Electronic Portal Imaging Device System Research Articles

Direct dose to water dosimetry for pretreatment IMRT verification using a modified EPID

Electronic portal imaging devices (EPIDs) are high resolution systems that produce electronic dose maps with minimal time required for equipment setup, and therefore potentially present a time-saving alternative for intensity modulated radiation therapy (IMRT) pretreatment verification. A modified commercial EPID was investigated operated with an opaque sheet blocking the optical signal produced in the phosphor layer as a precursor to a switched mode dual dosimetry-imaging EPID system. The purpose of this study was to investigate the feasibility of using this system for direct dose to water dosimetry for pretreatment IMRT verification. A Varian amorphous silicon EPID was modified by placing an opaque sheet between the Gd(2)S(2)O:Tb phosphor layer and the photodiode array to block the optical photons. The EPID was thus converted to a direct-detecting system (dEPID), in which the high energy radiation deposits energy directly in the photodiode array. The copper build-up was replaced with d(max) solid water. Sixty-one IMRT beams of varying complexity were delivered to the EPID, to EDR2 dosimetric film and to a 2D ion chamber array (MapCheck). EPID data was compared to film and MapCheck data using gamma analysis with 3%, 3mm pass criteria. The fraction of points that passed the gamma test was on average 98.1% and 98.6%, for the EPID versus film and EPID versus MapCheck comparisons, respectively. In the case of comparison with film, the majority of observed discrepancies were associated with problems related to film sensitivity or processing. The very close agreement between EPID and both film and MapCheck data demonstrates that the modified EPID is suitable for direct dose to water measurement for pretreatment IMRT verification. These results suggest a reconfigured EPID could be an efficient and accurate dosimeter. Alternatively, optical switching methods could be developed to produce a dual-mode EPID with both dosimetry and imaging capabilities.

Dosimetric verification of IMAT delivery with a conventional EPID system and a commercial portal dose image prediction tool

The electronic portal imaging device (EPID) is a system for checking the patient setup; as a result of its integration with the linear accelerator and software customized for dosimetry, it is increasingly used for verification of the delivery of fixed-field intensity-modulated radiation therapy (IMRT). In order to extend such an approach to intensity-modulated arc therapy (IMAT), the combined use of an EPID system and a portal dose image prediction (PDIP) tool has been investigated. The dosimetric behavior of an EPID system, mechanically reinforced to maintain its positional stability during the accelerator gantry rotation, has been studied to assess its ability to measure portal dose distributions for IMAT treatment beams. In addition, the PDIP tool of a commercial treatment planning system, commonly used for static IMRT dosimetry, has been validated for simulating the PDIs of IMAT treatment fields. The method has been applied to the delivery verification of 23 treatment fields that were measured in their dual mode of IMRT and IMAT modalities. The EPID system has proved to be appropriate for measuring the PDIs of IMAT fields; additionally the PDIP tool was able to simulate these accurately. The results are quite similar to those obtained for static IMRT treatment verification, although it was necessary to investigate the dependence of the EPID signal and of the accelerator monitor chamber response on variable dose rate. Our initial tests indicate that the EPID system, together with the PDIP tool, is a suitable device for the verification of IMAT plan delivery; however, additional tests are necessary to confirm these results.

Virtual simulation and treatment verification–merits and demerits: experience at Sindh Institute of Urology and Transplantation (SIUT), Pakistan

Background: The rapid changes in practice of radiotherapy have taken place over the past 5 years in Pakistan. With advent of computed tomography simulator, and multileaf collimators–assisted linear accelerators and electronic portal imaging system, few centres in Pakistan have switched from conventional radiotherapy to modern computer-based technology. Our hospital is first centre in Pakistan which is using virtual simulation since March 2006. We present our experience with list of merits and demerits.Design: Retrospective study.Patient collection: Medical records of all patients who received radiotherapy in our centre were reviewed. Parameters included were type of malignancy, type radiotherapy (curative/palliative), simulation and planning process time and the displacement of the beam-axis from the planning isocentre in clinical situations during three-dimensional conformal radiotherapy using electronic portal imaging device (EPID). Data were collected on written proforma. Percentages, frequencies, measures of central tendency and dispersion were calculated using SPSS version 17.0.Results: A total of 289 patients were treated from March 2006 to November 2008. Transitional cell carcinoma of urinary bladder was most common malignancy seen (42.4%) followed by prostate (28.62%) and renal cell carcinoma (14.14%). Of these 34.26% patients were treated on curative basis. The virtual simulation process could be completed in an average time of 5 min (SD 3.5). Under many cases, the treatment portals could be designed and the patient marked in one session. The displacements were recorded for 43 portals for early prostate cancer using an EPID system. The mean displacement was found 2.44 ± 0.8 mm in x (transverse), y (craniocaudal), and z (anteroposterior) directions during treatment. Standard deviation (SD) was 0.87 (90% CI 2.21–2.66). Average number of portals taken was 10 (6–27) per treatment session.Conclusion: Computer-based simulation and treatment over conventional methods is appropriate for curative patients, achieving more accurate tumour localisation, sparing normal organs at risk, reduced field sizes and a film free environment; however efforts are required to achieve maximum immobilisation during treatment.

SU-FF-J-110: Geometrical Verification of Real-Time Tumor Tracking Using Electronic Portal Imaging Device (EPID)

Purpose: We have developed a novel method to verify the geometrical accuracy of real‐time tumor tracking. An Electronic Portal Imaging Device(EPID) is used as a virtual phantom system. Methods and Materials: This virtual phantom system uses virtual target motion in the absence of a moving phantom and fluoroscopic mode of EPID for monitoring the motion of the tracking aperture. The expected apertures are designed from real‐time detection of tumor motion, and the delivered apertures are monitored from the EPID system. These two apertures are compared for geometrical accuracy of a real‐time tracking technique. The virtual phantom system was tested with several tracking patterns with irregular motion. Target motion was designed for an oval‐shaped tumor with 40 mm and 55 mm in diameters for four cases: (1) stationary; (2) rigid‐body two‐dimensional (2‐D) displacement with a period variation from 3 to 10 s and 20‐mm peak‐to‐peak distance in the superior‐inferior direction and 10 mm in the left‐right direction; (3) 2‐D displacement with deformation, and (4) 2‐D displacement with both deformation and rotation. In the continuous acquisition mode of EPID,MLC aperture images were acquired at ∼8 Hz. Root‐mean‐square (RMS) deviation between the designed and delivered aperture motions were calculated for each case. Results: The RMS deviation is 0.005 mm for the stationary case, 0.5 mm for the 2‐D displacement, 0.7 mm for the 2‐D displacement with deformation, and 0.7 mm for the 2‐D displacement combined with both deformation and rotation. Conclusion: The EPID‐based virtual system can measure geometrical accuracy of real‐time tumor tracking without a moving phantom. Besides, it can be used even for complex target motion (i.e., 2‐D displacement combined with deformation and rotation), which is not possible with any existing moving phantom. Therefore, this EPID‐based virtual system shows great potential as a quality‐assurance process to verify real‐time tumor tracking.

MO‐FF‐A1‐01: Fluoroscopic Verification of Intensity‐Modulated Rotational Arc Therapy Delivery

Purpose: Intensity modulated rotational arc technique requires verification of leaf positions, gantry angle and dose rate in the entire arc. This study shows how to achieve this with a detailed verification of Varian RapidArc using a fluoroscopic electronic portal imaging device (EPID). Materials and Methods: Three Rapid Arc plans (prostate 1, whole pelvis 1, and head and neck 1) are delivered on a Triology linac (Varian Medical Systems, CA). During delivery, approximately 600 fluoroscopic portal images are acquired (∼8 images/s) per arc with a PV‐aS1000 EPID, without use of secondary phantoms or blocks. Each leaf position of each gantry angle is calculated from the acquired EPID images offline. Gantry angle information of each portal image is acquired from the dynalog file generated during beam delivery. Leaf positions from the dynalog file are compared to scheduled positions from the DICOM RT plan file. Results: Online EPID image acquisition of Rapid Arc delivery is prompt, involving extension of the EPID system and beam delivery time. The measurement error depends on the displacement of EPID system relatively to the center of rotation, which is only 1mm–1.5 mm. Offline analyses show the accuracy of leaf positions for static leaf and gantry field are better than 1 mm. More than 98.5% of leaf sequences exhibit less than 3mm deviations, 83 % show 2mm and 56% for 1mm. Conclusions: Position of each leaf of each gantry angle for Rapid Arc delivery is verified within 1 mm accuracy with fluoroscopic portal images. Use of fluoroscopic EPID images can be considered as a practical QA tool for the verification of the Rapid Arc delivery.This study is partially supported by Varian Medical Systems.

Use of a line‐pair resolution phantom for comprehensive quality assurance of electronic portal imaging devices based on fundamental imaging metrics

Image guided radiation therapy solutions based on megavoltage computed tomography (MVCT) involve the extension of electronic portal imaging devices (EPIDs) from their traditional role of weekly localization imaging and planar dose mapping to volumetric imaging for 3D setup and dose verification. To sustain the potential advantages of MVCT, EPIDs are required to provide improved levels of portal image quality. Therefore, it is vital that the performance of EPIDs in clinical use is maintained at an optimal level through regular and rigorous quality assurance (QA). Traditionally, portal imaging QA has been carried out by imaging calibrated line-pair and contrast resolution phantoms and obtaining arbitrarily defined QA indices that are usually dependent on imaging conditions and merely indicate relative trends in imaging performance. They are not adequately sensitive to all aspects of image quality unlike fundamental imaging metrics such as the modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) that are widely used to characterize detector performance in radiographic imaging and would be ideal for QA purposes. However, due to the difficulty of performing conventional MTF measurements, they have not been used for routine clinical QA. The authors present a simple and quick QA methodology based on obtaining the MTF, NPS, and DQE of a megavoltage imager by imaging standard open fields and a bar-pattern QA phantom containing 2 mm thick tungsten line-pair bar resolution targets. Our bar-pattern based MTF measurement features a novel zero-frequency normalization scheme that eliminates normalization errors typically associated with traditional bar-pattern measurements at megavoltage x-ray energies. The bar-pattern QA phantom and open-field images are used in conjunction with an automated image analysis algorithm that quickly computes the MTF, NPS, and DQE of an EPID system. Our approach combines the fundamental advantages of linear systems metrics such as robustness, sensitivity across the full spatial frequency range of interest, and normalization to imaging conditions (magnification, system gain settings, and exposure), with the simplicity, ease, and speed of traditional phantom imaging. The algorithm was analyzed for accuracy and sensitivity by comparing with a commercial portal imaging QA method (PIPSPRO, Standard Imaging, Middleton, WI) on both first-generation lens-coupled and modern a-Si flat-panel based clinical EPID systems. The bar-pattern based QA measurements were found to be far more sensitive to even small levels of degradation in spatial resolution and noise. The bar-pattern based QA methodology offers a comprehensive image quality assessment tool suitable for both commissioning and routine EPID QA.

Verification of setup errors in external beam radiation therapy using electronic portal imaging

The objective of this study was to conduct an audit on QA aspects of treatment delivery by the verification of the treatment fields′ position on different days to document the efficiency of immobilization methods and reproducibility of treatment. A retrospective study was carried out on 60 patients, each 20 treated for head and neck, breast, and pelvic sites; and a total of 506 images obtained by electronic portal imaging device (EPID) were analyzed. The portal images acquired using the EPID systems attached to the Varian linear accelerators were superimposed on the reference images. The anatomy matching software (Varian portal Vision. 6.0) was used, and the displacements in two dimensions and rotation were noted for each treated field to study the patient setup errors. The percentages of mean deviations more than 3 mm in ‘lateral (X) and longitudinal (Y)’ directions were 17.5%, 11.25%, and 7.5% for breast, pelvis, and head and neck cases respectively. In all cases, the percentage of mean deviation with more than 5 mm error was 0.83%. The maximum average mean deviation in all the cases was 1.87. The average mean SD along X and Y directions in all the cases was less than 2.65. The results revealed that the ranges of setup errors are site specific and immobilization methods improve reproducibility. The observed variations were well within the limits. The study confirmed the accuracy and quality of treatments delivered to the patients.

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.

Comparison of ghosting effects for three commercial EPIDs

Many studies have reported dosimetric characteristics of amorphous silicon electronic portal imaging devices (EPIDs). Some studies ascribed a non-linear signal to gain ghosting and image lag. Other reports, however, state the effect is negligible. This study compares the signal-to-monitor unit (MU) ratio for three different brands of EPID systems. The signal was measured for a wide range of monitor units (5-1000), dose-rates, and beam energies. All EPIDs exhibited a relative under-response for beams of few MUs; giving 4 to 10% lower signal-to-MU ratios relative to that of 1000 MUs. This under-response is consistent with ghosting effects due to charge trapping.

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

SU-FF-I-59: MTF Measurement of Digital Imaging Systems Used In Radiotherapy-

Purpose: The pre-sampled MTF was used to investigate the resolution of a storage phosphor plate. Three commercial computed radiography (CR) cassettes (Kodak EC-L fast and regular, and Agfa fast) were used. The MTF of the CR system was compared to the MTF of an electronic portal imaging device (EPID) system. Method and Materials: The MTF was measured using the edge method. For kilo-voltage simulation images, the edge device was a 180×100×2 mm3 lead plate with a 104 mm polished edge, and the standard spectrum (RQA3) and geometry specified in IEC 62220-1 were used. The edge device for 6 MV portal images was a 75×50×50 mm3 lead block with one of the 75×50 mm2 surfaces polished. A new method based on multiple over-sampled edge profiles was developed to verify the edge angle determined by the other two methods, which were based on edge detection techniques. The line spread function was calculated by the finite-element difference method and then Fourier transformed to obtain the MTF. Results: For simulation images: the MTFs obtained with the Kodak regular cassette were slightly lower than those obtained using the two fast cassettes. The MTFs in the line scan direction fell off slightly faster than those in the perpendicular direction. The MTFs obtained with large and small pixel sizes were almost identical. For the portal images: the MTFs for the EPID and CR systems were similar at frequencies lower than 0.4cycles/mm. Conclusion: Although the cassettes were designed for film use, the fast cassette allows exposure reduction for the use of CR plates, and results in slightly better MTFs. For CR systems, the laser spot size remains the same regardless of the pixel size, so changes in pixel size have little impact on the MTF. The smaller-pixel-size CR system gives better MTFs than the EPID system.

SU‐FF‐J‐88: Portal Imaging Capability of Motion Detection

Purpose: Quantitative determination and validation of tumor motion is required for Image‐Guided Radiotherapy (IGRT) of mobile tumors. One method for validating motion is portal imaging using electronic portal imaging devices (EPIDs). Ability to detect and delineate moving tumors depends on EPID specifications and image acquisition protocol. Requirements include accurate object definition and the capability to localize objects throughout the breathing cycle, at different locations within the body. We devised a straightforward test to assess EPID performance. Method and Materials: Water‐filled spheres of different diameters were attached to a computer‐controlled linear stage programmed to move spheres at constant velocities over a range of distances (up to 60mm) and speeds (from 5–40mm/s), within a stationary phantom. Images were captured during motion cycles using Varian PortalVision and Elekta iView amorphous silicon EPIDs. Accuracy of the projected target position was evaluated for images consisting of 1–3 averaged frames. We tested the image quality across the EPID while maintaining fixed source‐to‐object (isocenter) distances. Results: Both imaging systems accurately reproduced projected target shapes. The accuracy of range and speed measurements were 0.5mm and 0.1mm/s, respectively. The iView system detected the 60mm range of motion to within 2.5mm, for all tested speeds. The PortalVision system detected 60mm range of motion to within 3.5mm, up to 20mm/s, but performance degraded at greater speeds, with images missing in parts of motion cycles. At 30mm/s, only 45mm was detected. For both systems, range measurement errors were proportionally smaller for shorter ranges. Both imagers performed consistently throughout their imaging surface and both were comparable for image quality. Conclusion: We've devised a process that can simultaneously assess motion detection capability and test EPID systems. Both imagers performed well in static conditions, implying accelerator feedback system led to missing images. We'll investigate methods to improve the dynamic imaging characteristics of these EPID systems.

Verification of step-and-shoot IMRT delivery using a fast video-based electronic portal imaging device.

We present an investigation into the use of a fast video-based electronic portal-imaging device (EPID) to study intensity modulated radiation therapy (IMRT) delivery. The aim of this study is to test the feasibility of using an EPID system to independently measure the orchestration of collimator leaf motion and beam fluence; simultaneously measuring both the delivered field fluence and shape as it exits the accelerator head during IMRT delivery. A fast EPID that consists of a terbium-doped gadolinium oxysulphide (GdO2S:Tb) scintillator coupled with an inexpensive commercial 30 frames-per-second (FPS) CCD-video recorder (16.7 ms shutter time) was employed for imaging IMRT delivery. The measurements were performed on a Varian 2100 C/D linear accelerator equipped with a 120-leaf multileaf-collimator (MLC). A characterization of the EPID was performed that included measurements of spatial resolution, linac pulse-rate dependence, linear output response, signal uniformity, and imaging artifacts. The average pixel intensity for fields imaged with the EPID was found to be linear in the delivered monitor units of static non-IMRT fields between 3x3 and 15x15 cm2. A systematic increase of the average pixel intensity was observed with increasing field size, leading to a maximum variation of 8%. Deliveries of a clinical step-and-shoot mode leaf sequence were imaged at 600 MU/min. Measurements from this IMRT delivery were compared with experimentally validated MLC controller log files and were found to agree to within 5%. An analysis of the EPID image data allowed identification of three types of errors: (1) 5 out of 35 segments were undelivered; (2) redistributing all of the delivered segment MUs; and (3) leaf movement during segment delivery. Measurements with the EPID at lower dose rates showed poor agreement with log files due to an aliasing artifact. The study was extended to use a high-speed camera (1-1000 FPS and 10 micros shutter time) with our EPID to image the same delivery to demonstrate the feasibility of imaging without aliasing artifacts. High-speed imaging was shown to be a promising direction toward validating IMRT deliveries with reasonable image resolution and noise.

Clinical use of electronic portal imaging: report of AAPM Radiation Therapy Committee Task Group 58.

AAPM Task Group 58 was created to provide materials to help the medical physicist and colleagues succeed in the clinical implementation of electronic portal imaging devices (EPIDs) in radiation oncology. This complex technology has matured over the past decade and is capable of being integrated into routine practice. However, the difficulties encountered during the specification, installation, and implementation process can be overwhelming. TG58 was charged with providing sufficient information to allow the users to overcome these difficulties and put EPIDs into routine clinical practice. In answering the charge, this report provides; comprehensive information about the physics and technology of currently available EPID systems; a detailed discussion of the steps required for successful clinical implementation, based on accumulated experience; a review of software tools available and clinical use protocols to enhance EPID utilization; and specific quality assurance requirements for initial and continuing clinical use of the systems. Specific recommendations are summarized to assist the reader with successful implementation and continuing use of an EPID.

Electronic portal imaging (EPI) on linear accelerator

The EPI has become available recently in the Oncoradiological Centre of Budapest. The purpose of this paper is to review the construction and operation of the electronic portal imaging devices (EPIDs). The different EPID systems as well the EPID technique vs. portal films are compared. The advantages in patient set-up and the detection of the set-up errors are discussed. The use of the EPID technique in the clinical everyday practice is detailed. Recommendations of the set-up error correction for the most often occurring failures is given.

Electronic portal imaging with an avalanche‐multiplication‐based video camera

The aim of this study is to investigate the degree to which the imaging quality of an existing (video-based) electronic portal imaging device (EPID) system may be improved by using an avalanche-multiplication-based video camera (called the avalanche-gain method). Due to avalanche multiplication in the target of the video camera tube, the new camera can be made up to several hundred times more sensitive than a camera using a conventional video (e.g., Saticon) tube. As a result, the camera noise which limits the performance of current video-based EPIDs should be overwhelmed and made negligible. The detective quantum efficiency (DQE) of an EPID using the avalanche-gain method has been measured with 6 MV and 18 MV beams obtained using a linear accelerator. It is shown that the camera noise is indeed much smaller than quantum noise and that the DQE of the system is significantly increased compared to conventional video-based EPIDs. Variation of DQE of the avalanche-gain video portal system with a change of demagnification was also investigated. It has been shown that the improvement of optical coupling has less effect in this system than that in a conventional video-based EPID system.

Évolution des concepts d'utilisation de l'imagerie électronique: étude prospective sur trois périodes

To describe the evolution of the use of the electronic portal imaging device (EPID) over three periods. From 1990, as part of the quality assurance research programs, the radiotherapy department of the G.-F. Leclerc Centre of Dijon used EPID systems in a prospective fashion. During the first of the three periods (PER 1:1990-1993), the study consisted of analysis criteria determination, software efficiency improvement and a selection of patients who could benefit from the method. Eight hundred and forty-five images of 40 patients were analysed qualitatively and quantitatively. Two verifications per week were planned, and the action level for correction was 10 mm. Head and neck images were also displayed in 'cinema' presentation for internal movements analysis. From 1994 to 1995 (PER 2), off-line procedure (OLP) based upon early correction of the systematic error and the rules calculated from our previous experience were tested for checking the brain, head and neck (LOC 1: 396 images) and many of the pelvic irradiations (LOC 2: 260 images). A double-exposure procedure and/or movie loop presentation was reserved for other patients. During the last period (PER 3: 1996-1997), the OLP procedure was routinely performed in 54 patients (images: 321 LOC 1, 680 LOC 2). LOC 1: deviations of < 3 mm increased from 75.5% during PER 1 to 81% during PER 2 to 83% during PER3. Conversely, deviations of 3-5 mm dropped from 19.5 to 13%, while deviations of more than 5 mm remained stable, around 5%. The actual standard error of the mean deviation observed was 2 mm. LOC 2: deviations of < 5 mm were observed in 81% of the cases during PER 1 and in 91% during PER 3 (89.5% in PER 2). These good results led to a decrease in deviation of 5 to 7 mm (11 to 6%) and also to a significant drop in deviations of more than 7 mm, 8 to 3% respectively. The actual precision obtained was 2.5 mm +/- 3 mm SD. The OLP based upon the early correction of the systematic error led to a significant increase of setup accuracy of patients irradiated for the brain, head and neck, and especially for pelvic lesions.

Portal dosimetry using a liquid ion chamber matrix: Dose response studies

Current intensive investigations of electronic portal imaging devices (EPIDs) have prompted their potential application to portal dosimetry. In this paper, the progress made in using a commercial liquid ion chamber matrix EPID for portal dosimetry is discussed. The pixel value of the liquid ion chamber element was calibrated against dose by exposing the imager to 6-MV x-ray beams of various intensities obtained with various thicknesses of lead attenuators and a range of source to detector distances. Absolute dose values were determined using an ion chamber on the central axis at the depth of maximum dose in a solid water phantom. The pixel values of the matrix were determined for various field sizes in order to evaluate the dependence of pixel value on dose at those field sizes. It was confirmed that the pixel value was proportional to the square root of the dose rate and was nearly independent of the field size. The 2D pixel values were converted to 2D dose maps in the water phantom after applying a correction for the effect of horns in the flood calibration field. The flood calibration field was used to obtain the relative sensitivity of each pixel. Good agreement was observed (normally better than 1% in relative standard deviation) between the converted dose distribution obtained from the pixel matrix and the direct dose measurement using an ion chamber scanned in a water phantom in regions of shallow dose gradient. For application to on-line portal dosimetry, both the short- and long-term stability of this EPID system were found to be within 1% relative standard deviation.(ABSTRACT TRUNCATED AT 250 WORDS)