125 cm3 ionization chamber(PTW31010) in water at zref for SSD = 100 cm and dose profiles obtained from DM EpiDream, considering: homogeneity and symmetry parameters, penumbra and correlation coefficient. Results Data analysis is in progress. However, first results are encouraging: At 9 MeV and for a DSD = 100 + zref, the mean gamma index value for 6 × 6, 10 × 10, and 15 × 15 cm2 field sizes is respectively: 99.9% (σ = 0.1%), 99.8% (σ = 0.32%), 96.3% (σ = 2.2%). Conclusions The EpidDream method would permit to perform electrons beams QA using EPID for all gantry angles by maximizing measurement reproducibility. The aim of the study is to evaluate the use of EPID for electron beams QA defined by the 07/27/2007 French decision with a new approach in absorbed dose to water reconstruction: the EpiDream method. The study was performed on two Varian LINACs, equipped with EPID (either with the AS500 or AS1000) for electron beams of 6, 9, 12, 15 and 18 MeV. Each EPID was calibrated with EpiDream application in order to obtain from the Grey Level (GL) image a dose matrix computed in water at the reference depth(zref) of the energy studied. For each energy, GL integrated images were corrected of dark field and acquired using AM Maintenance system(Varian) at the detector source distance (DSD) = 100 cm and DSD = (100 + zref) cm for different parameters : field sizes(6 × 6–20 × 20 cm2, delivered MU (3-300 MU), dose rate(100–300 MU/min). For each irradiation condition, a gamma index evaluation was performed between dose matrix computed by Eclipse TPS algorithm (eMC v13.6, Varian) and dose matrix computed by EpiDream method (DM EpiDream). For a 15 × 15 cm2 field size, a comparison was performed between radial and transverse dose profiles measured with a 0.125 cm3 ionization chamber(PTW31010) in water at zref for SSD = 100 cm and dose profiles obtained from DM EpiDream, considering: homogeneity and symmetry parameters, penumbra and correlation coefficient. Data analysis is in progress. However, first results are encouraging: At 9 MeV and for a DSD = 100 + zref, the mean gamma index value for 6 × 6, 10 × 10, and 15 × 15 cm2 field sizes is respectively: 99.9% (σ = 0.1%), 99.8% (σ = 0.32%), 96.3% (σ = 2.2%). The EpidDream method would permit to perform electrons beams QA using EPID for all gantry angles by maximizing measurement reproducibility.

SU‐F‐T‐229: A Novel Method for EPID‐Based In‐Vivo Exit Dose Verification for Intensity Modulated Radiotherapy

Purpose:Electronic portal imaging device (EPID) can be used to acquire a two‐dimensional exit dose distribution during treatment delivery, thus allowing the in‐vivo verification of the dose delivery through a comparison of measured portal images to predicted portal dose images (PDI). The aim of this study was to present a novel method to easily and accurately predict PDI, and to establish an EPID‐based in‐vivo dose verification method during IMRT treatments.Methods:We developed a model to determine the predicted portal dose at the same plane of the EPID detector location. The Varian EPID (aS1000) positions at 150cm source‐to‐detector‐distance (SDD), and can be used to acquire in‐vivo exit dose using Portal Dosimetry (PD) function. Our model was generated to make an equivalent water thickness represent the buildup plate of EPID. The exit dose at extend SDD plane with patient CT data in the beam can be calculated as the predicted PDI in the treatment planning system (TPS). After that, the PDI was converted to the fluence at SDD of 150cm using the inverse square law coded in MATLAB. Five head‐and‐neck and prostate IMRT patient plans contain 32 fields were investigated to evaluate the feasibility of this new method. The measured EPID image was compared with PDI using the gamma analysis.Results:The average results for cumulative dose comparison were 81.9% and 91.6% for 3%, 3mm and 4%, 4mm gamma criteria, respectively. Results indicate that the patient transit dosimetry predicted algorithm compares well with EPID measured PD doses for test situations.Conclusion:Our new method can be used as an easy and feasible tool for online EPID‐based in‐vivo dose delivery verification for IMRT treatments. It can be implemented for fast detecting those obvious treatment delivery errors for individual field and patient quality assurance.

A self-sufficient method for calibration of Varian electronic portal imaging device

Electronic portal imaging device (EPID) is currently used for dosimetric verification of IMRT fields and linac quality assurance (QA). It is critical to understand the dosimetric response and perform an accurate and robust calibration of EPID. We present the implementation of an efficient method for the calibration and the validation of a Varian EPID, which relies only on data collected with that specific device. The calibration method is based on images obtained with five shifts of EPID panel. With this method, the relative gain (sensitivity) of each element of a detector matrix is calculated and applied on top of the calibration determined with the flood-field procedure. The calibration procedure was verified using a physical wedge inserted in the beam line and the corrected profile shows consistent results with the measurements using a calibrated 2D array. This method does not rely on the beam profile used in the flood-field calibration process, which allows EPID calibration in 10 minutes with no additional equipment compared to at least 2 hours to obtain beam profile and scanning beam equipment requirement with the conventional method.

An EPID-based method for comprehensive verification of gantry, EPID and the MLC carriage positional accuracy in Varian linacs during arc treatments.

BackgroundIn modern radiotherapy, it is crucial to monitor the performance of all linac components including gantry, collimation system and electronic portal imaging device (EPID) during arc deliveries. In this study, a simple EPID-based measurement method has been introduced in conjunction with an algorithm to investigate the stability of these systems during arc treatments with the aim of ensuring the accuracy of linac mechanical performance.MethodsThe Varian EPID sag, gantry sag, changes in source-to-detector distance (SDD), EPID and collimator skewness, EPID tilt, and the sag in MLC carriages as a result of linac rotation were separately investigated by acquisition of EPID images of a simple phantom comprised of 5 ball-bearings during arc delivery. A fast and robust software package was developed for automated analysis of image data. Twelve Varian linacs of different models were investigated.ResultsThe average EPID sag was within 1 mm for all tested linacs. All machines showed less than 1 mm gantry sag. Changes in SDD values were within 1.7 mm except for three linacs of one centre which were within 9 mm. Values of EPID skewness and tilt were negligible in all tested linacs. The maximum sag in MLC leaf bank assemblies was around 1 mm. The EPID sag showed a considerable improvement in TrueBeam linacs.ConclusionThe methodology and software developed in this study provide a simple tool for effective investigation of the behaviour of linac components with gantry rotation. It is reproducible and accurate and can be easily performed as a routine test in clinics.

Patient Specific Pre-Treatment QA Verification Using an EPID Approach

A software program [MU-EPID], has been developed to perform patient specific pre-treatment quality assurance (QA) verification for intensity modulated radiation therapy (IMRT) using fluence maps measured with an electronic portal imaging device (EPID). The software converts the EPID acquired images of each IMRT beam, to fluence maps that are equivalent to those calculated by the treatment planning system (TPS). The software has the capability to process Varian, Elekta and Siemens EPID DICOM images. In the present investigation, several IMRT plans for different treatment sites were used to validate the software using the Varian a-Si 1000 EPID with the Pinnacle TPS. A total of 20 IMRT plans of different treatment sites were analyzed. Isodose distributions, dose profiles, dose volume histograms (DVH's) and gamma analysis comparisons were performed to evaluate the accuracy of our method. A gamma index analysis of the isocenter coronal plane was done for each plan and showed an average of 97.44% of gamma passing rate using a 3% and 3 mm gamma criterion. Isodose, DVH and dose profile comparisons were conducted between the original calculated plan and the measured reconstructed plan from the EPID images processed through the MU-EPID software. The results suggest that MU-EPID can be used clinically for patient specific IMRT QA, providing a comprehensive 3D dosimetric evaluation through DVH comparison as well as an option for a 2D gamma analysis.

Quality assurance of electron beams using a Varian electronic portal imaging device

The feasibility of utilizing an electronic portal imaging device (EPID) for the quality assurance of electron beams was investigated. This work was conducted on a Varian 2100iX machine equipped with an amorphous silicon (aS1000) portal imager. The linearity of the imager pixel response as a function of exposed dose was first confirmed. The short-term reproducibility of the EPID response to electron beams was verified. Low (6 MeV), medium (12 MeV) and high (20 MeV) energies were tested, each along with small (6 × 6 cm2), medium (10 × 10 cm2) and large (20 × 20 cm2) applicators. Acquired EPID images were analyzed using an in-house MATLAB code for radiation field size, penumbra, symmetry and flatness. Field sizes and penumbra values agreed with those from film dosimetry to within 1 mm. Field symmetry and flatness constancies were measured over a period of three weeks. The results indicate that EPID can be used for routine quality assurance of electron beams.

A method for removing arm backscatter from EPID images

To develop a method for removing the support arm backscatter from images acquired using current Varian electronic portal imaging devices (EPIDs). The effect of arm backscatter on EPID images was modeled using a kernel convolution method. The parameters of the model were optimized by comparing on-arm images to off-arm images. The model was used to develop a method to remove the effect of backscatter from measured EPID images. The performance of the backscatter removal method was tested by comparing backscatter corrected on-arm images to measured off-arm images for 17 rectangular fields of different sizes and locations on the imager. The method was also tested using on- and off-arm images from 42 intensity modulated radiotherapy (IMRT) fields. Images generated by the backscatter removal method gave consistently better agreement with off-arm images than images without backscatter correction. For the 17 rectangular fields studied, the root mean square difference of in-plane profiles compared to off-arm profiles was reduced from 1.19% (standard deviation 0.59%) on average without backscatter removal to 0.38% (standard deviation 0.18%) when using the backscatter removal method. When comparing to the off-arm images from the 42 IMRT fields, the mean γ and percentage of pixels with γ < 1 were improved by the backscatter removal method in all but one of the images studied. The mean γ value (1%, 1 mm) for the IMRT fields studied was reduced from 0.80 to 0.57 by using the backscatter removal method, while the mean γ pass rate was increased from 72.2% to 84.6%. A backscatter removal method has been developed to estimate the image acquired by the EPID without any arm backscatter from an image acquired in the presence of arm backscatter. The method has been shown to produce consistently reliable results for a wide range of field sizes and jaw configurations.

Automated x‐ray/light field congruence using the LINAC EPID panel

X-ray/light field alignment is a test described in many guidelines for the routine quality control of clinical linear accelerators (LINAC). Currently, the gold standard method for measuring alignment is through utilization of radiographic film. However, many modern LINACs are equipped with an electronic portal imaging device (EPID) that may be used to perform this test and thus subsequently reducing overall cost, processing, and analysis time, removing operator dependency and the requirement to sustain the departmental film processor. This work describes a novel method of utilizing the EPID together with a custom inhouse designed jig and automatic image processing software allowing measurement of the light field size, x-ray field size, and congruence between them. The authors present results of testing the method for aS1000 and aS500 Varian EPID detectors for six LINACs at a range of energies (6, 10, and 15 MV) in comparison with the results obtained from the use of radiographic film. Reproducibility of the software in fully automatic operation under a range of operating conditions for a single image showed a congruence of 0.01 cm with a coefficient of variation of 0. Slight variation in congruence repeatability was noted through semiautomatic processing by four independent operators due to manual marking of positions on the jig. Testing of the methodology using the automatic method shows a high precision of 0.02 mm compared to a maximum of 0.06 mm determined by film processing. Intraindividual examination of operator measurements of congruence was shown to vary as much as 0.75 mm. Similar congruence measurements of 0.02 mm were also determined for a lower resolution EPID (aS500 model), after rescaling of the image to the aS1000 image size. The designed methodology was proven to be time efficient, cost effective, and at least as accurate as using the gold standard radiographic film. Additionally, congruence testing can be easily performed for all four cardinal gantry angles which can be difficult when using radiographic film. Therefore, the authors propose it can be used as an alternative to the radiographic film method allowing decommissioning of the film processor.

Dosimetric properties and clinical application of an a-Si EPID for dynamic IMRT quality assurance

Dosimetric properties of an amorphous silicon electronic portal imaging device (EPID) for verification of intensity-modulated radiation therapy (IMRT) were investigated as a replacement for conventional verification tools. The portal dosimetry system of Varian's EPID (aS1000) has an integrated image mode for portal dosimetry (PD). The source-to-imager distance was 105 cm, and there were no extra buildup materials on the surface of the EPID in this study. Several dosimetric properties were examined. For clinical dosimetry, the dose distributions of dynamic IMRT beams for prostate cancer (19 patients, 97 beams) were measured by EPID and compared with the results of ionization chamber (IC) measurements. In addition, pretreatment measurements for prostate IMRT (50 patients, 309 beams) were performed by EPID and were evaluated by the gamma method (criterion: 3 mm/3 %). The signal-to-monitor unit ratio of PD showed dose dependence, indicating ghosting effects. Tongue-and-groove effects were observed as a result of the dose difference in the measured EPID images. The results of PD for clinical IMRT beams were in good agreement with the predicted dose image with average values of 1.37 and 0.25 for γ (max) and γ (ave), respectively. The point doses of PD were slightly, but significantly, higher than the results of IC measurements (p < 0.05 paired t test). However, this small difference seems clinically acceptable. This portal dosimetry system is useful as a rapid and convenient verification tool for dynamic IMRT.

TU-E-BRA-02: A Method to Remove Support Arm Backscatter from EPID Images.

To develop a method for removing the effect of support arm backscatter from Varian electronic portal imaging devices (EPIDs), improving the dosimetric abilities of the imager. A physical, kernel based model of the backscatter signal produced during an exposure was developed. The model parameters were determined through an optimization process, comparing measured images without arm backscatter (EPID removed from arm) to measured images that include arm backscatter. The backscatter model was used to develop a backscatter correction process that removes the support arm backscatter from measured EPID images. The correction process was tested by applying the method to measured images of 17 rectangular asymmetric fields and comparing the Result to off-arm images. The same process was repeated with 42 IMRT fields. The backscatter removal process was able to effectively remove the arm backscatter from all of the measured images and accurately predict the measured off-arm images. Comparing the corrected images to the measured off-arm images, the mean absolute difference at the centre of each rectangular field was 0.29% (standard deviation 0.18%). This is an improvement over the uncorrected images which gave a mean difference of 1.01% (standard deviation 0.73%). The largest discrepancy observed with the corrected images was 0.6%, compared to 2.8% for the uncorrected images. Comparing the corrected IMRT images to the measured off-arm images, an overall mean gamma value of 0.28 (standard deviation 0.04) was found using 2%, 2mm criteria. Comparison of the uncorrected images to the measured off-arm images resulted in an overall mean gamma of 0.40 (standard deviation 0.10). A method for accurately and reliably removing the effect of support arm backscatter from EPID images has been developed and extensively tested. The method can be applied to any measured EPID image and does not require any additional information about the exposure.

Using an EPID for patient‐specific VMAT quality assurance

A patient-specific quality assurance (QA) method was developed to verify gantry-specific individual multileaf collimator (MLC) apertures (control points) in volumetric modulated arc therapy (VMAT) plans using an electronic portal imaging device (EPID). VMAT treatment plans were generated in an Eclipse treatment planning system (TPS). DICOM images from a Varian EPID (aS1000) acquired in continuous acquisition mode were used for pretreatment QA. Each cine image file contains the grayscale image of the MLC aperture related to its specific control point and the corresponding gantry angle information. The TPS MLC file of this RapidArc plan contains the leaf positions for all 177 control points (gantry angles). In-house software was developed that interpolates the measured images based on the gantry angle and overlays them with the MLC pattern for all control points. The 38% isointensity line was used to define the edge of the MLC leaves on the portal images. The software generates graphs and tables that provide analysis for the number of mismatched leaf positions for a chosen distance to agreement at each control point and the frequency in which each particular leaf mismatches for the entire arc. Seven patients plans were analyzed using this method. The leaves with the highest mismatched rate were found to be treatment plan dependent. This in-house software can be used to automatically verify the MLC leaf positions for all control points of VMAT plans using cine images acquired by an EPID.

SU-DD-A1-01: Using an Electronic Portal Imaging Device for Patient-Specific Volumetric Modulated Arc Therapy Quality Assurance

Purpose: To implement a patient specific Quality Assurance (QA) method to verify gantry specific individual Multi‐Leaf Collimator (MLC) apertures (control points) in Volumetric Modulated Arc Therapy (VMAT) plans using an Electronic Portal Imaging Device (EPID). Method and Materials: RapidArc treatment plans were generated on Eclipse treatment planning system (TPS). DICOM images from a Varian EPID (as 1000) acquired in continuous acquisition mode were used for pretreatment QA. Each DICOM file contains the grey scale image of the MLC aperture related to its specific control point and the corresponding gantry angle. The TPS MLC file of this RapidArc plan contains the leaf positions for all the 177 control points. In‐house software was developed that interpolates the measured images and overlays them with the MLC pattern at all control points. The 50% isodose line was used as the edge of MLCs on the portal images. Results: The software generates graphs and tables that provide analysis for the number of mismatched leaf positions for a chosen distance to agreement (DTA) at each control point and the frequency in which each particular leaf mismatches for the entire arc. Five patients QAs were analyzed using this method. Even for a very complex plan 80% of the active leaves passed the 3 mm DTA criteria. We found that maximum and minimum number of mismatched leaves occurred at a gantry angle of 180° (IEC) and 0° respectively. The leaves with the highest mismatched rate are treatment plan dependent. Conclusion: This in‐house software automatically verifies the MLC leaf positions for all the control points of Rapid Arc plans. The leaf edge positions of the detected segment are compared with the calculated positions specified by the TPS and it was found that number of failures depends on complexity of the treatment plan and gantry angle.

SU‐GG‐J‐42: Development of an EPID‐Based Machine and IMRT QA System

Purpose: Interests in electronic portal imaging devices (EPIDs) for radiation dosimetry increase over the past decade, particularly with recent technique advances in amorphous‐silicon EPID. For a clinical quality assurance (QA) program, EPIDs may provide a useful tool with benefits of reduced cost and added convenience, e.g., shortening setup time. This study is to investigate the clinical application of EPID in routine machine and IMRT QA and develop an EPID‐based QA system. Materials and Methods: EPID images were acquired using Varian aS1000 EPID (integrated mode; 6 MV beam; 600 MU/min dose rate). Signal intensity of EPID images were converted to dose maps using a calibrated linear signal‐dose relationship. An integrated EPID‐based QA system was designed, which include functions for machine QA and IMRT QA. For routine machine QA, recommendations by TG‐40 and TG‐45 reports were used. Specific EPID‐based QA procedures and image analysis algorithms were designed and developed for basic QA tasks, e.g., beam flatness/symmetry test and Winston‐Lutz isocenter test. For IMRT QA, a phantom‐less method was proposed, which used fluence maps reconstructed from EPID‐measured fluence as input to treatment plan system for dose calculation and comparison. Results: EPID‐measured machine QA results were comparable to those measured monthly using films in our institute. For example, EPID‐measured parameters were: flatness = 0.9–1.45%; symmetry = 0.48–1.31%; penumbra = 3.2–4 mm at SSD = 150 cm. Star‐shot images were well reconstructed for isocenter check from EPID images acquired at different gantry or collimator angles. The EPID‐based IMRT QA method was able to verify the treatment plan and capture the reasonable plan‐versus‐QA dose difference. Conclusion: The results show that EPID may be properly used as a tool for clinical QA tasks and the developed EPID‐based QA system may be used for machine and IMRT QA. (Research sponsored by Oncology Data Systems, Inc, Oklahoma City, OK)

SU‐GG‐T‐186: Fast and Generalized Dose Reconstruction Method Using Virtual Beamlet Dose Responses

Purpose: We have previously developed an accurate dose‐reconstruction algorithm that is based on non‐iterative inverse solution of deposited dose using Monte Carlo‐generated responses for known multi‐leaf collimator (MLC) positions. The method was time‐consuming in generating the responses. The objective of this study is to develop a faster algorithm that does not require MLC positions by using pre‐calculated virtual beamlet‐specific dose responses for an open beam, and thus can be used at the end of radiation delivery on‐the‐spot. Methods: The success of the method is based on the hypothesis that the ratio of dose responses in phantom and electronic portal imaging device (EPID) of delivered beam intensities is approximated by the ratio of virtual beam intensities. The possibility is mathematically shown by convolving reconstructed intensities from EPID to both numerator and denominator of the latter ratio. The inversion algorithm for the reconstruction is unchanged. To test the hypothesis, we have calculated virtual beamlet dose responses in Rando phantom and EPID for intensity modulated beams determined for prostate treatment. We have measured in EPID under the phantom and performed dose reconstruction for the two cases. A 6MV photon beam from a Varian 2100EX accelerator and aS500 EPID (integrated acquisition) was used. Results: When compared with a forwardly calculated dose in an isocentric plane within the phantom for an IMRT beam, the reconstructed doses of the two showed similar pass rates of &gt;94.7%, given 3% dose difference or 3mm distance‐to‐agreement. The calculation used 5 minutes per beam using four 2.8GHz Intel XEON CPUs. Automation and the use of a higher data processor can potentially reduce the calculation time to less than a minute. Conclusion: A fast method of dose reconstruction was developed and more results will be presented. This method is applicable to detect delivery error. In part supported by Varian Medical Systems.

An EPID response calculation algorithm using spatial beam characteristics of primary, head scattered and MLC transmitted radiation

We have developed an independent algorithm for the prediction of electronic portal imaging device (EPID) response. The algorithm uses a set of images [open beam, closed multileaf collimator (MLC), various fence and modified sweeping gap patterns] to separately characterize the primary and head-scatter contributions to EPID response. It also characterizes the relevant dosimetric properties of the MLC: Transmission, dosimetric gap, MLC scatter [P. Zygmansky et al., J. Appl. Clin. Med. Phys. 8(4) (2007)], inter-leaf leakage, and tongue and groove [F. Lorenz et al., Phys. Med. Biol. 52, 5985-5999 (2007)]. The primary radiation is modeled with a single Gaussian distribution defined at the target position, while the head-scatter radiation is modeled with a triple Gaussian distribution defined downstream of the target. The distances between the target and the head-scatter source, jaws, and MLC are model parameters. The scatter associated with the EPID is implicit in the model. Open beam images are predicted to within 1% of the maximum value across the image. Other MLC test patterns and intensity-modulated radiation therapy fluences are predicted to within 1.5% of the maximum value. The presented method was applied to the Varian aS500 EPID but is designed to work with any planar detector with sufficient spatial resolution.

Optimization of image quality and dose for Varian aS500 electronic portal imaging devices (EPIDs)

This study was carried out to investigate whether the electronic portal imaging (EPI) acquisition process could be optimized, and as a result tolerance and action levels be set for the PIPSPro QC-3V phantom image quality assessment. The aim of the optimization process was to reduce the dose delivered to the patient while maintaining a clinically acceptable image quality. This is of interest when images are acquired in addition to the planned patient treatment, rather than images being acquired using the treatment field during a patient's treatment. A series of phantoms were used to assess image quality for different acquisition settings relative to the baseline values obtained following acceptance testing. Eight Varian aS500 EPID systems on four matched Varian 600C/D linacs and four matched Varian 2100C/D linacs were compared for consistency of performance and images were acquired at the four main orthogonal gantry angles. Images were acquired using a 6 MV beam operating at 100 MU min−1 and the low-dose acquisition mode. Doses used in the comparison were measured using a Farmer ionization chamber placed at dmax in solid water. The results demonstrated that the number of reset frames did not have any influence on the image contrast, but the number of frame averages did. The expected increase in noise with corresponding decrease in contrast was also observed when reducing the number of frame averages. The optimal settings for the low-dose acquisition mode with respect to image quality and dose were found to be one reset frame and three frame averages. All patients at the Northern Ireland Cancer Centre are now imaged using one reset frame and three frame averages in the 6 MV 100 MU min−1 low-dose acquisition mode. Routine EPID QC contrast tolerance (±10) and action (±20) levels using the PIPSPro phantom based around expected values of 190 (Varian 600C/D) and 225 (Varian 2100C/D) have been introduced. The dose at dmax from electronic portal imaging has been reduced by approximately 28%, and while the image quality has been reduced, the images produced are still clinically acceptable.

SU-FF-T-293: Modeling An ASi Portal Imager Response Using a Commercial Treatment Planning Software

Purpose: Quality assurance (QA) for intensity modulated radiation therapy (IMRT) plans is substantially more complex and time consuming than for conventional plans. Electronic portal imaging devices (EPIDs) can be employed to improve the efficiency of the QA process. The major objective of this work was to model the EPID as a machine, which is used to calculate the planar dose from an IMRT field. This planar dose can be compared directly to the measured EPID image for the purpose of pre-treatment dosimetric verification of an IMRT field. Materials and methods: The investigations were performed for a 6MV photon beam and an aS500 type EPID attached to a Varian Clinac 21EX linear accelerator (Varian Medical Systems, Palo Alto, CA). Planar portal images were acquired, with the EPID positioned at 100 cm source to axis distance, for a variety of fields defined by jaws and multi-leaf collimators. From the measured planar portal images, cross-plane and in-plane profiles were derived. Similarly, relative output factors, defined as the ratio of greyscale in a central 1 cm square region of interest for an arbitrary field to the greyscale in a reference 10 cm square field, were extracted. Using physics commissioning tools, the data were used in Pinnacle3® (Philips Medical Systems, Madison, WI) treatment planning software to model the response of the EPID at a single depth. The EPID model was tested and then clinically used to calculate pre-treatment portal images of individual beams for the IMRT test plan. Results and conclusions: The Varian EPID was successfully modeled as a machine in Pinnacle3® to calculate a planar dose map of an IMRT field at isocentre under 3cm of water that corresponds directly to the EPID image. The model was tested with various IMRT patterns, giving good agreement between calculated and imaged dose maps.

Performance optimization of the Varian aS500 EPID system

Today, electronic portal imaging devices (EPIDs) are widely used as a replacement to portal films for patient position verification, but the image quality is not always optimal. The general aim of this study was to optimize the acquisition parameters of an amorphous silicon EPID commercially available for clinical use in radiation therapy with the view to avoid saturation of the system. Special attention was paid to selection of the parameter corresponding to the number of rows acquired between accelerator pulses (NRP) for various beam energies and dose rates. The image acquisition system (IAS2) has been studied, and portal image acquisition was found to be strongly dependent on the accelerator pulse frequency. This frequency is set for each “energy — dose rate” combination of the linear accelerator. For all combinations, the image acquisition parameters were systematically changed to determine their influence on the performances of the Varian aS500 EPID system. New parameters such as the maximum number of rows (MNR) and the number of pulses per frame (NPF) were introduced to explain portal image acquisition theory. Theoretical and experimental values of MNR and NPF were compared, and they were in good agreement. Other results showed that NRP had a major influence on detector saturation and dose per image. A rule of thumb was established to determine the optimum NRP value to be used. This practical application was illustrated by a clinical example in which the saturation of the aSi EPID was avoided by NRP optimization. Moreover, an additional study showed that image quality was relatively insensitive to this parameter.PACS numbers: 87.53.Oq; 87.59.Jq

Performance optimization of the Varian aS500 EPID system

Today, electronic portal imaging devices (EPIDs) are widely used as a replacement to portal films for patient position verification, but the image quality is not always optimal. The general aim of this study was to optimize the acquisition parameters of an amorphous silicon EPID commercially available for clinical use in radiation therapy with the view to avoid saturation of the system. Special attention was paid to selection of the parameter corresponding to the number of rows acquired between accelerator pulses (NRP) for various beam energies and dose rates. The image acquisition system (IAS2) has been studied, and portal image acquisition was found to be strongly dependent on the accelerator pulse frequency. This frequency is set for each “energy — dose rate” combination of the linear accelerator. For all combinations, the image acquisition parameters were systematically changed to determine their influence on the performances of the Varian aS500 EPID system. New parameters such as the maximum number of rows (MNR) and the number of pulses per frame (NPF) were introduced to explain portal image acquisition theory. Theoretical and experimental values of MNR and NPF were compared, and they were in good agreement. Other results showed that NRP had a major influence on detector saturation and dose per image. A rule of thumb was established to determine the optimum NRP value to be used. This practical application was illustrated by a clinical example in which the saturation of the aSi EPID was avoided by NRP optimization. Moreover, an additional study showed that image quality was relatively insensitive to this parameter. PACS numbers: 87.53.Oq; 87.59.Jq