Compensator-based Intensity Modulated Radiotherapy With Telecobalt Machine Using Missing Tissue Approach

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.