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

Abstract

Siemens Virtual Wedge factor may drift significantly due to increase in output as dose rate decreases.  This variation of output is caused by defective sample and hold boards in the dosimetry circuit due to radiation damage.  In this note, we present a simple model based on the principle of Virtual Wedge operation and the measured output variation with dose rate to quantitatively describe Virtual Wedge factor drift as a function of field size and wedge angle.  Our results support the recommendation that VWF be measured for large field sizes (e.g., 20 ´ 20 cm²) and large wedge angles (e.g., 60°) as a part of routine quality assurance.