The relation between wedge factors in air and water

The wedge factor (WF) for an x-ray beam is defined as the ratio of doses with and without the wedge at a point on the central axis, in water. It is a function of depth d, field size s, and collimator setting c. It is demonstrated that WF can be calculated as WF(d,s,c) = WFairAw(d)Hw(c)/A0(d)H0(c), where WFair is a constant factor, namely, the ratio of the incident fluences measured "in air" with and without the wedge for reference conditions, e.g., at isocenter for a 10x10 cm2 collimator setting. Aw(d) and A0(d) are attenuation factors, and Hw(c) and H0(c) head-scatter factors, with and without the wedge and normalized to reference conditions. This expression is based on the approximation that the scatter factor is the same with and without the wedge. The field size, which affects the scatter factor, is therefore not an important variable for the wedge factor using this approach. The above relation has been evaluated at 6 and 25 MV for various depths (d < or = 25 cm) and field sizes (s < or = 25 cm) for built-in wedges (wedge angle approximately 60 degrees). The agreement was better than +/- 1.5%. This formulation of the wedge factor is easily implemented in calculations of dose and monitor settings.