Flattening-filter-based empirical methods to parametrize the head scatter factor

Parametrizing the collimator scatter factor, S-c (or head scatter factor), of a linear accelerator by the side of the equivalent square of the collimator setting at the isocenter does not accurately predict the change in S-c when the width and length of a rectangular field are exchanged. We have studied two methods based on measurements of square fields to predict S-c's of rectangular fields more accurately. The first method parametrizes S-c by the side of the equivalent square of the flattening filter region visible from the point of calculation. The S-c's of rectangular fields were predicted to an accuracy of 1% from measurements with square fields. The second method computes S-c of rectangular configurations by integrating radiation that can reach the point of calculation from a point source at the target and a second extended source at the flattening filter. The radial distribution of the extended source at the level of the flattening filter is computed from S-c of square fields measured at the isocenter. Effects of extended distance are modeled by separately performing inverse square law corrections for the two sources. This method also predicted the measured values to within 1% accuracy. (C) 1996 American Association of Physicists in Medicine.