Wedge factor changes with depth and field size on a linear accelerator with a motorized wedge.

The wedge factor was experimentally determined as a function of field size and phantom depth for 6-MV X-rays; measurements were performed on a linear accelerator with a motorized wedge filter (universal wedge, nominal wedge angle = 60 degrees). As a result of our experimental work, we can state that: 1) The wedge factor increases linearly with phantom depth (0.275%/cm), almost independent of field size; this change was experimentally related to the beam "hardening" caused by the high Z value of wedge material, as shown by the increase in wedged-field PDD with respect to the corresponding open-field PDD. Neglecting this variation can induce systematic errors in delivered dose, especially when deep-seated tumors are treated with wedge fields. 2) The wedge factor changes, for rectangular fields, according to the "equivalent square law", and not to the dimension in the wedged direction.