BLOCKED FIELD EFFECTS ON COLLIMATOR SCATTER FACTORS

In routine dosimetry we assume separability of the collimator (S-c) an d phantom (S-p) scatter components that together comprise the total sc atter factor(S-c.p) In practice, the addition of blocking also affects the photon fluence attributable to the treatment head and flattening filter in a complicated way. The reduced aperture blocks out some of t he head scatter contribution, while the block and tray add back second ary scatter. In the following we present techniques for directly measu ring the aperture effect on S-c in air or in a full-scatter phantom. T he change in S-c is found to be a scaleable quantity that can be model led as a simple linear fit to the ratio of projected open-to-blocked e quivalent square fields. Measurements have been made for 6, 18 and 24 MV photon beams on one Varian 2500 and two Varian 2100c accelerators. Results indicate a progressive loss of collimator scatter contribution with increased field blocking that is amplified with increasing energ y. Block and tray scatter only contribute significantly to S-c for lar ge fields and treatment distances of 80 cm or less. Application of the se corrections in monitor unit calculations is presented.