Dependence of virtual wedge factor on dose calibration and monitor units

One of the important features of the Siemens Virtual Wedge (VW) is that the VW factor (VWF) is approximately equal to unity for all beams with a total deviation for a given wedge no greater than 0.05, as specified by Siemens. In this note we report the observed dependence of VWF on dose calibration (cGy/MU), monitor units (MU), and beam tuning for a Primus, a linear accele rator with two dose-rate ranges available for VW operation. The VWF is defi ned as the ratio of doses measured on the beam central axis for the wedge f ield to the open field; the open field dose is always measured with the nom inal high dose-rate beam. When VW operates in the high dose-rate range, the VWF is independent of calibration (cGy/MU). When VW works in the low dose- rate range, the VWF varies linearly with the calibration of the low dose-ra te mode. For a linear accelerator that has only one dose-rate range for VW, there is no observable dependence of VWF on the calibration. We also studi ed the monitor unit dependence of VWF. A discontinuity in VWF was observed at the switching point between the high and low dose-rate ranges. Working w ith Siemens, we have investigated causes of this discontinuity. As a result of this investigation, the discontinuity in VWF as a function monitor unit is practically removed. (C) 2001 American Association of Physicists in Med icine.