Düzleştirici Filtresiz Tedavi Sistemleri İçin Kalem Huzme Kerneli Geliştirilmesi
Ertürk , Mehmet Ertuğrul
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Pencil beam algorithm is a fast and accurate method for dose computation at Radiotherapy. This algorithm was developed for filtered x-ray energies. The aim of this study was developing pencil beam kernels by utilizing x-ray characteristics of flattening filter free energy, and build a calculation model working at inhomogeneous medium. It was also purposed to make dose calculation at inhomogeneous medium kernel of each medium. Dose distribution data of flattening filter free x-ray at different mediums were gotten with water phantom measurements and Monte Carlo modeling. Phase-space data provided by Varian were used for Monte Carlo modeling. Optimization method was used to obtain kernel parameters. Gama analysis method was used as penalty function at optimization. Dose distribution was obtained with two dimensional discrete convolutions by using these kernel parameters. Gama values of each measurement points were calculated on dose profile by using calculated dose distribution. Accuracy of dose distributions, which were calculated with evaluated kernels, was controlled with three different methods by using gamma analysis technique. At the first, measured profiles used for kernel evaluation were compared with profiles that were calculated with evaluated kernels. Criteria’s of gamma analysis were 1 mm distance to agreement, 1% dose difference and the threshold was 10%. Multileaf collimator shaped fields were used at the second stage of calculation accuracy control. Validity of evaluated kernel was controlled with shaped field with multileaf collimators (MLC) at the second phase. Dose maps of irradiated MLC shaped fields at different depths were measured with two dimensional detector. Criteria’s of gamma analysis were 3 mm 3% dose difference criteria and with 10% threshold. Passing ratio of each comparison was found greater than 95%. Acquired dose maps by irradiation of intensity modulated radiation therapy (IMRT) field were used at the third phase of the comparison. Criteria were used at MLC shaped fields were also used in this phase, and a passing ratio greater than 99% were achieved. At the second stage, capability of dose to medium calculation ability of pencil beam algorithm was investigated. Thus, dose distributions were calculated with the kernels evaluated from different homogeneous mediums. In order to calculate dose distributions at heterogeneous medium, two different kernels were prepared for calculate dose distribution at depth and lateral directions. Dose distributions evaluated at heterogeneous mediums pencil beam calculation were compared with dose distributions evaluated with Monte Carlo simulation by using gamma analysis method. Criteria used for comparison of lateral dose distribution were 2 mm 2% with 10% threshold. A passing ratio greater than 95% was achieved. At the second part of the second stage a percentage depth dose calculation model with forward scatter and backward scatter kernel was suggested. Percentage depth dose curves evaluated with pencil beam algorithm and Monte Carlo methods for different phantoms. These curves were compared with gamma analysis method. Passing ratio’s greater than 95% were achieved by the criteria of 3 mm 3%. As a result, the kernels can be redefined by considering the beam characteristics of the flattening filter free x-ray energies. With this redefinition, the number of parameters in the kernel can be reduced. Thus, the calculation time can be shortened. In addition, dose to medium approach can be used in the pencil beam algorithm.