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

Abstract

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 10x10 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 (5x5 to 20x20 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 10x10 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.

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

“…The assumption behind this procedure is that each pixel is being exposed to the same radiation intensity. As a consequence, the non-flat beam profile of a clinical linear accelerator introduces some problems in typical flood-field calibrations [14]. Chief among these is that the beam profile is removed from all EPID images.…”

“…The floodfield calibration image could be acquired with a flattened beam if a solid water slab was placed in the beam [13,16]. Alternatively, acquiring a series of images with the imager moved to different locations in the beam would allow the beam profile to be separated from the pixel response [14,17]. Monte Carlo simulations of the EPID response can also be used for this purpose [16,18].…”

Sensitivity of an Elekta iView GT a-Si EPID model to delivery errors for pre-treatment verification of IMRT fields

“…A pixel-by-pixel sensitivity correction was performed on all EPID images. The pixel sensitivity map was obtained using a method that involved measuring a series of overlapping 10 9 10 cm 2 fields (considered to be uniform) [17,18]. During the course of the work a routine MLC calibration was performed.…”

Experimental validation of a MCNP-based model to predict portal dose images from MLC beams

“…In this study it has been fixed a 100 MU dose rate, nevertheless, as the response of the EPID is dose rate-dependent, alternatives calibration matrix (using the same methodology) should be perform to each dose rate settings [8].…”