Investigation of dose distributions of high energy beams used in small field radiotherapy treatment in water medium

The aim of this study is to investigate the voxel size effects of Monte Carlo-based dose calculation algorithms in small field radiotherapy applications and treatment planning system (TPS) for more accurate dose estimation. In this thesis study, voxel size effects were investigated by calculating dose distributions in skin tissue and subcutaneous field close to the surface. Physical parameters such as treatment field size, media material and photon energy were determined by considering the conditions in their TPS. Dose calculations were carried out in three separate stages. In all stages, the EGSnrc Monte Carlo code-based DOSXYZnrc user code was used and the patient's body was considered as a homogeneous water phantom with a volume of 30x30x5 cm3. Dose distributions were calculated separately in different scenarios for different treatment field sizes, voxel sizes and photon energies at different depths up to z=1.0 cm, horizontally and in the plane in the X-ray incident direction. When the results obtained in the first stage were evaluated, it was revealed that the ideal voxel edge length value at 1 MeV photon energy varies considerably depending on the field size and depth. In the second stage, dose values were calculated in 0.5x0.5 cm2 and 3x3 cm2 treatment areas and at different points up to 1.0 cm depth for different photon energies (2-10 MeV). Ideal voxel sizes were found to vary significantly depending on field size, incident photon energy and depth. In the third stage, dose calculations were carried out separately for the X-ray spectrum emanating from a 4 MV accelerator head in 0.5x0.5 cm2 and 3x3 cm2 treatment areas. Although the dose values obtained using monoenergetic and spectrum photon sources are different, the ideal voxel edge length values were shown to be the same. It was concluded that 0.01 cm voxel edge length value up to 0.1 cm depth is the ideal voxel size in the horizontal plane that can be used in all calculations using the Monte Carlo technique and under the selected energy and geometry conditions for each treatment area. It has been shown that the voxel edge length value that can be used safely for small field can be selected for larger lengths for both spectrum and monoenergetic photon sources, depending on the increase in the incident photon energy, the size of the treated area and the depth values. According to the geometry conditions, the treatment area width, incident photon energy and ideal voxel edge length values depending on the depth were presented in tables. It was concluded that if the data in these tables in detail are taken into account in clinical use, dose values can be calculated more precisely, and accordingly, a significant contribution can be made to the prevention of vital side effects that may occur in patients after treatment.