[Method of estimating 10 MV X-ray irregular field dose using the collimator scatter factor (Sc) and phantom scatter factor (Sp)].

It has been found that in general 10 MV X-ray dose calculation can be made accurately for multi-leaf collimator irregular fields by using the total scatter factor (S(cp)), collimator scatter factor (S(c)), and phantom scatter factor (S(p)) proposed by Khan et al. With respect to the collimator scatter factor (S(c)), we used the field-mapping method of Kim et al. to obtain equivalent square fields of irregular fields (the collimator reverse effect can be accurately dissolved using the field-mapping method). Even for extremely small multi-leaf fields compared with the main collimator opening, X-ray output calculations could be made accurately by introducing the small segment correction (SSC) factor. With respect to the phantom scatter factor (S(p)), highly accurate calculations could be made for irregular field irradiation by applying an F(MLC) (MLC radiation leakage) factor to the equivalent square field (in cases in which the ratio of the multi-leaf equivalent square field side to the main collimator equivalent square field side is less than 0.6). However, it has been found that highly accurate dose calculations can, in general, be performed when the main collimator is limited just at the opening determined by the multi-leaf collimator field.