Amorphous Silicon Electronic Portal Imaging Devices Research Articles

Evaluation of Perkin Elmer Amorphous Silicon Electronic Portal Imaging Device for Small Photon Field Dosimetry.

Electronic portal imaging devices (EPIDs) are applied to measure the dose and verify patients' position. The present study aims to evaluate the performance of EPID for measuring dosimetric parameters in small photon fields. In this experimental study, the output factors and beam profiles were obtained using the amorphous silicon (a-Si) EPID for square field sizes ranging from 1×1 to 10×10 cm2 at energies 6 and 18 mega-voltage (MV). For comparison, the dosimetric parameters were measured with the pinpoint, diode, and Semiflex dosimeters. Additionally, the Monaco treatment planning system was selected to calculate the output factors and beam profiles. There was a significant difference between the output factors measured using the EPID and that measured with the other dosimeters for field sizes lower than 8×8 cm2. In the energy of 6 MV, the gamma passing rates (3%/3 mm) between EPID and diode profile were 98%, 98%, 95%, 94%, 93%, and 94% for 1×1, 2×2, 3×3, 4×4, 5×5, and 10×10 cm2, respectively. The measured penumbra width with EPID was higher compared to that measured by the diode dosimeter for both energies. The EPID can measure the dosimetric parameters in small photon fields, especially for beam profiles and penumbra measurements.

The feasibility of the pretreatment verification of 2D dose distributions in radiation therapy with small fields using the electronic portal imaging device.

The present study aims to evaluate the performance of an Electronic portal imaging device (EPID) for measuring dosimetric parameters and for verification of dose in small photon fields. In this study, the beam profiles were obtained using the amorphous silicon (a-Si) EPID for field sizes ranging from 1 × 1 to 10 × 10 cm 2 at energies 6 and 18 mega-voltage (MV). For comparison, the dosimetric parameters, including penumbra widths and field sizes, were measured with the pinpoint, diode, and Semiflex dosimeters. Finally, Rando Phantom was used to compare the two-dimensional (2D) Dose distribution between EPID and Treatment Planning System (TPS). In both 5 cm and 10 cm depths, there were large differences between the measured doses obtained from TPS, Pinpoint detector, and Farmer detector in 1 × 1 field size. The differences become negligible as the field sizes increase and from 3 × 3 field size to 10 × 10 field size, the maximum observed differences are 2 cGy and 2.4 cGy for 5 cm and 10 cm depths, respectively. The results indicate that the penumbra widths are smaller in the Gantry-Target (GT) direction compared to the Right-Left (RL) direction. The maximum difference (47.6%) was observed for EPID in the 10 × 10 field size, and the minimum difference (16.6%) was observed for TPS in the 1 × 1 field size. Finally, 2D dose distributions obtained by EPID and TPS exhibit excellent agreement. EPID is an excellent tool for the measurement of dosimetry parameters such as dose profiles, penumbra widths, field sizes, and pretreatment verification of 2D dose distributions, especially in small fields.

Development of a Fluence Generating Programme for Patient-Specific Quality Assurance and Calibration of an Amorphous Silicon EPID

Patient-specific quality assurance (QA) is an essential part of intensity-modulated radiotherapy (IMRT) and volumetric arc therapy (VMAT) treatments. This study aims to develop a Patient-specific QA system through the development of a fluence generating programme and utilization of amorphous silicon (aSi) electronic portal imaging device (EPID). The EPID was investigated on the dependency of dose rate, uniformity of detector sensitivity and output factor. A fluence generating programme was written using MATLAB. Five simple dynamic arc plans and one patient VMAT plan were created using the Monaco treatment planning system which were used in the programme to generate fluence maps. The accuracy of the programme was evaluated by performing gamma analysis between the generated and measured fluence. EPID signal was found to be linearly proportional to dose (1-1000 MU) and independent of dose rate. The mean uniformity ratio was 1.07 ± 0.04%. The measured fluence maps were corrected for uniformity before gamma analysis. Output factor and dose deposition were taken into account in the process. The average result of the gamma analysis passed the 95% threshold. A fluence generating programme was developed, serving as the foundation of the Patient-specific QA system using aSi EPID for IMRT/VMAT delivery in this centre.

Pretreatment quality control of single isocenter half- beam treatment planning technique using an amorphous silicon electronic portal-imaging device (EPID)

Introduction: Electronic portal imaging devices (EPIDs) are fundamentally used for instantaneous verification of the patient set‐up, block shape, and leaf positions during radiation therapy. In radiotherapy, situations arise in which an inclined PTV must be treated mutually with adjacent nodal regions. This methodology is most widely used for matching tangential/lateral breast/head and neck fields to supraclavicular and axillary nodal fields. The single-isocenter half-beam technique (SIHB) is the commonly used technique for field matching in the three-dimensional radiation therapy (3DCRT) in which abutting fields asymmetrically with no divergence at the central axis are matched. Linear accelerator asymmetric jaw junctioning is used in single isocenter half-beam treatment planning (SIHB) technique. In practice, the major difficulty of the SIHB technique lies in creating the dose accuracy at the central axis and dose homogeneity in the abutting region of half beams with sufficient precision represents. The aim of this work was to investigate the use of amorphous silicon EPIDs for pretreatment quality control of the SIHB-based treatment plans. Materials and Methods: The zero jaw position was studied at the gantry angle of 0o. All measurements were made for 6, 10, and 15MV photon beams. The EPID used in this study was a CCD camera‐based iView EPID mounted to an Elekta synergy platform. The EPID has been calibrated to convert a grayscale EPID image into a two‐dimensional absolute dose distribution. In addition, calculations of the central field dose were performed at a fixed EPID distance of 157.5cm. The double-exposure technique and EPID was used for measuring the junction doses for each asymmetric jaw. The junction dose was obtained as a function of jaw position. Then, the EPID- based junction dose measurements were compared with radiographic film measurements of junction dose. Results: The results, obtained for 6, 10, and 15MV photon beams, showed that there was an approximately linear relationship between doses and gap or overlap. Because of the non- water equivalent EPID scattering properties, there were a different in the junction doses measured with the EPID and film. A correction factor was obtained to convert the EPID measured junction dose to film measured equivalent. Conclusions: The inaccurate abutting junction between the half beams results in under or overdose of the normal tissue between the two volumes. In the present study, we carried out a procedure for daily EPID-based quality assurance of the SIHB technique of treatment planning. The utilization of this method can be significant enough to warrant the accuracy and safety of the SIHB technique in clinical procedures. Conclusion: our study showed that the aSi EPID can be used as a transit dosimeter. The technique to reconstruct the dose delivered in the patient is straightforward, reproducible, and rapid for repeated in vivo dose verification.

A comparative analysis of Matrixx and EPID for dosimetric verification of intensity-modulated radiotherapy

Objective To compare the dosimetric verification results of Varian Portal Dosimetry and Matrixx, and to assess the reliability of the clinical application of electronic portal imaging device (EPID) verification. Methods Varian TrueBeam linear accelerator, which was equipped with a 120-leaf multileaf collimator and an amorphous silicon EPID, as well as portal dose prediction software. IBA I′mRT Matrixx ion chamber array was used. EPID algorithm configuration, dose calibration, and testing before use were performed. The sliding-window protocol was used. There were 77 patients with tumors involving the head and neck (mainly nasopharyngeal carcinoma), mediastinum, abdomen, and pelvic cavity were selected. The verification plan of the portal dose was created with a source-detector distance of 100 cm, and the gantry angle was kept the same as the treatment plan. The verification plan was carried out in the TrueBeam machine, and the data were collected at the same time by EPID. Comparison between the measured and calculated dose images was performed, and the evaluation standard was gamma index (3%/3 mm). The paired t-test was used for difference analysis. Results For the 77 patients, the Gamma passing rates of both methods were above 97%. Except for head and neck carcinoma were a significant difference between the results of dosimetric results using EPID and Matrixx in intensity-modulated radiotherapy (P=0.018) other remaining all P> 0.05. Conclusions The dosimetric verification results of EPID are consistent with those of Matrixx. EPID can be used for dosimetric verification, and Matrixx ion chamber array can be used only in case of a low Gamma passing rate. Key words: Dosimetric verification; Portal dosimetry; Matrixx

Clinical Experience and Evaluation of Patient Treatment Verification With a Transit Dosimeter

To prospectively evaluate a protocol for transit dosimetry on a patient population undergoing intensity modulated radiation therapy (IMRT) and to assess the issues in clinical implementation of electronic portal imaging devices (EPIDs) for treatment verification. Fifty-eight patients were enrolled in the study. Amorphous silicon EPIDs were calibrated for dose and used to acquire images of delivered fields. Measured EPID dose maps were back-projected using the planning computed tomographic (CT) images to calculate dose at prespecified points within the patient and compared with treatment planning system dose offline using point dose difference and point γ analysis. The deviation of the results was used to inform future action levels. Two hundred twenty-five transit images were analyzed, composed of breast, prostate, and head and neck IMRT fields. Patient measurements demonstrated the potential of the dose verification protocol to model dose well under complex conditions: 83.8% of all delivered beams achieved the initial set tolerance level of ΔD of 0±5cGy or %ΔD of 0%±5%. Importantly, the protocol was also sensitive to anatomic changes and spotted that 3 patients from 20 measured prostate patients had undergone anatomic change in comparison with the planning CT. Patient data suggested an EPID-reconstructed versus treatment planning system dose difference action level of 0%±7% for breast fields. Asymmetric action levels were more appropriate for inversed IMRT fields, using absolute dose difference (-2±5cGy) or summed field percentage dose difference (-6%±7%). The invivo dose verification method was easy to use and simple to implement, and it could detect patient anatomic changes that impacted dose delivery. The system required no extra dose to the patient or treatment time delay and so could be used throughout the course of treatment to identify and limit systematic and random errors in dose delivery for patient groups.

Beam's-eye-view imaging during non-coplanar lung SBRT.

Beam's-eye-view (BEV) imaging with an electronic portal imaging device (EPID) can be performed during lung stereotactic body radiation therapy (SBRT) to monitor the tumor location in real-time. Image quality for each patient and treatment field depends on several factors including the patient anatomy and the gantry and couch angles. The authors investigated the angular dependence of automatic tumor localization during non-coplanar lung SBRT delivery. All images were acquired at a frame rate of 12 Hz with an amorphous silicon EPID. A previously validated markerless lung tumor localization algorithm was employed with manual localization as the reference. From ten SBRT patients, 12 987 image frames of 123 image sequences acquired at 48 different gantry-couch rotations were analyzed. δ was defined by the position difference of the automatic and manual localization. Regardless of the couch angle, the best tracking performance was found in image sequences with a gantry angle within 20° of 250° (δ = 1.40 mm). Image sequences acquired with gantry angles of 150°, 210°, and 350° also led to good tracking performances with δ = 1.77-2.00 mm. Overall, the couch angle was not correlated with the tracking results. Among all the gantry-couch combinations, image sequences acquired at (θ = 30°, ϕ = 330°), (θ = 210°, ϕ = 10°), and (θ = 250°, ϕ = 30°) led to the best tracking results with δ = 1.19-1.82 mm. The worst performing combinations were (θ = 90° and 230°, ϕ = 10°) and (θ = 270°, ϕ = 30°) with δ > 3.5 mm. However, 35% (17/48) of the gantry-couch rotations demonstrated substantial variability in tracking performances between patients. For example, the field angle (θ = 70°, ϕ = 10°) was acquired for five patients. While the tracking errors were ≤1.98 mm for three patients, poor performance was found for the other two patients with δ ≥ 2.18 mm, leading to average tracking error of 2.70 mm. Only one image sequence was acquired for all other gantry-couch rotations (δ = 1.18-10.29 mm). Non-coplanar beams with gantry-couch rotation of (θ = 30°, ϕ = 330°), (θ = 210°, ϕ = 10°), and (θ = 250°, ϕ = 30°) have the highest accuracy for BEV lung tumor localization. Additionally, gantry angles of 150°, 210°, 250°, and 350° also offer good tracking performance. The beam geometries (θ = 90° and 230°, ϕ = 10°) and (θ = 270°, ϕ = 30°) are associated with substantial automatic localization errors. Overall, lung tumor visibility and tracking performance were patient dependent for a substantial number of the gantry-couch angle combinations studied.

Overview of 3-year experience with large-scale electronic portal imaging device–based 3-dimensional transit dosimetry

PurposeTo assess the usefulness of electronic portal imaging device (EPID)-based 3-dimensional (3D) transit dosimetry in a radiation therapy department by analyzing a large set of dose verification results. Methods and materialsIn our institution, routine in vivo dose verification of all treatments is performed by means of 3D transit dosimetry using amorphous silicon EPIDs. The total 3D dose distribution is reconstructed using a back-projection algorithm and compared with the planned dose distribution using 3D gamma evaluation. Dose reconstruction and gamma evaluation software runs automatically in our clinic, and analysis results are (almost) immediately available. If a deviation exceeds our alert criteria, manual inspection is required. If necessary, additional phantom measurements are performed to separate patient-related errors from planning or delivery errors. Three-dimensional transit dosimetry results were analyzed per treatment site between 2012 and 2014 and the origin of the deviations was assessed. ResultsIn total, 4689 of 15,076 plans (31%) exceeded the alert criteria between 2012 and 2014. These alerts were patient-related and attributable to limitations of our back-projection and dose calculation algorithm or to external sources. Clinically relevant deviations were detected for approximately 1 of 430 patient treatments. Most of these errors were because of anatomical changes or deviations from the routine clinical procedure and would not have been detected by pretreatment verification. Although cone beam computed tomography scans yielded information about anatomical changes, their effect on the dose delivery was assessed quantitatively by means of 3D in vivo dosimetry. ConclusionsEPID-based transit dosimetry is a fast and efficient dose verification technique. It provides more useful information and is less time-consuming than pretreatment verification measurements of intensity modulated radiation therapy and volumetric modulated arc therapy. Large-scale implementation of 3D transit dosimetry is therefore a powerful method to guarantee safe dose delivery during radiation therapy.

Time dependent pre-treatment EPID dosimetry for standard and FFF VMAT

Methods to calibrate Megavoltage electronic portal imaging devices (EPIDs) for dosimetry have been previously documented for dynamic treatments such as intensity modulated radiotherapy (IMRT) using flattened beams and typically using integrated fields. While these methods verify the accumulated field shape and dose, the dose rate and differential fields remain unverified. The aim of this work is to provide an accurate calibration model for time dependent pre-treatment dose verification using amorphous silicon (a-Si) EPIDs in volumetric modulated arc therapy (VMAT) for both flattened and flattening filter free (FFF) beams.A general calibration model was created using a Varian TrueBeam accelerator, equipped with an aS1000 EPID, for each photon spectrum 6 MV, 10 MV, 6 MV-FFF, 10 MV-FFF. As planned VMAT treatments use control points (CPs) for optimization, measured images are separated into corresponding time intervals for direct comparison with predictions. The accuracy of the calibration model was determined for a range of treatment conditions. Measured and predicted CP dose images were compared using a time dependent gamma evaluation using criteria (3%, 3 mm, 0.5 sec).Time dependent pre-treatment dose verification is possible without an additional measurement device or phantom, using the on-board EPID. Sufficient data is present in trajectory log files and EPID frame headers to reliably synchronize and resample portal images. For the VMAT plans tested, significantly more deviation is observed when analysed in a time dependent manner for FFF and non-FFF plans than when analysed using only the integrated field.We show EPID-based pre-treatment dose verification can be performed on a CP basis for VMAT plans. This model can measure pre-treatment doses for both flattened and unflattened beams in a time dependent manner which highlights deviations that are missed in integrated field verifications.

Transit dosimetry in dynamic IMRT with an a-Si EPID.

Using an amorphous silicon (a-Si) EPID for transit dosimetry requires detailed characterization of its dosimetric response in a variety of conditions. In this study, a measurement-based model was developed to calibrate an a-Si EPID response to dose for transit dosimetry by comparison with a reference ionization chamber. The ionization chamber reference depth and the required additional buildup thickness for electronic portal imaging devices (EPID) transit dosimetry were determined. The combined effects of changes in radiation field size, phantom thickness, and the off-axis distance on EPID transit dosimetry were characterized. The effect of scattered radiation on out-of-field response was investigated for different field sizes and phantom thicknesses by evaluation of the differences in image profiles and in-water measured profiles. An algorithm was developed to automatically apply these corrections to EPID images based on the user-specified field size and phantom thickness. The average phantom thickness and an effective field size were used for IMRT fields, and images were acquired in cine mode in the presence of an anthropomorphic phantom. The effective field size was defined as the percentage of the jaw-defined field that was involved during the delivery. Nine head and neck dynamic IMRT fields were tested by comparison with a MatriXX two-dimensional array dosimeter using the Gamma (3%, 3 mm) evaluation. A depth of 1.5 cm was selected as the ionization chamber reference depth. An additional 2.2 mm of copper buildup was added to the EPID. Comparison of EPID and MatriXX dose images for the tested fields showed that using a 10% threshold, the average number of points with Gamma index <1 was 96.5%. The agreement in the out-of field area was shown by selection of a 2% threshold which on average resulted in 94.8% of points with a Gamma index <1. The suggested method is less complicated than previously reported techniques and can be used for all a-Si EPIDs regardless of the manufacturer.

Evaluation of MegaVoltage Cone Beam CT image quality with an unmodified Elekta Precise Linac and EPID: a feasibility study

In order to increase the accuracy of patient positioning for complex radiotherapy treatments various 3D imaging techniques have been developed. MegaVoltage Cone Beam CT (MVCBCT) can utilise existing hardware to implement a 3D imaging modality to aid patient positioning. MVCBCT has been investigated using an unmodified Elekta Precise Linac and iView amorphous silicon electronic portal imaging device (EPID). Two methods of delivery and acquisition have been investigated for imaging an anthropomorphic head phantom and quality assurance phantom. Phantom projections were successfully acquired and CT datasets reconstructed using both acquisition methods. Bone, tissue and air were clearly resolvable in both phantoms even with low dose (22 MU) scans. The feasibility of MVCBCT was investigated using a standard linac, amorphous silicon EPID and a combination of a free open source reconstruction toolkit as well as custom in-house software written in Matlab. The resultant image quality has been assessed and presented. Although bone, tissue and air were resolvable in all scans, artifacts are present and scan doses are increased when compared with standard portal imaging. The feasibility of MVCBCT with unmodified Elekta Precise Linac and EPID has been considered as well as the identification of possible areas for future development in artifact correction techniques to further improve image quality.

2D lag and signal nonlinearity correction in an amorphous silicon EPID and their impact on pretreatment dosimetric verification

This investigation provides measurements of signal lag and nonlinearity separately for the Varian aS500 electronic portal imaging device (EPID), and an algorithm to correct for these effects in 2D; their potential impact on intensity modulated radiation therapy (IMRT) verification is also investigated. The authors quantify lag, as a function of both delivered monitor units (MU) and time, by using a range of MUs delivered at a clinically used rate of 400 MU∕min. Explicit cumulative lag curves are thus determined for a range of MUs and times between the end of irradiation and the end of image acquisition. Signal nonlinearity is also investigated as a function of total MUs delivered. The family of cumulative lag curves and signal nonlinearity are then used to determine their effects on dynamic multileaf collimator (MLC) (IMRT) deliveries, and to correct for theses effects in 2D. Images acquired with an aS500 EPID and Varis Portal-Vision software were used to quantify detector lag and signal-nonlinearity. For the signal lag investigation, Portal-Vision's service monitor was used to acquire EPID images at a rate of 8 frames/s. The images were acquired during irradiation and 66 s thereafter, by inhibiting the M-holdoff-In signal of the Linac for a range of 4.5-198.5 MUs. Relative cumulative lag was calculated by integrating the EPID signal for a time after beam-off, and normalizing this to the integrated EPID signal accumulated during radiation. Signal nonlinearity was studied by acquiring 10 × 10 cm(2) open-field EPID images in "integrated image" mode for a range of 2-500 MUs, and normalized to the 100 MU case. All data were incorporated into in-house written software to create a 2D correction map for these effects, using the field's MLC file and a field-specific calculated 2D "time-map," which keeps track of the time elapsed from the last fluence delivered at each given point in the image to the end of the beam delivery. Relative cumulative lag curves reveal that the lag alone can deviate the EPID's perceived dose by as large as 6% (1 MU delivery, 60 s postirradiation). For signal nonlinearity relative to 100 MU, EPID signals per MU of 0.84 and 1.01 were observed for 2 and 500 MUs, respectively. Correction maps were applied to a 1 cm sweeping-window 14 × 14 cm(2) field and clinical head-and-neck IMRT field. A mean correction of 1.028 was implemented in the head-and-neck field, which significantly reduced lag-related asymmetries in the EPID images, and restored linearity to the EPID imager's dose response. Corrections made to the sweeping-field showed good agreement with the treatment planning system-predicted field, yielding an average percent difference of 0.05% ± 0.91%, compared to the -1.32% ± 1.02% before corrections, or 1.75% ± 1.04% when only a signal nonlinearity correction is made. Lag and signal-nonlinearity have been quantified for an aS500 EPID imager, and an effective 2D correction method has been developed which effectively removes nonlinearity and lag effects. Both of these effects were shown to negatively impact IMRT verifications. Especially fields that involve prolonged irradiation and small overall MUs should be corrected for in 2D.

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.

Accurately simulating the production of radiotherapy portal images using non-zero beam angles

In this study, the delivery and portal imaging of one square-field and one conformal radiotherapy treatment was simulated using the Monte Carlo codes BEAMnrc and DOSXYZnrc. The treatment fields were delivered to a humanoid phantom from different angles by a 6 MV photon beam linear accelerator, with an amorphous-silicon electronic portal imaging device (a-Si EPID) used to provide images of the phantom generated by each field. The virtual phantom preparation code CTCombine was used to combine a computed-tomography-derived model of the irradiated phantom with a simple, rectilinear model of the a-Si EPID, at each beam angle used in the treatment. Comparison of the resulting experimental and simulated a-Si EPID images showed good agreement, within \[gamma](3%, 3 mm), indicating that this method may be useful in providing accurate Monte Carlo predictions of clinical a-Si EPID images, for use in the verification of complex radiotherapy treatments.

Simplifying EPID dosimetry for IMRT treatment verification

Electronic portal imaging devices (EPIDs) are increasingly used for IMRT dose verification, both pretreatment and in vivo. In this study, an earlier developed backprojection model has been modified to avoid the need for patient-specific transmission measurements and, consequently, leads to a faster procedure. Currently, the transmission, an essential ingredient of the backprojection model, is estimated from the ratio of EPID measurements with and without a phantom/patient in the beam. Thus, an additional irradiation to obtain "open images" under the same conditions as the actual phantom/patient irradiation is required. However, by calculating the transmission of the phantom/patient in the direction of the beam instead of using open images, this extra measurement can be avoided. This was achieved by using a model that includes the effect of beam hardening and off-axis dependence of the EPID response on photon beam spectral changes. The parameters in the model were empirically obtained by performing EPID measurements using polystyrene slab phantoms of different thickness in 6, 10, and 18 MV photon beams. A theoretical analysis to verify the sensitivity of the model with patient thickness changes was performed. The new model was finally applied for the analysis of EPID dose verification measurements of step-and-shoot IMRT treatments of head and neck, lung, breast, cervix, prostate, and rectum patients. All measurements were carried out using Elekta SL20i linear accelerators equipped with a hydrogenated amorphous silicon EPID, and the IMRT plans were made using PINNACLE software (Philips Medical Systems). The results showed generally good agreement with the dose determined using the old model applying the measured transmission. The average differences between EPID-based in vivo dose at the isocenter determined using either the new model for transmission and its measured value were 2.6 +/- 3.1%, 0.2 +/- 3.1%, and 2.2 +/- 3.9% for 47 patients treated with 6, 10, and 18 MV IMRT beams, respectively. For the same group of patients, the differences in mean gamma analysis (3% maximum dose, 3 mm) were 0.16 +/- 0.26%, 0.21 +/- 0.24%, and 0.02 +/- 0.12%, respectively. For a subgroup of 11 patients, pretreatment verification was also performed, showing similar dose differences at the isocenter: -1.9 +/- 0.9%, -1.4 +/- 1.2%, and -0.4 +/- 2.4%, with somewhat lower mean gamma difference values: 0.01 +/- 0.09%, 0.01 +/- 0.07%, and -0.09 +/- 0.10%, respectively. Clinical implementation of the new model would save 450 h/yr spent in measurement of open images. It can be concluded that calculating instead of measuring the transmission leads to differences in the isocenter dose generally smaller than 2% (2.6% for 6 MV photon beams for in vivo dose) and yielded only slightly higher gamma-evaluation parameter values in planes through the isocenter. Hence, the new model is suitable for clinical implementation and measurement of open images can be omitted.

Evaluation of an a-Si EPID in direct detection configuration as a water-equivalent dosimeter for transit dosimetry

A major problem associated with amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) for transit dosimetry is the presence of a phosphor layer, which can introduce large deviations from water-equivalent behavior due to energy-dependent response and visible light scattering. In this study, an amorphous silicon EPID was modified to a direct detection configuration by removing the phosphor layer, and the accuracy of using it for transit dosimetry measurements was investigated for 6 and 18 MV treatment beams by comparison to ion-chamber in water measurements. Solid water and copper were both evaluated as buildup materials. Using the optimum buildup thickness in each case, effects of changes in radiation field size, source to detector distance, and patient/phantom thickness were investigated by comparison to reference measurements made by an ionization chamber on the central axis. The off-axis response of the imager was also investigated by comparison of EPID image profiles to dose profiles obtained by a scanning ionization chamber in a water tank with various thicknesses of slab phantoms, and an anthropomorphic phantom in the beam using Gamma evaluation (3%, 3 mm criteria). The imaging characteristics of the direct EPID were investigated by comparison to a commercial EPID using QC3V phantom, and by taking images of an anthropomorphic pelvic phantom containing fiducial gold markers. Either 30 mm of solid water or 3.3 mm of copper were found to be the most suitable buildup thicknesses with solid water providing more accurate results. Using solid water buildup, the EPID response compared to the reference dosimeter within 2% for all conditions except phantom thicknesses larger than 25 cm in 6 MV beams, which was up to 6.5%. Gamma evaluation results comparing EPID profiles and reference ionization chamber profiles showed that for 6 and 18 MV beams, at least 91.8% and 90.9% of points had a Gamma <1 for all phantoms, respectively. But using copper buildup, the EPID response had more discrepancies from the ionization chamber reference measurements, including: More than 2% difference for small air gaps using 6 MV beams, up to 8% difference for phantom thicknesses larger than 25 cm in 6 MV beams, and large differences (up to 9.3%) for increasing phantom thicknesses in 18 MV beams. The percentage of points with Gamma <1 with copper buildup were at least 96.6% and 99.8% in 6 and 18 MV beams, respectively. The direct EPID performs as an ion-chamber detector for transit dosimetry applications in all geometries studied except for small discrepancies at 6 MV for thick phantoms. This can be ameliorated by the calibration of the EPID to dose at an intermediate phantom thickness. The major current limitation of the direct EPID is poor quality of images compared with the clinical configuration, which could be overcome by a method to interchange between imaging and dosimetry setups.

Dosimetric properties of an amorphous‐silicon EPID used in continuous acquisition mode for application to dynamic and arc IMRT

Dosimetric properties of an amorphous-silicon electronic portal imaging device (EPID) operated in a real-time acquisition mode were investigated. This mode will be essential for time-resolved dose verification of dynamic (sliding window) intensity modulated radiation therapy (IMRT) and intensity modulated arc radiation therapy (arc-IMRT). The EPID was used in continuous acquisition mode (i.e., "cine" mode) where individual sequential image frames are acquired in real time. The properties studied include dose linearity, reproducibility of response, and image stability. Results of using the continuous acquisition mode with several example treatments including dynamic IMRT, arc treatment, and single-arc-IMRT are compared to results using the well-studied integrated acquisition mode (i.e., "frame averaging" or "IMRT" mode). Real-time EPID response was also compared to real-time ion-chamber data for selected points in the deliveries. The example treatment deliveries in both continuous and integrated acquisition modes were converted to arbitrary EPID dose units via a calibration field. The summation of all acquired continuous mode images was compared using percentage dose difference to the single image acquired in the integrated mode using in-field pixels only (defined as those pixels > 10% of maximum, in-field signal). Using the continuous acquisition mode, the EPID response was not linear with dose. It was found that the continuous mode dose response corresponded approximately to dropping one image per acquisition session. Reproducibility of EPID response to low monitor units (MUs) was found to be poor but greatly improved with increasing MU. Open field profiles were found to be stable in the cross-plane direction but required several frames to become stable in the in-plane direction. However, both of these issues are clinically insignificant due to arc-IMRT deliveries requiring relatively large monitor units (> 100 MU). Analysis of the five IMRT, arc, and arc-IMRT tests revealed that all examples compared to within 2% of maximum dose for more than 95% of in-field pixels. The continuous acquisition mode is suited to time-resolved dosimetry applications including arc-IMRT and dynamic IMRT, giving comparable dose results to the well-studied integrated acquisition mode, although caution should be used in low MU applications. Time-resolved EPID dose information also compared well to time-resolved ion-chamber measurements.

USE OF ELECTRONIC PORTAL IMAGE DEVICE IN SOME QA TESTS

Introduction: In the present study a method to investigate light and radiation field congruence utilizing an amorphoussilicon electronic portal imaging device (EPID) was developed. An alternative method for routine dynamic multi-leafcollimator (DMLC) quality assurance (QA) using EPID was also described.Materials and Methods: This study employed an amorphous silicon (aSi) EPID, the associated EPI software and adiamond-shaped template. The collimator jaws of the linac (Elekta iViewGT) were aligned such that the light field fell directly on the corners of the diamond. A radiation detection algorithm within the EPI software determined the extent of the radiation field. The light and radiation field congruence was evaluated by comparing the vertices of the diamond reference structure to the detected radiation field. In addition, the digital jaw settings were recorded and later compared to the light field detected on Kodak x-omat films and EPIs. In this study an alternative method for routine dynamic multi-leaf collimator (DMLC) quality assurance (QA) using (EPID) was described. This QA was done by producing apattern of five 0.5-cm bands 5-cm apart and performing a visual spot-check for dose peak location, dose peak values,interband distance and band gaps measured at Full Width at Half-Maximum (FWHM) of the test pattern for this QA experiment. An aSi EPID and films are used for the DMLC QA to test the practicality and efficacy of EPID vis film.Results: Light radiation field congruence tests with films and EPIs were comparable, yielding a difference of less than 1.2 mm, well within the allowed 2-mm tolerance. A disparity was observed in the magnitude of the detected light field.The X and Y dimensions of the light field measured with film differed by less than or equal to 1.6 mm from the digital collimator settings, whereas the values extracted from the EPIs differed by up to 1.8 mm. The FWHM of the multi-leaf collimator (MLC) obtained by the EPID is 6.34 mm, which is slightly higher than the 6.18 mm for the film. The lowest dose at the valley is significantly lower for the EPID than for the film. The dose peak value of the EPID is found to be97.9 with standard deviation of ±1.59 and the dose peak value of film is found to be 96.5 with standard deviation of±2.41, respectively. Film and EPID track the distance between peaks with a mean of 49.8 and 49.53 mm and standarddeviation of ±0.3 and ±1.02 mm, respectively. EPID is able to track the location of individual dose peaks for all the leafpairs consistently to a standard deviation of ±0.12 to ±0.47 mm, slightly better than that of ±1.00 to ±1.26 mm for thefilm.Conclusion: EPIs were found to be a quick and reliable alternative to film for qualitative and relative analyses. AlsoEPID is as good as the film in resolving the band pattern of the DMLC test field. Although the resolution of the EPIDis lower than that of the film, it is high enough to faithfully reproduce the band pattern without significant distortion.Since EPID images can be acquired, analyzed and stored much more conveniently than film, EPID is a good alternativeto film for routine DMLC QA.

Quality Control of Portal Imaging with PTW EPID QC PHANTOM®

Quality assurance (QA) and quality control (QC) of different electronic portal imaging devices (EPID) and portal images with the PTW EPID QC PHANTOM. Characteristic properties of images of different file formats were measured on Siemens OptiVue500aSi, Siemens BeamView Plus, Elekta iView, and Varian PortalVision and analyzed with the epidSoft 2.0 program in four radiation therapy centers. The portal images were taken with Kodak X-OMAT V and the Kodak Portal Localisation ReadyPack films and evaluated with the same program. The optimal exposition both for EPIDs and portal films of different kind was determined. For double exposition, the 2+1 MU values can be recommended in the case of Siemens OptiVue500aSi Elekta iView and Kodak Portal Localisation ReadyPack films, while for Siemens BeamView Plus, Varian PortalVision and Kodak X-OMAT V film 7+7 MU is recommended. The PTW EPID QC PHANTOM can be used not only for amorphous silicon EPIDs but also for images taken with a video-based system or by using an ionization chamber matrix or for portal film. For analysis of QC tests, a standardized format (used at the acceptance test) should be applied, as the results are dependent on the file format used.

Analysis of the measurement precision of an amorphous silicon EPID used for MLC leaf position quality control and the long-term calibration stability of an optically controlled MLC

Electronic portal imaging devices (EPIDs) have been shown to be suitable for multileaf collimator (MLC) leaf positioning quality control (QC). In our centre, a continuous dataset is available of 2 years of film measurements followed by 3 years of EPID measurements on five MLC-equipped linear accelerators of identical head design. The aim of this work was to analyse this unique dataset in order to determine the relative precision of film and EPID for MLC leaf positioning measurements and to determine the long-term stability of the MLC calibration. The QC dataset was examined and periods without MLC adjustments that contained at least four successive collimator position measurements (a minimum of 6 months) were identified. By calculating the standard deviations (SD) of these results, the reproducibility of the measurements can be determined. Comparison of the film and EPID results enables their relative measurement precision to be assessed; on average film gave an SD of 0.52 mm compared to 0.13 mm for EPIDs. The MLC and conventional collimator results were compared to assess MLC calibration stability; on average, for EPID measurements, the MLC gave an SD of 0.12 mm compared to 0.14 mm for a conventional collimator. The long-term relative individual leaf positions were compared and found to vary between 0.07 and 0.15 mm implying that they are stable over long time periods. These results suggest that the calibration of an optically controlled MLC is inherently very stable between disturbances to the optical system which normally occur on service days.