3-Dimensional Dosimetry of Small Field Photon Beam / 의학물리

A polymer gel dosimeter was fabricated. A 3-dimensional dosimetry experiment was performed in the small field of the photon of the cyberknife. The dosimeter was installed in a head and neck phantom. It was manufactured from the acrylic and it was used in dosimetry. By using the head and neck CT protocol of the CyberKnife system, CT images of the head and neck phantom were obtained and delivered to the treatment planning system. The irradiation to the dosimeter in the treatment planning was performed, and then, the image was obtained by using 3.0T magnetic resonance imaging (MRI) after 24 hours. The dose distribution of the phantom was analyzed by using MATLAB. The results of this measurement were compared to the results of calculation in the treatment planning. In the isodose curve on the axial direction, the dose distribution coincided with the high dose area, 0.76mm difference on 80%, rather than the low dose area, 1.29 mm difference on 40%. In this research, the fact that the polymer gel dosimeter and MRI can be applied for analyzing a small field in a 3 dimensional dosimetry was confirmed. Moreover, the feasibility of using these for the therapeutic radiation quality control was also confirmed.

Estimation of Inhomogeneity Correction Factor in Small Field Dosimetry / 의학물리

In this study, we estimated inhomogeneity correction factor in small field. And, we evaluated accuracy of treatment planning and measurement data which applied inhomogeneity correction factor or not. We developed the Inhomogeneity Correction Phantom (ICP) for insertion of inhomogeneity materials. The inhomogeneity materials were 12 types in each different electron density. This phantom is able to adapt the EBT film and 0.125 cc ion chamber for measurement of dose distribution and point dose. We evaluated comparison of planning and measurement data using ICP. When we applied to inhomogeneity correction factor or not, the average difference was 1.63% and 10.05% in each plan and film measurement data. And, the average difference of dose distribution was 10.09% in each measurement film. And the average difference of point dose was 0.43% and 2.09% in each plan and measurement data. In conclusion, if we did not apply the inhomogeneity correction factor in small field, it shows more great difference in measurement data. The planning system using this study shows good result for correction of inhomogeneity materials. In radiosurgery using small field, we should be correct the inhomogeneity correction factor, more exactly.

Dosimetry and Three Dimensional Planning for Stereotactic Radiosurgery with SIEMENS 6-MV LINAC / 대한치료방사선과학회지

Radiosurgery requires integral procedure where special devices and computer systems are needed for localization, dose planning and treatment. The aim of this work is to verify the overall mechanical accuracy of our LINAC and develop dose calculation algorithm for LINAC radiosurgery. The alignment of treatment machine and the performance testing of the entire system were extensively carried out and the basic data such as percent depth dose, off-axis ratio and output factor were measured. A three dimensional treatment planning system for stereotactic radiosurgery has been developed. We used an IBM personal computer with C programming language (IBM personal system/2, Model 80386, 1BM Co., USA) for calcu1ating the dose distribution. As a result, deviations at isocenter on gantry and table rotation for our treatment machine were acceptable since they were less than 2 mm. According to the phantom experiments, the focusing isocenter were successful by the error of less than 2 mm. Finally, the mechanical accuracy of our three dimensional planning system was confirmed by film dosimetry in sphere phantom.