Abstract:
ABSTRACT
The present study sought to generate intensity-modulated beams with
compensating filter, constructed from medium density materials, for a
conventional telecobalt machine, based on dose distributions generated with a
treatment planning system (TPS) performing forward planning, and cannot
directly simulate a compensating filter. Bolus with varying thicknesses placed
on the surface of a tissue equivalent phantom were used to achieve beam
intensity modulation during treatment planning with the treatment planning
system, and the treatment plans replicated on the telecobalt machine with the
bolus represented with compensating filters placed at certain distances from
the phantom surface. A semi-empirical equation, which could account for the
influences of treatment parameters, was developed for converting a bolus
thickness to a compensating filter thickness such that dose at any point within
the phantom would be the same as planned. The equation was established by
acquiring beam data along the beam central axis in a full scatter water
phantom for various irradiation geometries with and without a compensating
filter within beams from the telecobalt machine, such that the heights of water
within the phantom adjusted to get the same dose as before for respective
measurements. Dosimetric verification of outputs of the developed approach
in a solid water phantom with calibrated Gafchromic EBT2 films were found
to be comparable to those of the treatment planning system with deviations
less or equal to ± 3.00% (mean of ±(2.22 ± 0.68)%). The use of the proposed
approach for clinical application is recommended, and could facilitate the
generation of intensity-modulated beams with limited resources using the
missing tissue approach rendering encouraging results.
