飽和モデルによる矩形照射野のコリメータ散乱係数S_cの推定

概要

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We estimated collimator scatter factor, Sc, of symmetric rectangular fields of any size by applying a two-component scatter model to measured in-air output data in width and length directions of measured rectangles. The in-air output was measured for symmetric rectangles with combined width and length sizes of 7 × 7 and 6 × 6 using 10 MV and 4 MV X-rays of Varian’s Clinac 2100 C/D, respectively. The model consists of scatter components from the primary collimator and flattening filter and from the collimator jaws: the former shows a saturation curve and the latter increases linearly with enlarging field size. This model was fitted to the measured dataset firstly in the width and secondly in the length directions of rectangles; then, by compiling interpolated matrix data, the Sc table of symmetric rectangles was constructed. In addition, using this Sc table, values of Sc were calculated for a few asymmetric rectangles by Day’s method, and were in good agreement with measured values. Therefore, we think that our method is practical and precise for constructing the Sc table of symmetric rectangles from measured data, and that using this table, the Sc of any asymmetric rectangles may be calculated.
2008-10-20