Dependence of wedge transmission factors on blocking in high energy photon beams

The proper use of wedge factors (WF) in point dose and monitor unit (MU) calculations has become a well published topic in the last several years. It has become a common knowledge that WFs are definitely a function of both the beam field size and also a function of the depth. The field size dependent WF is defined to be the ratio of ionization readings measured with and without the wedge at the normalization depth and for the same field size (r). With the Introduction of multileaf collimators (MLCs) and the ability to place the wedge below custom Cerrobend blocks, the question of what field size to use MU calculations has become important. This is because the field size projected on the wedge is either the field defined by the collimator or the field size defined by the field shaping device (MLC or custom block). Consequently, the authors initiated this study to determine if the wedge factor used in MU calculations should be selected as a function of the collimator setting or the size of the field projected on the surface of the wedge. A Farmer type ion chamber in a water phantom at the normalization depth was used to perform the measurements. WFs were measured for several MLC blocked equivalent squares with the wedges placed below the MLC, keeping the collimator settings constant. In addition, WFs were measured for custom blocked fields with the wedges placed both above and below the custom blocks while holding the collimator setting constant. 6 and 18 MV photons from a Varian 2300CD with both upper and lower wedges were used for the investigation. The authors´ results clearly show that the WF are closer to those determined using the effective field size as long as the wedges reside always directly below the blocked field. The magnitude of the errors for using the incorrect wedge factor are presented