Impact of the locking function of hybrid inverse planning optimization on the treatment plan quality of intracavitary/interstitial brachytherapy for locally advanced cervical cancer
To investigate the dosimetric characteristics of intracavitary/interstitial brachytherapy (IC/ISBT) plans created via the hybrid inverse planning optimization (HIPO) algorithm with the dwell time Lock function. Sixteen patients with locally advanced cervical cancer treated with high-dose-rate IC/ISBT were evaluated. Based on the clinical plan data, five plans were retrospectively created: Manchester-based HIPO for needles, HIPO for all applicators, HIPO for needles after HIPO for tandem/ovoid, HIPO for ovoid after HIPO for tandem/needle, and HIPO for ovoid/needle after HIPO for tandem. The target coverage, organs at risk (OARs) doses, therapeutic ratios, and the dwell time contributions of the needles were analyzed to evaluate the plan quality. Dice similarity coefficients (DSCs) between clinical plan and each created plan were calculated to evaluate the similarity of the shape of the dose distribution. All plans created using HIPO had a sufficient target coverage, while the OAR dose for the Manchester-based HIPO plans was considerably higher than the other plans. The plan with HIPO for all applicators and with HIPO for the ovoid applicator after HIPO for tandem/needles had comparable or superior therapeutic ratios than those of the clinical plan while the dwell time contributions of the needle were much larger. For DSCs, an intermediate to low correlation was observed between the clinical plan and all HIPO plans. The HIPO algorithm could create high-quality IC/ISBT plans, although the dosimetric consequences were affected by the locking function.