MEASURING COLLIMATOR SCATTER COMPONENT OF MEGAVOLTAGE PHOTON BEAMS BY USING CYLINDRICAL PHANTOM

The separation of the total scatter correction factor Sc,p in a collimator scatter component, Sc, and a phantom scatter component, Sp has proven to be an useful concept in megavoltage photon beam dose calculations in situations which differ from the standard treatment geometry. At this study a clinically applicable method to determine Sc is described. Measurements are carried out with an ionization chamber, placed at a depth beyond the range of contaminant electrons, in a narrow cylindirical persfex phantom with a diameter of 4 cm of which the axis coincides with the beam axis. Sc,p is measured in a full-scatter phantom and Sp can be derived from Sc,p and Sc. The Sc measurements taken at different SSD, brought us to the point that Sc is independent from SSD. The Sc,p and Sc curves at the ATC C9 Cobalt 60 unit are much steeper than the one at the AECL Cobalt 60 unit. At this work Sc component were not measured for the different models of linear accelerator and our opinion is that these unit will be dissimilar at these model of linear accelerator at the same beam quality. The method can easily be used in existing procedures for the calculation of the number of monitor units to deliver a specified dose to a target volume.