Tissue Compensation Approach with Perspex Slab for External Beam Radiotherapy
Résumé
Skin sparing is retained by representing bolus for tissue deficit compensation with a compensator which is placed at certain distance from the skin. Owing to the position of the compensator, the shape of the compensator needs to be adjusted to account for beam divergence and reduction in scattered radiation contribution to dose at any point within the patient, such that dose distribution within the patient would be the same as that obtained with the bolus. Procedures for determining the shape and constructing Perspex (Polymethylmethaneacrylate) missing tissue compensators had been outlined for a telecobalt machine based on outputs of a forward planning treatment planning system, used to simulate irradiation geometries with boluses. The proposed approach considered the influences of treatment parameters: field size, treatment depth and bolus thickness to be applied in the determination of the required compensator material thickness along a particular ray-line. A semi-empirical algorithm based on beam data measured in full scatter water phantom with and without the compensator material, was proposed for the conversion of a bolus thickness to compensator material thickness such the dose distribution within a patient remain the same. Outputs of the proposed approach compared favourably well with those of the treatment planning system with discrepancies less than or equal to ± 3% (mean of 1.99 ± 0.64 %). The use of the proposed approach for clinical application is recommended.
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