AbstractID: 9696 Title: Determination of scatter loss factor for clinical high energy xray beams near tissue-air intrerface
In many treatment situations like tangential breast fields, the radiation beam passes through a
volume of tissue surrounded by air. The presence of air near the tissue leads to a reduction
in the dose to the point of interest relative to an equivalent homogeneous volume of tissue
mainly due to the loss of scatter dose. The aims of the investigation are to determine the
extent of this scatter loss due to the presence of tissue – air interface and to check the
accuracy of the theoretical models used by treatment planning systems (TPS) for dose
calculations. The extent of scatter loss was determined experimentally by simulating the
tissue – air interface by slabs of plastic water surrounded by air. The dose was measured at a
fixed depth using an ionization chamber. The scatter loss factor was determined by taking
the ratio of dose at the point of interest with some volume of air near it to the dose without
any air, and was studied as a function of distance of the point of interest from the tissue – air
interface. The preliminary results of this investigation indicate that the convolution algorithm
of the CMS Focus and the pencil beam model of Varian CadPlan TPS predict the scatter loss
reasonably well whereas the Clarkson algorithm of CMS Focus ignores the loss of scatter
due to the presence of air. Work is under progress to study the dependence of scatter loss
with depth by film dosimetry. Results of this investigation will be presented.