DISTANCE OPTIMATION BETWEEN TWO IRRADIATION FIELDS FOR ELECTRON BEAM JUNCTION OF LINEAR ACCELERATOR

Abstract

The optimal distance between two electron-beam irradiation fields which experienced beam junction by the variations of electron energy and distance between those fields with the ICRU tolerance of dose distribution (95% - 107%) has been determined. The measurement of relative absorbed dose was done for the electron energy as much as 5, 7, 8, 10, 12, and 14 MeV in the area of 10 x 10 cm2 within distance variation of 0; 0,2; 0,4; 0,6; and 0,8 cm between the electron-beam irradiation fields. The data was then analyzed and described in graph to illustrate the correlation between relative absorbed dose to the axis points of the electron-beam irradiation fields at the crossline scanning (X-axis) and inline scanning (Y-axis). The result has shown that the dose distribution at the beam junction was optimum. To the crossline direction, the electron energy 5, 7, 8, and 10 MeV had the optimum distance at 0,2 cm and the electron energy 12 and 14 MeV had the optimum distance at 0,4 cm. While to the inline direction, the electron energy 5 MeV had the optimum distance at 0 cm, the electron energy 7, 8, 10, and 12 MeV had the optimum distance at 0,2 cm and the electron energy 14 Mev had the optimum distance at 0,4 cm. Key words : beam junctions, electron beam, relative absorbed dose, radiotherapy, Linac.