| Summary: | 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. === Science, Faculty of === Physics and Astronomy, Department of === Graduate
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