Field size dependence of wedge factors.

The radiation output in the presence of wedge filters is characterized by the wedge transmission factor and open beam field size factors. Conventionally, the wedge factor for high-energy photons is measured in a water phantom at depth of maximum dose for a reference field size. Experimental measurements on different wedges indicate that the wedge factors are a function of field size. An analysis of these data show that this is primarily caused by the change in scattered radiation from the treatment head in the presence of wedge filters. The change in phantom scatter and radiation backscattered to the monitor chamber are minimal. For 4- or 6-MV x rays with a 60 degrees wedge, the use of a single wedge factor measured for 10 cm X 10 cm field introduces errors of up to 3.5%, for a 16-cm-wide field. For a 20-cm-wide field with this wedge, the error is 7%. Thinner wedges exhibit less differences.