The effects of a universal wedge and beam obliquity upon the central axis dose buildup for 6-MV x rays

In a number of clinical situations, the dose to the skin and the superficial tissues is of concern. Both beam obliquity and a beam modifier will modify the dose delivered to these regions due to changes in the scattering geometry, scattered photon and secondary electron production, and changes in the energy spectrum of a polyenergetic beam. Some linear accelerators use a single universal wedge mounted within the treatment head. Because such a wedge is at an extended distance from the patient, its contribution to the beam contaminants incident to the skin will be limited. Measurements of the ionization in the buildup region have been performed in a polystyrene phantom irradiated with a 6-MV x-ray beam from a linear accelerator equipped with a universal wedge. The variation of the buildup dose with obliquity, universal wedging, and distance from the source has been measured for angles of incidence between 0 degrees and 60 degrees and for effective wedge angles between 0 degrees and 45 degrees. The results indicate that the percentage buildup has a much stronger dependence upon the angle of incidence than upon the effective wedge angle. For distances approaching the treatment head, it is shown that the universal wedge generates secondary electrons that elevate the surface dose, but that this contribution decreases with distance.