Sci‐Sat AM (2) Therapy‐08: Influence of time dose rate calibration on Siemens Virtual Wedge dosimetry

We have investigated the influence of time dose rate calibration on the dosimetry of Siemens Virtual Wedge (VW). The linear accelerator (LINAC) uses the ionization current generated in two dose monitoring ion chambers (MONITOR 1 and MONITOR 2) to dynamically control the three phases of VW delivery (initial gap, sweep portion, and open field). We intentionally offset the calibration of MONITOR 2, which controls the time dose rate delivered on the LINAC, by up to ± 3% and observed fractional changes in the wedge dose profile of approximately 10% for a 6MV beam with a 60° wedge at a field size of 20 × 20 cm2. We present an analytical expression to calculate the fractional change in wedge dosimetry as a function of time dose rate calibration of MONITOR 2. A comparison with measurement showed that the expression was accurate to within 1%. We will show how changes in the calibration of time dose rate (MONITOR 2) affect both the central axis wedge factor and wedge angle in order to assist in the diagnosis of wedge problems encountered in routine quality control measurements. The effects are most prominent at low beam energies, large wedge angles and large field sizes.