SU-E-T-143: Effect of Physical and Virtual Wedges on the Surface Dose at Various SSD for 6 and 15 MV Photon Beam.

Abstract

To study the effect of the virtual wedge and physical wedge filters on the surface and build-up region doses for 6 and 15MV high-energy photon beams for different field sizes and various source to surface distance(SSD). The measurements were made in water equivalent (PMMA) solid phantom in the build-up region at various SSD for various field sizes using virtual and physical wedge filters having different angles. A parallel-plate ion chamber (Markus) was used to measure the percent depth doses at surface and buildup region. Plane parallel ion chamber with fixed plate separation on the surface and buildup region would perturbate the dose measured, to get the proper dose over response correction factor was used. The percentage depth dose at surface (PDD0) increased as the field size increased for open, virtual, and physical wedged beams. For open, 30 degree physical, and virtual wedged beams, the surface doses were found to be 15.4%, 11.2%, and 15.2% with 6-MV photons and 11.2%, 9.4%, 11.2% with 15-MV photons, respectively, at 10 × 10 cm2 field size at 100cm SSD.As SSD increases percentage depth dose at surface (PDD0) decreases for open,physical and virtual wedge field. Percentage depth dose at surface (PDD0) of virtual wedged beams were similar to those of open beams. PDD0 of physical wedged beams were lower than those of open and virtual wedged beams. Surface doses for both PW and VW increases with field size and small increase in surface dose for both PW and VW fields as wedge angle increases especially for large fields.