Effect of output variation with dose rate on the Virtual Wedge factor

The Siemens Virtual Wedge factor (VWF: Siemens Medical Solutions, Malvern, PA) may drift significantly because of an increase in output as the dose rate declines. This variation in output is caused by sample and hold boards in the dosimetry circuit that become defective because of radiation damage. Here, we present a simple model based on the principle of Virtual Wedge operation and the measured output variation with dose rate to quantitatively describe VWF drift as a function of field size and wedge angle. Our results support the recommendation that VWF be measured for large field sizes (for example, [Formula: see text] cm) and large wedge angles (for example, 60 degrees) as a part of routine quality assurance. PACS numbers: 87.55.Qr, 87.55.N‐