AbstractID: 6784 Title: Virtual and Physical Wedge Factors: Contributions of Head and Phantom Scatters
In recent literature, it has been shown that for Siemens accelerators, the virtual wedge factor (VWF)
increases with field size especially for large virtual wedge (VW) angles such as 60°. It also has been
reported the VWF increases slightly with depth. It has been debated whether phantom scatter has any
significant effect on the observed increase of VWF with the field size and depth. In this work, we
measured the total output factors in a solid water phantom and the head scatter factors in a cylindrical miniphantom as a function of field size and depth. Phantom scatter factors are derived from the measured total
output factors and head scatter factors. We provided an analysis to relate the wedge factor to the head
scatter and phantom scatter. Measurements and analysis are performed for 6 MV and 15 MV x-ray beams
from a Siemens MD2 linear accelerator. The same type measurements and analysis are also performed for
physical wedges (PW). The results for VW and PW are compared. It is demonstrated that head scatter
dominates the contribution to the increase of VWF and PW factor (PWF) with field size, while phantom
scatter has insignificant (<1%) contributions to 60° VWF at the depth of 10 cm. Phantom scatter is also
insignificant for depth dependence of VWF (<2% variation for depth from 3 cm to 25 cm). Beam
hardening, which is quantified by the attenuation factor, is the only cause for the increase of PWF with
depth for 6 and 15 MV x-ray beams.