AbstractID: 6582 Title: Calculations of Dose Profiles Under
Wedges, Using Spatially Dependent Attenuation Coefficients
Wedge filters are routinely used in radiation therapy to
modify x-ray beams to produce more uniform dose
distributions in target tissue. These wedge filters are
generally made out of steel or lead and have a sloped
surface in the wedge direction. In the non-wedge direction
the wedge filter has uniform thickness. Generally it is
assumed that the dose distribution in the non-wedge
direction is the same as an open field. Numerous reports
have shown that dose away from the central axis is 5 to 10%
below that of an open-field. This so-called sagging of
dose is due to the oblique path through the wedge and the
softening of the beam energy-spectra for position away from
the central axis. In this work the correct dose
distribution in a wedged field is predicted by calculating
the transmission through the wedge, using the thickness of
the wedge and the attenuation coefficient. The attenuation
coefficient is modeled for its dependence on wedge
thickness and on the distance of the calculation point away
from the central axis of the field. This modeling is based
on commonly available wedge factors and thickness measured
on the central axis and transmission measured along the
non-wedge direction of a 30º wedge. Measured data and
calculations in the wedge and non-wedge direction are
compared for a 6 MV beam with 15, 30, 45 and 60º wedges.
Differences between measurements and calculations are less
than 2% for fields as large as 25 cm x 25 cm.