[Method for estimating 4 MV X-ray irregular field dose using the collimator scatter factor (Sc) and phantom scatter factor (Sp)].

Calculation of in-air or in-water dose for 4 MV X-ray irregular fields could be accurately performed using the collimator scatter factor (S(c)) and phantom scatter factor (S(p)) concepts. It has been revealed that the equivalent square field for a multi-leaf collimator (MLC) irregular field can be evaluated accurately by using the S(p)-Clarkson or S(c)-Clarkson integration method; however, the S(c)-Clarkson integration method is more straightforward because the S(c) factor expresses the in-air X-ray output factor. It has been found that when the MLC field is relatively much smaller than the main collimator field, the Sc factor can be accurately evaluated by introducing the small segment correction (SSC) factor (except for the case in which the MLC field is less than 1 x 1 cm(2)). It has also been found that both the S(p) factor and the tissue-phantom ratio (TPR) can be precisely evaluated by introducing the F(MLC) factor in cases in which the ratio of the MLC equivalent square field side to the main collimator equivalent square field side is less than about 0.7.