AbstractID: 7974 Title: DETERMINATION OF ABSOLUTE DOSE USING AN aS500
PORTAL DETECTOR
Determination of absolute dose using an aS500 Portal
detector
This study was done to evaluate the use of an aS-500 portal
imager for dose verification. We adopted a methodology
presented by Chang et al. for dose verification. The pixel
value of the portal imager was calibrated against dose rates
by exposing the imager to various intensities by varying the
Source to Detector Distance(SDD).For 6MV, pixel values
varied linearly with dose rate with R2 values of 0.9993,
and 0.9890 measured at 400 MU/min, 100 MU/min
respectively. The study also involved correcting for the
scatter from the EPID. Extensive measurements for Sc and
Sp were done using an ion chamber in air and in a solid
water phantom for various field sizes at two different
SDD’s(104cm and 140 cm). The scatter from EPID (Spe)
was measured by correlating the dose rates measured in the
full phantom to pixel values using a correction factor F
which depended on Sp and Spe. An iterative procedure was
adopted from Chang et al. to correct for the Spe . This
study was done to bench mark the method for determining
the Spe to facilitate Monte Carlo simulations using the
BEAM code.The Spe values were used to calculate the
absolute dose to a phantom, which we intend to use for
routine IMRT treatment verification.