Virtual Wedge (VW) is a Siemens treatment modality which generates wedge-shaped 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 treatment planning (RTP) system is presented. The VW implementation models the dose delivered by VW using the Siemens monitor units (MU) analytic formalism which determines the number of MU required to generate a wedge-fluence profile at points across the VW beam. For any set of treatment parameters, the VW algorithm generates an "intensity map" that is used to model the modification of fluence emanating from the collimator. The intensity map is calculated as the ratio of MU delivered on an axis point, divided by the monitor units delivered on the central-axis MU(0). The dose calculation is then performed using either the Clarkson or Convolution/ Superposition algorithms. The VW implementation also models the operational constraints for the delivery of VW due to dose rate and jaw speed limits. Dose verifications with measured profiles were performed using both the Clarkson and Convolution/Superposition algorithms for three photon beams; Siemens Primus 6 and 23 MV, and Mevatron MD 15 MV. Agreement within 2% or 2 mm was found between calculated 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 supplied mu and c values for each beam.
Radiation therapy treatment planning for many clinical situations requires wedge-shaped isodose distributions. The wedged dose distributions can be generated through the use of physical wedges, motorized wedges, and the synchronization of jaw or multi-leaf collimator (MLC) dynamic motion with accelerator dose output. The Siemens Vritual Wedge (VW) delivers wedged shaped dose distributions by combining computer controlled dose delivery with collimator motion.In this paper, we describe a model to calculate dose for VW using either the Clarkson or ConvolutiodSuperposition algorithms. The VW implementation models the dose generated by VW using the Siemens monitor units (MU) analytic formalism which determines the number of MU required to deliver dose at points across the VW beam. For any treatment setup, the VW algorithm generates an "intensity matrix" that will be used to model the modification of fluence emanating from the source. The intensity (transmission) map is calculated as the ratio of the MU delivered on the axis divided by the monitor units delivered on the central-axis MU(0); MU(0) is also the number of monitor units that is entered at the machine console. The MU analytic formalism used for calculating the intensity matrix (which affects the calculation of dose) may be modified through the fine tuning of the attenuation correction factor c.The VW implementation models the operational constraints due to radiation delivery sequence in the different Siemens linear accelerators. Specifically, for a given VW beam setup, after the dose and MU calculations are performed, the VW model checks the machine dose rate and jaw speed constraints for the delivery of VW.We have performed dose calculations for 6, 15, and 23 MV beams, 15, 30, 45 and 60 degree VW angles, symmetric and asymmetric fields using the Clarkson and ConvolutiodSuperposition algorithms. Figures 1 displays measured and ConvolutiodSuperposition calculated dose profiles at depths of dmax, 10 and 20 cm for Primus 6 M V , 20x20 cm, 15 and 45 degree VW beams. Figure 2 shows measured and ConvolutiodSuperposition calculated dose profiles at depths of dmax, 10 and 20 cm for Mevatron MD 15 MV, 10x10 cm, 15 and 45 degree VW beams. We found good agreement within 2% or 2 mm (distance to agreement) between calculations and measurements for 15,30,45 and 60 degree VW angles, and 6 M V , 15 and 23 M V beams, using the Siemens supplied c and , u (linear attenuation coefficient)values. Similar agreement was observed for asymmetric fields. Also similar results were observed using the Clarkson algorithm for both symmetric and asymmetric fields. The results show that using the Siemens MU formalism with the supplied c and ,u to generate an intensity map yields accurate dose results, without the need for measured data to model VW as a physical filter. We noticed that the V W model overestimates the dose in the toe region of the wedge by a maximum of 5%, especially for large wedge angles and large field sizes, as shown in Figures 1 and 2. The reason for this difference is th...
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