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2d dose measurement using a flat panel EPID Lim, Seng Boh


The increasing use of intensity modulated radiation therapy (IMRT) to deliver conformal radiation treatment has prompted the search for a faster and more cost effective quality assurance (QA) system. The standard technique relies on the use of film for two-dimensional dose distribution verification. Although film is considered the gold standard and is widely used for this purpose, the procedures involved are relatively lengthy, labour intensive and costly for a multiple field IMRT verification. In this study, we investigate the use of an amorphous silicon electronic portal imaging device (a-Si EPID) to complement the film. The dosimetric behaviour of the device is studied both experimentally and numerically using the EGSnrc Monte Carlo simulation routine. The intrinsic build-up of the flat panel EPID was found to be 1.1 cm of water equivalent material. The response of the flat panel EPID was found to be linear between 0 and 300 cGy. To calibrate the flat panel EPID for two dimensional dose measurements, the deconvolution method was chosen. The scatter dose kernel required for this calibration method was calculated and characterized by varying the energy, spectrum and phantom material using a 6MV pencil beam. We found that flat panel EPID scatter kernel has as much as 80% more scattering power than the water scatter kernel in the region 1 cm away from the center of a 6MV pencil beam. This confirms that a flat panel EPID behaves significantly differently from water dosimetrically and requires an accurate dose scatter kernel for calibration. A 1.0 cm wide picket fence test pattern was used to test the accuracy of the kernel. Using the deconvolution method with the calculated dose kernels, the measurements from the flat panel EPID show improved agreement with the films.

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