A Study on Effective Source-Skin Distance using Phantom in Electron Beam Therapy

In this study, for 6-20 MeV electron beam energy occurring in a linear accelerator, the authors attempted to investigate the relation between the effective source-skin distance and the relation between the radiation field and the effective source-skin distance. The equipment used included a 6-20 MeV electron beam from a linear accelerator, and the distance was measured by a ionization chamber targeting the solid phantom. The measurement method for the effective source-skin distance according to the size of the radiation field changes the source-skin distance (100, 105, 110, 115 cm) for the electron beam energy (6, 9, 12, 16, 20 MeV). The effective source-skin distance was measured using the method proposed by Faiz Khan, measuring the dose according to each radiation field ($6{\times}6$, $10{\times}10$, $15{\times}150$, $20{\times}20cm^2$) at the maximum dose depth (1.3, 2.05, 2.7, 2.45, 1.8 cm, respectively) of each energy. In addition, the effective source-skin distance when cut-out blocks ($6{\times}6$, $10{\times}10$, $15{\times}15cm^2$) were used and the effective source-skin distance when they were not used, was measured and compared. The research results showed that the effective source-skin distance was increased according to the increase of the radiation field at the same amount of energy. In addition, the minimum distance was 60.4 cm when the 6 MeV electron beams were used with $6{\times}6$ cut-out blocks and the maximum distance was 87.2 cm when the 6 MeV electron beams were used with $20{\times}20$ cut-out blocks; thus, the largest difference between both of these was 26.8 cm. When comparing the before and after the using the $6{\times}6$ cut-out block, the difference between both was 8.2 cm in 6 MeV electron beam energy and was 2.1 cm in 20 MeV. Thus, the results showed that the difference was reduced according to an increase in the energy. In addition, in the comparative experiments performed by changing the size of the cut-out block at 6 MeV, the results showed that the source-skin distance was 8.2 cm when the size of the cut-out block was $6{\times}6$, 2.5 cm when the size of the cut-out block was $10{\times}10$, and 21.4 cm when the size of the cut-out block $15{\times}15$. In conclusion, it is recommended that the actual measurement is used for each energy and radiation field in the clinical dose measurement and for the measurement of the effective source-skin distance using cut-out blocks.