Abstract
In order to extend the application of the conformation radiotherapy, a linear accelerator was equipped with over-centerable multi-leaf collimators. The off-center ratio in a 10 MV Xray off-center beam with over-centerable collimators was measured. For the measurement, the same saurce situation, phantom, chamber between the off-center beam and normal beam with symmetric collimators was made and compared with the dose profile curves of both them. The dose profiles of the off-center beam are inclined to the rotational center of gantry. With the increase distance from the center, the dose profiles showed an increase in the gradient but with the increase depth of the phantom, a decrease was shown. Dose of off-center beam showed a decrease of about 3〜5% at the off-center distance of 52 and 87mm. Energy of the off-center beam had almost no change because the proportion of dose decrease had no change with the depth of the phantom. The off-center ratio data of the off-center beam has not been used in the algorism of the radiation treatment planning system. So the calculated dose values are estimated high at the rotation center side. The calculated values compared with the measured values concided with a 4% error.
