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[Estimation of the phantom scatter factor (Sp) of rectangular fields].

This study proposes a method to accurately estimate the phantom scatter factor (Sp) of arbitrary rectangular fields. We measured output doses in water and air; these measured values were based on square fields and a limited number of symmetric rectangular fields using 4 MV and 10 MV X-rays of a Varian Clinac-iX. We calculated Sp from these measured values. Then, using these Sp values, we estimated equations of Sp on square fields consisting of the primary dose, Day's scatter, and forward scatter. This equation may be used to estimate the Sp value on a square field, but it cannot estimate the Sp value on a rectangular field. We investigated the calculation method for an equivalent square of a rectangular field. As a result, this study's calculation method for an equivalent square, the area ratio correction method, was more accurate than the conventional Bjärngard's method. Therefore, when using the approximate equation of Sp on a square field and the equivalent square calculated by the area ratio correction method, a Sp value of an arbitrary rectangular field may be accurately estimated.