The variation of wedge factors with field size on a linear accelerator

When a wedge filter is used to modify the dose distribution in a beam of megavoltage X-rays, the beam intensity is reduced. The reduction of beam intensity is quantified by a factor called the wedge factor (WF) defined as follows: where measurements are performed in water at the depth of maximum build-up. It is assumed by the authors of several treatment planning systems that the wedge factor doses do not vary with field size. Palta et al (1988) presented measurements for a range of square field sizes with 4 MV and 6MV X rays, showing that there can be considerable variation of wedge factor with field size, especially for a 60° wedge, and that errors in dose of up to 7% can occur by ignoring this variation. This technical note presents the results of similar measurements on 8 MV and 16 MV beams with a 60° wedge. The variation of wedge factor for a range of rectangular field sizes has also been investigated; this was done to see whether the variation follows an equivalent square law, or whether one should follow the practice of one commercial planning system by considering only the dimension in the wedged direction.