Telecobalt Machine Beam Intensity Modulation with Aluminium Compensating Filter Using Missing Tissue Approach

Introduction: The present study aimed to generate intensity-modulated beams with Aluminium compensating filters for a conventional telecobalt machine based on the outputs of a treatment planning system (TPS) performing forward planning and cannot simulate directly the compensating filter. Materials and Methods: In order to achieve the beam intensity modulation during treatment planning with the TPS, we used a bolus placed on the surface of a tissue-equivalent phantom. The treatment plans replicated on the telecobalt machine with the bolus were represented with compensating filters placed at a certain distance from the phantom surface. An equation was proposed for the conversion of the bolus thickness to the compensating filter thickness such that any point within the phantom would receive the planned dose. Correction factors were introduced into the proposed equation to account for the influences of field size, treatment depth, and applied bolus thickness. The proposed equation was obtained based on the analyses of empirical data measured in a full scatter water phantom with and without the compensating filter.  Results: According to the results, the dosimetric verification of the proposed approach outputs in a solid water phantom with calibrated Gafchromic EBT2 films were comparable to that of the TPS with deviation of ±4.73% (mean: 2.98±1.05%). Conclusion: As the findings of the present study indicated, the discrepancy between the measured doses and TPS-estimated doses was within the tolerance of ±5%, which is recommended for dose delivery in external beam radiotherapy. Therefore, the proposed approach is recommended for clinical application.