AbstractID: 6661 Title: Film Dosimetry of Mixed Beams
In film dosimetry, a separate H&D curve is used for each beam energy because degree of film
darkening depends on radiation beam modality and energy. Therefore, film dosimetry cannot be
used to evaluate the dose distribution when beams of different energies are used to irradiate the
same target. In this work, a method that resolves this limitation of film dosimetry was
developed. This method was based on comparison of H&D curves for electron and photon
beams. It was found that, for example, 9 MeV electron beam produced ~10% higher optical
density than the 6 MV photon beam for the same delivered dose. This difference was
compensated by artificially increasing dose delivered by the photon beam by the 10% in the
process of creation of H&D curve. This makes H&D curves for both beams virtually identical.
Therefore, the following method of film dosimetry of mixed beams is proposed and tested.
Based on H&D curves, the ratio of optical density produced for the same dose by beams of all
energies and modalities is determined. Then, one of the beams is selected as a reference beam
and dose is delivered as prescribed. Doses delivered by beams of other energies (and/or
modalities) are modified by the ratio of optical densities between the H&D curves of the
reference beam and other beams. Finally, the dose distributions are obtained using the H&D
curve of the reference beam. Thus, this method will allow the verification of treatment plans
using film dosimetry when mixed beams are utilized.