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UBC Theses and Dissertations

Monte Carlo simulation of x-ray dose distributions for direct aperture optimization of intensity modulated treatment fields Bergman, Alanah Mary

Abstract

This thesis investigates methods of reducing radiation dose calculation errors as applied to a specialized x-ray therapy called intensity modulated radiation therapy (IMRT). There are three major areas of investigation. First, limits of the popular 2D pencil beam kernel (PBK) dose calculation algorithm are explored. The ability to resolve high dose gradients is partly related to the shape of the PBK. Improvements to the spatial resolution can be achieved by modifying the dose kernel shapes already present in the clinical treatment planning system. Optimization of the PBK shape based on measured-to-calculated test pattern dose comparisons reduces the impact of some limitations of this algorithm. However, other limitations remain (e.g. assuming spatial invariance, no modeling of extra-focal radiation, and no modeling of lateral electron transport). These limitations directed this thesis towards the second major investigation - Monte Carlo (MC) simulation for IMRT. MC is considered to be the "gold standard" for radiation dose calculation accuracy. This investigation incorporates MC calculated beamlets of dose deposition into a direct aperture optimization (DAO) algorithm for IMRT inverse planning (MC-DAO) . The goal is to show that accurate tissue inhomogeneity information and lateral electronic transport information, combined with DAO, will improve the quality/accuracy of the patient treatment plan. MC simulation generates accurate beamlet dose distributions in traditionally difficultto- calculate regions (e.g. air-tissue interfaces or small (≤ 5 cm² ) x-ray fields). Combining DAO with MC beamlets reduces the required number of radiation units delivered by the linear accelerator by ~30-50%. The MC method is criticized for having long simulation times (hours). This can be addressed with distributed computing methods and data filtering ('denoising'). The third major investigation describes a practical implementation of the 3D Savitzky-Golay digital filter for MC dose 'denoising'. This thesis concludes that MC-based DAO for IMRT inverse planning is clinically feasible and offers accurate modeling of particle transport and dose deposition in difficult environments where lateral electronic dis-equilibrium exists.

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