[Examination of formula for enhanced dynamic wedge factors in half fields]

The method presented by Liu et al. in 1998 has generally been used in Japan to calculate the wedge factor of the enhanced dynamic wedge (EDW). When the points used to evaluate the dose exist outside the central beam axis in the half field (HF), however, a difference of about 3% can occur between the values calculated by this method and the measured values. In 2003, Liu et al. presented a new general formula for calculating MU using the EDW. We modified the formula for calculating the wedge factor by combining the conventional calculation formula and the formula of Liu et al., presented in 2003, and compared the calculated values of the wedge factor of the EDW in the HF with the measured values. Our formula employs a very simple method in which only the normalized golden segmented treatment table (NGSTT) on the center of the radiation field and the central beam axis are added to the existing formula. The values calculated by our formula and the measured values were consistent within 1% in most combinations of energy, wedge angle, and size of radiation field. When the EDW was used for irradiation to the tangent line of the HF breast, in particular, the values calculated by our formula and the measured values were consistent within 0.5%, and it was possible to calculate the wedge factor with a higher degree of accuracy than that calculated with the existing formula.