OPTIMIZED LOW-DOSE RATE PELLET CONFIGURATIONS FOR INTRAVAGINAL BRACHYTHERAPY

Purpose: The objective was to identify configurations of low dose rate pellet sources that optimize short treatment length brachytherapy dos e distributions for a set of four intravaginal applicators. Methods an d Materials: The method of simulated annealing was used. Dose rates at calculation points on the surface of 2.0, 2.5, 3.0, and 3.5 cm diamet er applicators along a fixed 3.0 cm treatment length were optimized fo r Cs-137sources of strengths 0.74 and 0.63 GBq in trains having maximu m lengths of 3.0, 3.5 and 4.0 cm. Variations in the optimization algor ithm involving two different objective functions and different combina tions of selectable parameters were investigated in an effort to stand ardize the approach. Results: An objective function based on the maxim um dose rate difference at the calculation points in conjunction with a single parameter set proved suitable for all applicators. Optimized solutions involving both a single configuration of sources and a combi nation of two such configurations were successfully identified. The la tter consistently afforded superior dose rate uniformity, particularly for the smaller diameter applicators. A maximum source train length o f 3.5 cm was found to provide a good compromise between attaining dose rate uniformity along the 3.0 cm treatment length and minimizing irra diation of adjacent normal tissues. For each applicator, an optimized 3.5 cm pellet train yielded better surface dose rate uniformity than a corresponding optimum-length linear source. Conclusion: Pellet config urations that optimize dose distributions for intravaginal brachythera py applicators can be reliably identified with modest computational ef fort using the method of simulated annealing. The method is therefore suitable for use in routine clinical treatment planning for this site.