Modeling of megavoltage and low energy focused x-ray beams using MCNP5

In this thesis 6 and 18 MV photon beams for a Varian linear accelerator were simulated using a general purpose Monte Carlo code MCNP5 (Monte Carlo N-Particle Transport Code). The potential of MCNP5 is explored to calculate the dose in high dose gradient regions. In addition, MCNP5 was used to model another geometry, low energy focused x ray beams created with polycapillary optics, to explore their potential to provide a different skin-sparing technique for radiation therapy.;Modeling was benchmarked by adjusting electron beam energy and diameter to match calculations with ionization chamber (with 0.125 cm3 sensitive volume) measurements of percent depth doses (PDDs) beyond d max and cross dose profiles at dmax and 10 cm depths for 5×5, 10×10 and 30×30 cm2 field sizes. For comparison in the buildup region, the MCNP5 voxel was reduced to 1 mm, with extrapolation to find surface dose. In this region a plane parallel chamber (with 0.055 cm3 sensitive volume) was used to measure PDDs at 0, 2 and 4 mm depths for the three field sizes, using the Khan over-response correction.;Calculations and cylindrical chamber measurements for PDDs beyond d max agree within 2% for all field sizes and energies. Dose profiles agree within 2% in the flat region and within 10% in the penumbra region for all field sizes and energies. In the buildup region, the maximum deviation between MCNP5 calculations and the plane parallel PDD measurements at 18 MV is 3.8% for 30×30 cm2 at 2 mm depth. The deviation is within 3% for all other field sizes and depths for both beam energies. For 18 MV, the extrapolation of the MCNP5 PDD overestimates the surface dose for the 30×30 cm2 field size, and is within 4.6% for all other field sizes at both beam energies.;A focused beam obtained with polycapillary optics was simulated using MCNP5. Low energy focused x ray beams could be used to irradiate the tumors inside soft tissue within 5 cm of the surface. A better skin sparring compared to megavoltage photon beams can be achieved by the low energy focused x ray beam inside soft tissues.