Analysis of the physical factors influencing portal imaging exit dosimetry using Monte Carlo modeling
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As the use of electronic imaging devices (EPIDs) increases, the emphasis which was initially on geometric verification, is shifting to portal exit dosimetry. In this latter concept, a portal dose image obtained during treatment with an EPID is compared with that calculated during treatment planning. The success of this approach will ultimately depend on a proper understanding of the factors that influence the measurement and calculation of dose distributions in the detector plane. The main objective of this work was to identify and analyze such factors using Monte Carlo methods. Firstly, the spectral characteristics of exit photon beams were studied. It was found that the intensity, energy spectrum, mean and effective energies of the beams are significantly affected by variations in the patient/detector geometry. These changes in the spectral characteristics translated into changes in the exit dose to a metal-phosphor detector. Studies performed with different detectors showed that the exit dose is significantly affected by the design and atomic composition of the detector. Existing dose calculation algorithms will need to be improved if they are to be used to predict the exit dose measured by the high atomic number metal phosphor detector. Interestingly, the effects of an increasing air gap and an increasing field size, and vice versa, were found to be roughly complimentary. This suggests the use of equivalent field size techniques to account for intensity and spectral changes arising from air gap variations. Based on Monte Carlo modeling, an equivalent field size technique has been proposed and confirmed experimentally. An advantage of this approach is that it can be integrated with the existing dose calculation algorithms with only minor modifications.