Flood-field Calibration Research Articles

13. Assessing long-term stability of Electronic Portal Image Device-based dosimetry

Introduction Over the past years, EPID has demonstrated to be an available easy-to-use device revealing a high spatial resolution for dosimetric applications such as both pretreatment controls in open conditions and in vivo dosimetry in transit conditions. Under normal functioning of beam delivery, the relevance of EPID-based dose measurements is inherently related to the quality and reproducibility of the portal image acquisitions which must be periodically controlled. The aim of this study is to investigate the long-term stability of the EPID, in common situation of quality control and maintenance of the device (periodic dark field and flood field calibrations for linearity and differential pixel sensibility respectively). Methods We studied a pool of 44 iViewGT EPID images (Elekta) acquired weekly between May 2016 and April 2017. EPID was irradiated with a 6MV beam (Elekta Synergy), 100 Monitor Units with a large aperture (240 × 240 mm at Source Axis Distance) to cover the panel detector at a Source Imager Distance of 1600 mm. The EPID images were converted to 2D dose-in-water distributions at isocentre level under 50 mm of water using ThinkQA/EpiBeam solution (Dosisoft S.A.). This system corrects well-known EPID image biases, especially using an ideal 2D beam map restoration and an explicit dose calibration factor. All dose images were compared to the first one, taken as the reference, and to the ThinkQA-EpiBeam 2D dose prediction (based on the planned dicom RT-Plan). That way, it was possible to quantify the dose image fluctuations in terms of relative and absolute dose pixel and of image uniformity. Results The EPID dose changes are within the normal tolerances of beam functioning (about 2% asymmetry and output variations). Nonetheless, the whole dose comparisons showed two major discrepancies with the initial reference, requiring special interpretations. The first dose deviation corresponds to a drift of the monitor unit chamber response of the treatment unit which has been corrected. The second one corresponds to a maintenance act concerning the EPID, requiring a new Flood Field and Dark Field calibration. That consequently induced a change in grey levels and a major variation of sensitivity of the detector (−40% for the central pixel values). Conclusions The overall long term stability of the EPID has been confirmed, making possible with confidence its use for dosimetric applications, providing quality controls are periodically performed. Manufacturer maintenance must be taken into account through the introduction of a specific control of the dose calibration factor.

An in‐house protocol for improved flood field calibration of TrueBeam FFF cine imaging

PurposeTrueBeams equipped with the 40 × 30 cm2 Electronic Portal Imaging Devices (EPIDs) are prone to image saturation at the image center when used with flattening filter free (FFF) photon energies. While cine imaging during treatment may not saturate because the beam is attenuated by the patient, the flood field calibration is affected when the standard calibration procedure is followed. Here, we describe the hardware and protocol to achieve improved image quality for this model of TrueBeam EPID.Materials & methodsA stainless steel filter of uniform thickness was designed to have sufficient attenuation to avoid panel saturation. The cine imaging flood field calibration was acquired with the filter in place for the FFF energies under the standard calibration geometry (SID = 150 cm). Image quality during MV cine was assessed with & without the modified flood field calibration using a low contrast resolution phantom and an anthropomorphic phantom.ResultsWhen the flood field is acquired without the filter in place, a pixel gain artifact is clearly present in the image center which may be mis‐attributed to panel saturation in the subject image. At the image center, the artifact obscured all low contrast inserts and was also visible on the anthropomorphic phantom. Using the filter for flood field calibration eliminates the artifact.ConclusionTrueBeams equipped with the 40 × 30 cm2 IDU can utilize a modified flood field calibration procedure for FFF photon energies that improves image quality for cine MV imaging.

SU‐C‐202‐07: Protocol and Hardware for Improved Flood Field Calibration of TrueBeam FFF Cine Imaging

Purpose:Flattening filter free photon energies are commonly used for high dose treatments such as SBRT, where localization accuracy is essential. Often, MV cine imaging may be employed to verify correct localization. TrueBeam Electronic Portal Imaging Devices (EPIDs) equipped with the 40×30cm2 Image Detection Unit (IDU) are prone to image saturation at the image center especially for higher dose rates. While saturation often does not occur for cine imaging during treatment because the beam is attenuated by the patient, the flood field calibration is affected when the standard calibration procedure is followed. Here we describe the hardware and protocol to achieve improved image quality for this model of TrueBeam EPID.Methods:A stainless steel filter of uniform thickness was designed to have sufficient attenuation to avoid panel saturation for both 6XFFF and 10XFFF at the maximum dose rates (1400 MU/min & 2400 MU/min, respectively). The cine imaging flood field calibration was then acquired with the filter in place for the FFF energies under the standard calibration geometry (SDD=150cm). Image quality during MV cine was assessed with & without the modified flood field calibration using a low contrast resolution phantom and an anthropomorphic phantom.Results:When the flood field is acquired using the standard procedure (no filter in place), a pixel gain artifact is clearly present in the image center (r=3cm for 10XFFF at 2400 MU/min) which appears similar to and may be mis‐attributed to panel saturation in the subject image. The artifact obscured all low contrast inserts at the image center and was also visible on the anthropomorphic phantom. Using the filter for flood field calibration eliminated the artifact.Conclusion:Use of a modified flood field calibration procedure improves image quality for cine MV imaging with TrueBeams equipped with the 40×30cm2 IDU.

Use of electronic portal imaging devices for electron treatment verification

This study aims to help broaden the use of electronic portal imaging devices (EPIDs) for pre-treatment patient positioning verification, from photon-beam radiotherapy to photon- and electron-beam radiotherapy, by proposing and testing a method for acquiring clinically-useful EPID images of patient anatomy using electron beams, with a view to enabling and encouraging further research in this area. EPID images used in this study were acquired using all available beams from a linac configured to deliver electron beams with nominal energies of 6, 9, 12, 16 and 20 MeV, as well as photon beams with nominal energies of 6 and 10 MV. A widely-available heterogeneous, approximately-humanoid, thorax phantom was used, to provide an indication of the contrast and noise produced when imaging different types of tissue with comparatively realistic thicknesses. The acquired images were automatically calibrated, corrected for the effects of variations in the sensitivity of individual photodiodes, using a flood field image. For electron beam imaging, flood field EPID calibration images were acquired with and without the placement of blocks of water-equivalent plastic (with thicknesses approximately equal to the practical range of electrons in the plastic) placed upstream of the EPID, to filter out the primary electron beam, leaving only the bremsstrahlung photon signal. While the electron beam images acquired using a standard (unfiltered) flood field calibration were observed to be noisy and difficult to interpret, the electron beam images acquired using the filtered flood field calibration showed tissues and bony anatomy with levels of contrast and noise that were similar to the contrast and noise levels seen in the clinically acceptable photon beam EPID images. The best electron beam imaging results (highest contrast, signal-to-noise and contrast-to-noise ratios) were achieved when the images were acquired using the higher energy electron beams (16 and 20 MeV) when the EPID was calibrated using an intermediate (12 MeV) electron beam energy. These results demonstrate the feasibility of acquiring clinically-useful EPID images of patient anatomy using electron beams and suggest important avenues for future investigation, thus enabling and encouraging further research in this area. There is manifest potential for the EPID imaging method proposed in this work to lead to the clinical use of electron beam imaging for geometric verification of electron treatments in the future.

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.

Dosimetric properties of an amorphous silicon EPID for verification of modulated electron radiotherapy

To investigate the dosimetric properties of an electronic portal imaging device (EPID) for electron beam detection and to evaluate its potential for quality assurance (QA) of modulated electron radiotherapy (MERT). A commercially available EPID was used to detect electron beams shaped by a photon multileaf collimator (MLC) at a source-surface distance of 70 cm. The fundamental dosimetric properties such as reproducibility, dose linearity, field size response, energy response, and saturation were investigated for electron beams. A new method to acquire the flood-field for the EPID calibration was tested. For validation purpose, profiles of open fields and various MLC fields (square and irregular) were measured with a diode in water and compared to the EPID measurements. Finally, in order to use the EPID for QA of MERT delivery, a method was developed to reconstruct EPID two-dimensional (2D) dose distributions in a water-equivalent depth of 1.5 cm. Comparisons were performed with film measurement for static and dynamic monoenergy fields as well as for multienergy fields composed by several segments of different electron energies. The advantageous EPID dosimetric properties already known for photons as reproducibility, linearity with dose, and dose rate were found to be identical for electron detection. The flood-field calibration method was proven to be effective and the EPID was capable to accurately reproduce the dose measured in water at 1.0 cm depth for 6 MeV, 1.3 cm for 9 MeV, and 1.5 cm for 12, 15, and 18 MeV. The deviations between the output factors measured with EPID and in water at these depths were within ±1.2% for all the energies with a mean deviation of 0.1%. The average gamma pass rate (criteria: 1.5%, 1.5 mm) for profile comparison between EPID and measurements in water was better than 99% for all the energies considered in this study. When comparing the reconstructed EPID 2D dose distributions at 1.5 cm depth to film measurements, the gamma pass rate (criteria: 2%, 2 mm) was better than 97% for all the tested cases. This study demonstrates the high potential of the EPID for electron dosimetry, and in particular, confirms the possibility to use it as an efficient verification tool for MERT delivery.

Long-term two-dimensional pixel stability of EPIDs used for regular linear accelerator quality assurance

The long-term stability of three clinical electronic portal imaging devices (EPIDs) was studied to determine if longer times between calibrations can be justified. This would make alternatives to flood-field calibration of EPIDs clinically feasible, allowing for more effective use of EPIDs for dosimetry. Images were acquired monthly for each EPID as part of regular clinical quality assurance over a time period of approximately 3 years. The images were analysed to determine (1) the long-term stability of the EPID positioning system, (2) the dose response of the central pixels and (3) the long term stability of each pixel in the imager. The position of the EPID was found to be very repeatable with variations less than 0.3 pixels (0.27 mm) for all imagers (1 standard deviation). The central axis dose response was found to reliably track ion chamber measurements to better than 0.5%. The mean variation in pixel response (1 standard deviation), averaged over all pixels in the EPID, was found to be at most 0.6% for the three EPIDs studied over the entire period. More than 99% of pixels in each EPID showed less than 1% variation. Since the EPID response was found to be very stable over long periods of time, an annual calibration should be sufficient in most cases. More complex dosimetric calibrations should be clinically feasible.

EPID dosimetry must soon become an essential component of IMRT quality assurance

EPID dosimetry must soon become an essential component of IMRT quality assurance

A simple approach to using an amorphous silicon EPID to verify IMRT planar dose maps

A simplified method of verifying intensity modulated radiation therapy (IMRT) fields using a Varian aS500 amorphous silicon electronic portal imaging device (EPID) is demonstrated. Unlike previous approaches, it does not involve time consuming or complicated analytical processing of the data. The central axis pixel response of the EPID, as well as the profile characteristics obtained from images acquired with a 6 MV photon beam, was examined as a function of field size. Ion chamber measurements at various depths in a water phantom were then collected and it was found that at a specific depth d(ref), the dose response and profile characteristics closely matched the results of the EPID analysis. The only manipulation required to be performed on the EPID images was the multiplication of a matrix of off axis ratio values to remove the effect of the flood field calibration. Similarly, d(ref) was found for 18 MV. Planar dose maps at d(ref) in a water phantom for a bar pattern, a strip pattern, and 14 clinical IMRT fields from two patient cases each being from a separate anatomical region, i.e., head and neck as well as the pelvis, for both energies were generated by the Pinnacle planning system (V7.4). EPID images of these fields were acquired and converted to planar dose maps and compared directly with the Pinnacle planar dose maps. Radiographic film dosimetry and a MapCHECK dosimetry device (Sun Nuclear Corporation, Melbourne, FL) were used as an independent verification of the dose distribution. Gamma analysis of the EPID, film, and Pinnacle planar dose maps generated for the clinical IMRT fields showed that approximately 97% of all points passed using a 3% dose/3 mm DTA tolerance test. Based on the range of fields studied, the author's results appear to justify using this approach as a method to verify dose distributions calculated on a treatment planning system, including complex intensity modulated fields.

SU-GG-T-215: Routine Linear Accelerator Electron Beam QA Using EPIDs

Purpose: To investigate the use of Electronic Portal Imaging Devices (EPIDs) for the routine quality assurance (QA) of linear accelerator (Linac) electron beams. Method and Materials: Routine electron beam QA is typically performed using radiographic film. The use of film for beam QA is time consuming and prone to many sources of error. EPIDs have become standard equipment on modern linacs, and their suitability for photon beam QA has been previously established. We have investigated the use of EPIDs for routing electron beam QA, including radiation field size, penumbra, flatness and symmetry. A Varian 2100iX with aSi imager was used. Service mode was used, as the linac does not support imaging with electron beams in clinical mode. Dark and flood field calibration of the EPID was obtained using the electron beams. EPID pixel response linearity with dose was verified for energies 6, 12, and 20 MeV with a 10×10 cm cone by exposing the imager to various MU. For radiation field size and beam flatness/symmetry QA, images were acquired using the 6×6 cm and 20×20 cm cones at energies of 6, 12 and 20 MeV. For comparison, Kodak XV films at a depth of 2 cm were exposed using the same configurations. The SDD for both film and EPID measurements was 105 cm. All films and images were analyzed using MATLAB code. Results: The EPID pixel response is linear with delivered dose. The radiation field size as measured by the EPID agreed with film to within 1.6 mm. Penumbra agreed to within 1.5 mm. It was not possible to obtain accurate flatness and symmetry results, but the EPID can be used for flatness and symmetry constancy. Conclusion: EPIDs can be used for the routine QA of electron beams. Radiation field size results are similar to those obtained with film.

Amorphous silicon EPID calibration for dosimetric applications: comparison of a method based on Monte Carlo prediction of response with existing techniques

For EPID dosimetry, the calibration should ensure that all pixels have a similar response to a given irradiation. A calibration method (MC), using an analytical fit of a Monte Carlo simulated flood field EPID image to correct for the flood field image pixel intensity shape, was proposed. It was compared with the standard flood field calibration (FF), with the use of a water slab placed in the beam to flatten the flood field (WS) and with a multiple field calibration where the EPID was irradiated with a fixed 10 × 10 field for 16 different positions (MF). The EPID was used in its normal configuration (clinical setup) and with an additional 3 mm copper slab (modified setup). Beam asymmetry measured with a diode array was taken into account in MC and WS methods. For both setups, the MC method provided pixel sensitivity values within 3% of those obtained with the MF and WS methods (mean difference <1%, standard deviation <2%). The difference of pixel sensitivity between MC and FF methods was up to 12.2% (clinical setup) and 11.8% (modified setup). MC calibration provided images of open fields (5 × 5 to 20 × 20 cm2) and IMRT fields to within 3% of that obtained with WS and MF calibrations while differences with images calibrated with the FF method for fields larger than 10 × 10 cm2 were up to 8%. MC, WS and MF methods all provided a major improvement on the FF method. Advantages and drawbacks of each method were reviewed.

Pretreatment verification of IMRT absolute dose distributions using a commercial EPID

A commercial amorphous silicon electronic portal imaging device (EPID) has been studied to investigate its potential in the field of pretreatment verifications of step and shoot, intensity modulated radiation therapy (IMRT), 6 MV photon beams. The EPID was calibrated to measure absolute exit dose in a water-equivalent phantom at patient level, following an experimental approach, which does not require sophisticated calculation algorithms. The procedure presented was specifically intended to replace the time-consuming in-phantom film dosimetry. The dosimetric response was characterized on the central axis in terms of stability, linearity, and pulse repetition frequency dependence. The a-Si EPID demonstrated a good linearity with dose (within 2% from 1 monitor unit), which represent a prerequisite for the application in IMRT. A series of measurements, in which phantom thickness, air gap between the phantom and the EPID, field size and position of measurement of dose in the phantom (entrance or exit) varied, was performed to find the optimal calibration conditions, for which the field size dependence is minimized. In these conditions (20 cm phantom thickness, 56 cm air gap, exit dose measured at the isocenter), the introduction of a filter for the low-energy scattered radiation allowed us to define a universal calibration factor, independent of field size. The off-axis extension of the dose calibration was performed by applying a radial correction for the beam profile, distorted due to the standard flood field calibration of the device. For the acquisition of IMRT fields, it was necessary to employ home-made software and a specific procedure. This method was applied for the measurement of the dose distributions for 15 clinical IMRT fields. The agreement between the dose distributions, quantified by the gamma index, was found, on average, in 97.6% and 98.3% of the analyzed points for EPID versus TPS and for EPID versus FILM, respectively, thus suggesting a great potential of this EPID for IMRT dosimetric applications.