Dosimetric evaluation of the Siemens Virtual Wedge.

Recently, Siemens has introduced its Virtual Wedge (VW). On a Mevatron accelerator, this option generates a wedge-like dose profile by moving a collimator jaw at constant speed while varying the dose rate. In this paper the formalism is given that is used to deliver a wedge profile and from that the expressions for possible combinations of wedge angle, field size and delivered MUs are derived. Also the time needed to deliver a VW field is calculated. An effective attenuation coefficient mu is used in the implementation. For three beam energies, values of mu are determined in order to get VW angles that are as close as possible to the hard wedge angles, over a wide range of field sizes and wedge angles. Linearity with number of MUs and gantry angle dependence of the generated dose profiles were checked. These factors did not have a significant influence on the VW dose profiles. Wedge factors should be close to unity in the VW implementation. We have measured a number of wedge factors and found that they start to deviate from 1 with more than 1% for large wedge angles and field sizes, up to 3.5% for a 19 x 19 cm2, 60 degrees VW field. The Virtual Wedge turned out to be a reliable tool that can be used clinically, provided that it can be handled by the treatment planning system. It provides extra flexibility and usually results in shorter beam on times.