Two Approximations to Obtain the Collimator Scatter Factor (S_c) for MLC Irregular Fields

Abstract

The collimator scatter factor (S_c^MLC) at MLC irregular fields for high-energy X-ray irradiation is generally assumed to be equal to the jaw collimator scatter factor (S_c^jaw) of the square field equivalent to the rectangular field produced using pairs of jaw collimators. However, this assumption becomes strained as the ratio of the MLC equivalent square field side to the jaw collimator equivalent square field side decreases. In this study, for 4 MV and 10 MV X-rays, the collimator scatter factor (S_c^MLC) for an MLC irregular field could be evaluated with a high degree of accuracy using the MLC irregular correction (F_MIC) factor or the jaw collimator correction (F_JCC) factor.