Monte Carlo modelling of a-Si EPID response: The effect of spectral variations with field size and position
Parent, L, Seco, J, Evans, PM, Fielding, A and Dance, DR (2006) Monte Carlo modelling of a-Si EPID response: The effect of spectral variations with field size and position MEDICAL PHYSICS, 33 (12). 4527 - 4540. ISSN 0094-2405
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Electronic portal imaging detectors (EPID) have initially been developed for imaging purposes but they also present a great potential for dosimetry. This is of special interest for intensity modulated radiation treatment (IMRT), where the complexity of the delivery makes quality assurance necessary. By comparing a predicted EPID image of an IMRT field with a measured image, it is possible to verify that the beam is properly delivered by the linear accelerator and that the dose is delivered to the correct location in the patient. This study focused on predicting the EPID image of IMRT fields in air with Monte Carlo methods. As IMRT treatments consist of a series of segments of various sizes which are not always delivered on the central axis, large spectral variations may be observed between the segments. The effect of these spectral variations on the EPID response was studied. A detailed description of the EPID was implemented in a Monte Carlo model. The EPID model was validated by comparing the EPID output factors for field sizes between 2x2 and 26x26 cm at the isocentre. The Monte Carlo simulations agreed with the measurements to within 0.5%. The effect of 1spectral variations on the EPID response, with field size and position, was studied for three field sizes (2x2, 6x6 and 10x10 cm at the isocentre) with various offsets (from 10 to 19 cm). The Monte Carlo model succeeded in predicting the EPID response to within 1% of the measurements. Large variations of the EPID response were observed between the various offsets: 29%, 29% and 25% for the 10x10, 6x6 and 2x2 cm fields respectively. The EPID response increased with field size and with field offset for most cases. The Monte Carlo model was then used to predict the image of an IMRT field consisting of four segments. The field was delivered on the beam axis and with offsets of 15 and 12 cm on X- and Y-axes respectively. Good agreement was found between the simulated and the measured images. A variation up to 30% was found between the on and off-axis delivery. The feasibility of using Monte Carlo methods to predict portal images is thus demonstrated. The model predicted accurately the EPID response in air for the large spectral variations observed with field offsets.
|Additional Information:||Copyright © 2006 by The American Association of Physicists in medicine.|
|Uncontrolled Keywords:||Science & Technology, Life Sciences & Biomedicine, Radiology, Nuclear Medicine & Medical Imaging, EPID, Monte Carlo, PORTAL IMAGING DEVICE, MULTILEAF COLLIMATOR, QUALITY-CONTROL, DOSE-RESPONSE, OPTIMAL BACKSCATTER, RADIATION-THERAPY, IMRT VERIFICATION, LEAF POSITIONS, CALIBRATION, MLC|
|Divisions:||Faculty of Engineering and Physical Sciences > Electronic Engineering > Centre for Vision Speech and Signal Processing|
|Deposited By:||Symplectic Elements|
|Deposited On:||03 Dec 2012 12:48|
|Last Modified:||13 Apr 2013 14:46|
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