ON 3-DIMENSIONAL DOSE CALCULATION OF PHOTON-BEAM WITH WEDGE FILTERS

A wedge filter is one of the most commonly used beam modifying devices. The introduction of a wedge filter alters both the distribution of the primary photon fluence and the first and second scatter patterns. The effect of the wedge filter is normally expressed in terms of an exponential attenuation function that is used to modify the primary photon fluence. This paper describes a new three-dimensional (3-D) wedged field dose calculation algorithm. This algorithm automatically takes into account (1) the scatter distribution pattern correction due to the effect of wedge filters and (2) the fixed primary photon fluence correction, by utilizing a tissue maximum ratio table calculated from the measured percent depth dose of the wedge, a measured in-plane scan and a measured cross-plane scan of the largest rectangular wedge field size. As a result, the calculation accuracy achieved by this algorithm is better than the calculation accuracy achieved by other algorithms. The results show better than 1% accuracy of the wedge central axis percent depth dose calculation and better than 3% accuracy of the off-axis dose calculation when compared with measurements.