Correction of pixel sensitivity variation and off‐axis response for amorphous silicon EPID dosimetry

Abstract

The aim of this work is to determine the pixel sensitivity variation and off-axis dose response of an amorphous silicon electronic portal imaging device (EPID), and develop a correction method to improve EPID dosimetry. The uncorrected or raw pixel response of the aS500 amorphous silicon EPID shows differences in response (sensitivity) of individual pixels as well as a large off-axis differential response with respect to an ion chamber in water. Both can be corrected by division of raw images by the flood-field (FF) image. However, this leads to two problems for dosimetry: (1) the beam profile is present in both the raw image and FF image, and hence is "washed out" of the corrected image, and (2) any mismatch of EPID position between dosimetry and FF calibration means that the beam profile and off-axis response in the raw image and FF are misaligned. This causes artifacts in FF division and dosimetric errors. A method was developed to measure the off-axis response and pixel sensitivity variation separately to allow correction of images at any EPID position while retaining beam profile information. The pixel sensitivity variation is applied to the imager plane and is independent of imager position. The off-axis response depends on the imager plane position relative to the beam central axis. The pixel sensitivities were derived from multiple images of the same symmetric field acquired with the detector displaced laterally between each image. The off-axis response was measured by acquiring off-axis raw images (FF correction removed) and dividing out the off-axis beam fluence and previously determined pixel sensitivity differences. The dosimetric errors due to lateral and vertical detector displacement with the conventional FF calibration method were measured and compared to the new method. Corrected EPID profiles were then compared to beam profiles measured with ion chamber in water for open fields. The EPID was found to have a large off-axis differential response with respect to an ion chamber in water, particularly for 6 MV. This increased to 13% at 15 cm off-axis for 6 MV, and 3.5% for 18 MV at the isocenter plane. The dosimetric errors introduced by detector displacement with conventional FF calibration were found to be approximately 1% per centimeter of lateral detector displacement and 0.1% per centimeter of vertical displacement. These were reduced to less than 1% for any position with the new correction method. Corrected EPID images agreed with ion-chamber measurements to within 2% (excluding penumbra and low-dose areas outside the field) for various field sizes. The new correction method gives consistent dosimetry for any EPID position and retains beam profile information in the image.

A proposed method for linear accelerator photon beam steering usingEPID

“…The concept of imaging with EPID displacement forms the basis of the PSM method of Greer 12. The concept is that the PSM can be isolated from the Beam Response by taking a series of images where the beam is kept constant (e.g., 5 × 5 cm 2 field at central axis), but imaged with different parts of the EPID panel and hence with different PSM.…”

“…We also further evaluate Chojnowski's method against an IC Profiler method. Thirdly, the work of Greer et al 12. provides the base theory for determining the PSM (i.e., moving the EPID panel); however, a simplified version of Greer's method is used in this study, which is appropriate to the application and makes the method more easily adoptable.…”

Evaluation of the TrueBeam machine performance check (MPC) beam constancy checks for flattened and flattening filter‐free (FFF) photon beams

“…A disadvantage of EPID as a detector for linac QA is the removal of the incident beam profile characteristics by the flood field correction. In addition, the high atomic number of the phosphor results in changed scattering properties compared to the equivalent depth in water14, 17 and also introduces an energy‐dependent response 13, 18, 19. Backscatter from the EPID positioning arm has also been a source of image nonuniformity 20.…”

Comparison between an in‐house 1D profile correction method and a 2D correction provided in Varian's PDPC Package for improving the accuracy of portal dosimetry images

“…The dark field calibration provides a correction for background signal in the EPID by collecting image frames without any beam on, and the food field calibration uses an open beam to provide a sensitivity correction for the EPID. Greer (27) provide a detailed explanation as to how the Varian software applies these two corrections to a raw image. The standard method provided by Varian used a food field size of $40\times 30\hspace{0.17em}{\text{cm}}^2$.…”