Monte Carlo modelling of a virtual wedge

Compared with a set of physical photon wedges, a non-physical wedge (virtua l or dynamic wedge), realized by a moving collimator jaw, offers an alterna tive that allows creation of a wedged field with any arbitrary wedge angle instead of the traditional four physical wedges (15 degrees, 30 degrees, 45 degrees and 60 degrees). It is commonly assumed that non-physical wedges d o not alter the photon spectrum compared with physical wedges that introduc e beam hardening and loss of dose uniformity in the unwedged direction. In this study, we investigated the influence of a vidual wedge on the photon s pectra of a 6-10 MV Siemens MD2 accelerator with the Monte Carlo code EGS4/ BEAM. Good agreement was obtained between calculated and measured lateral d ose profiles at the depth of maximum dose and at 10 cm depth for 20 x 20 cm (2) fields for 6 and 10 MV photon beams. By comparing Monte Carlo models of a physical wedge and the vidual wedge that was studied in this work, it is confirmed that the latter has an insignificant effect on the beam quality, whereas the former can introduce significant beam hardening.