AbstractID: 7319 Title: In-vivo dose reconstruction using a transimission factor and an effective field concept
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aim of this study is to develop a simple method which computes in-vivo doses from
transmission doses measured during patient treatment using an ionization chamber. Energy fluence
and the dose that reach the chamber positioned behind the patient is modified by three factors:
patient attenuation, inverse square attenuation, and scattering. We adopted a straightforward
empirical approach using a phantom transmission factor (PTF) which accounts for the contribution
from all three factors. In this model, we hypothesized that the presence of inhomogeneities and the
use of irregular fields could be effectively accommodated by adjusting effective field size, and thus
the scatter contribution. The factor was measured as a simple ratio of the chamber reading measured
with and without a homogeneous phantom in the radiation beam. As a result, a table of PTF values
was obtained as a function of field size, phantom thickness, and phantom-to-chamber distance. The
developed method was tested. With this method the absolute midline dose was predicted with an
error of 2.3% at 10 cm phantom-to-chamber distance for a 25 cm-thick homogeneous phantom; an
error of 3.9% at 10 cm phantom-to-chamber distance for a 20 cm-thick inhomogeneous phantom.
An X-ray beam with an energy of 6 MV and a field size of 20 x 20 cm2 was used. The phantom-tochamber distance was maintained to be 10 cm. The test with irregular fields showed a maximum
difference of 3.6% for the similar irradiation condition.
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