Implementation and verification of Virtual Wedge in a three-dimensional radiotherapy planning system

Virtual Wedge (VW) is a Siemens treatment modality which generates wedge sh aped dose distributions by moving a collimator jaw from closed to open at a constant speed while varying the dose rate in every 2 mm jaw position. In this work, the implementation and verification of VW in a radiotherapy trea tment planning (RTP) system is presented. The VW implementation models the dose delivered by VW using the Siemens monitor units (MU) analytic formalis m which determines the number of MU required to generate a wedge-fluence pr ofile at points across the VW beam. For any set of treatment parameters, th e VW algorithm generates an "intensity map" that is used to model the modif ication of fluence emanating from the collimator. The intensity map is calc ulated as the ratio of MU delivered on an axis point, divided by the monito r units delivered on the central-axis MU(0). The dose calculation is then p erformed using either the Clarkson or Convolution/Superposition algorithms. The VW implementation also models the operational constraints for the deli very of VW due to dose rate and jaw speed limits. Dose verifications with m easured profiles were performed using both the Clarkson and Convolution/Sup erposition algorithms for three photon beams: Siemens Primus 6 and 23 MV, a nd Mevatron MD 15 MV. Agreement within 2% or 2 mm was found between calcula ted and measured doses, over a large set of test cases, for 15, 30, 45, and 60 degree symmetric and asymmetric VW fields, using the manufacturer's sup plied mu and c values for each beam. (C) 2000 American Association of Physi cists in Medicine. [S0094-2405(00)01407-3].