Direction Modulated Brachytherapy Tandem Model Applicators for Treatment Planning of Multi-Institutional Cervical Cancer Cases

Abstract

Direction Modulated Brachytherapy (DMBT) offers a means of utilizing an anisotropic source to create more conformal dose distributions when integrated with image-guided adaptive brachytherapy (IGABT). Authors sought to validate the implementation of nine unique six-channeled, MRI-compatible, novel DMBT tandem applicators of varying physical dimensions within Varian's BrachyVision® (v16.1) treatment planning system (BV-TPS). A total of 110 retrospective clinically delivered high-dose-rate (HDR) cervical cancer brachytherapy plans, from three institutions, were re-planned for each of the nine DMBT tandem models within the BV-TPS, using the latest VEGO® inverse optimization algorithm, with dose heterogeneity accounted for through AcurosBV®. Plans consisted of both intracavitary (77 plans) and interstitial (33 plans) cases with an average prescription dose and high-risk clinical target volumes (CTVHR) of 607±113 cGy and 26.96±14.95 [range 6.70-69.58] cm3, respectively. During re-planning, the conventional tandems were replaced by one of the nine DMBT tandem models while leaving ovoids or rings, and needles (if present), in place. A two-step inverse optimization process was performed such that the lowest possible organs at risk (OAR) D2cc doses could be achieved while 1) keeping equivalent target coverage (ΔCTVHR-D90 to within ±0.5%) and, at the same time, 2) maintaining the general pear-shape dose distribution of the original plans. Noteworthy improvements in plan quality were achieved by all nine DMBT tandem models, which are presented in Table 1. Irrespective of the model, about ∼50 cGy reduction in D2cc across all OARs appear feasible. There is also a general trend of D2cc reductions' magnitude becoming smaller as the CTVHR volume increased due to loss in modulation at distance. Additionally, D2cc reductions in terms of EQD2 [Gy] were calculated assuming each re-plan was delivered throughout the course of treatment, which includes the external beam radiotherapy dose of 45 Gy and showed significant reductions of -6.29±4.38 Gy, -3.80±2.06 Gy, and -4.86±3.02 Gy for the bladder, rectum, and sigmoid, respectively, for DMBT model #9 for example. We have successfully incorporated nine DMBT tandem models into a commercial TPS and re-planned 110 cases, to a total of 990 plans. All nine DMBT tandem models were each able to generate notable D2cc reductions to OARs (∼50 cGy), without compromising target coverage, across plans from multiple institutions with various clinical/optimization practices. The results indicate both a promising impact and smooth integration of DMBT tandem technology into modern clinical IGABT workflow.