Influence of monitor chamber calibration on virtual Wedge dosimetry

Commissioning Siemens Virtual Wedges in the Oncentra MasterPlan treatment planning system using Gafchromic EBT film

PURPOSE

Virtual Wedges were introduced in Siemens LINACs to improve the treatment workflow. The aim of the present work is the validation of dose calculation by MasterPlan-Oncentra treatment planning system for virtual wedged beams.

METHODS

The Oncor Siemens accelerator installed in the authors' department produces 6 and 15 MV photon beams. At first, the consistency of VW LINAC production was tested and the EBT film measuring method was verified. This method is based on the scanner uniformity correction and absolute dose calibration as reported in literature. Then, the measured and calculated wedge factors and beam profiles are compared. For 15 degrees, 30 degrees, 45 degrees, and 60 degrees wedge angles, the wedge factors for different field sizes were measured by an ionization chamber and the dose profiles were acquired by Gafchromic EBT film. Both types of measurements were collected in isocentric condition.

RESULTS

The comparison between measured and calculated VW factors shows discrepancies that increase with field size and angle. The OTP Enhanced algorithm produces better agreement with measurements than the Classic one, with improvement overall visible for large angles. The agreement between measured and planned beam profiles is within limits reported by the ESTRO Booklet No. 7 in terms of confidence limits.

CONCLUSIONS

The MasterPlan-Oncentra treatment planning system determines wedge factors and VW profiles within the requested accuracy in the majority of treatment conditions. For big field dimensions and wedge angle, wedge factor accordance was worse, but it may be increased with an improvement of the LINAC dosimetric board calibration.

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, 20×20 cm) and large wedge angles (for example, 60 degrees) as a part of routine quality assurance.

PACS numbers: 87.55.Qr, 87.55.N‐