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Photon dose calculations in inhomogeneous media Shahine, Bilal Haidar

Abstract

A series of experiments were carried out to simulate air cavities in a polystyrene phantom. Results of experiments were compared to calculations done using three treatment planning systems employing Batho, modified Batho and the equivalent tissue-air-ratio (ETAR) methods for inhomogeneity corrections. The measured interface dose decreased by 55% for a 5 cm air gap, 5x5 cm² field size and 6 MV photons while only a 10% decrease was calculated by these methods. This points to the need for proper inclusion of electronic disequilibrium effects caused by the air cavities. In an attempt to account for electron transport, an inhomogeneity correction factor model is proposed based on separating the primary and scatter photon interaction effect with matter. The primary inhomogeneity correction factor was evaluated in phantoms containing air cavities following Klein-Nishina formalism and detailed electron transport. The Fermi-Eyges theory was adopted to transport recoil electrons based on multiple Coulomb scattering formalism. The scatter inhomogeneity correction factor is proposed as a semi-empirical model based on ratios of tissue-maximum-ratio with scaled effective beam radius. A total correction factor was derived by weighting the primary and scatter components and Monte Carlo simulation was used to verify these results. A calculation code was written and an optimization technique was devised reducing the computation time to a practical limit while maintaining accuracy. An additional topic on the subject of Monte Carlo treatment planning was discussed. The two Monte Carlo codes EGS4 and GEANT3 have been compared for calculations of photon and electron depth doses in water. Good agreement was seen for radiation beams relevant to radiation therapy. To conclude the comparison, a timing study was performed. GEANT3 was seen to be two times slower than EGS4 in one of its electron transport modes and up to three times faster when using its energy straggling mode of electron transport.

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