Calculation of dose distributions with constraint checking for Siemens Virtual Wedge

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 multileaf collimator dynamic motion with accelerator dose output. The Siemens Virtual 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 convolution/superposition 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(O); MU(O) is also the number of monitor units that is entered at the machine console. The MCT 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.