The Montreal split ring applicator: Towards highly adaptive gynecological brachytherapy using 3D-printed biocompatible patient-specific interstitial caps

Purpose: The addition of interstitial (IS) needles to intra-cavitary (IC) brachytherapy applicators is associated with improved outcomes in locally advanced cervical cancers involving parametrial tumor extensions. The purpose of this work was to validate a clinical workflow involving 3D-printed caps for a commercial IC split ring applicator that enable using IS needle trajectories tailored to each treatment. Material and methods: A dedicated software module was developed in this work allowing users to design patient-specific IS caps without knowledge of computer-aided design (CAD) software. This software module was integrated to 3D Brachy, a commercial software developed by Adaptiiv Medical Technologies Inc. For validation of the workflow, CAD models of ground truth caps with five IS needle trajectories were designed with Fusion 360TM, 3D-printed, assembled with a split ring applicator, and CT-scanned with radio-opaque markers. 3D Brachy was then applied to generate a replica based on trajectories reconstructed from the radio-opaque markers. A comparison between ground truth and replicated IS needle trajectories was done using intersection points with planes at the level of the cervix (z = 0 cm) and a representative needle depth (z = 3 cm). Results: Prototypes of interstitial caps 3D-printed in both BioMed Amber and BioMed Clear SLA resins were tested to be functional both pre- and post-sterilization for IS needles with obliquity angles <= 45 degrees. Distance-to-agreement at z = 0 cm and 3 cm as well as deviations in pitch and yaw angles of the five IS needle trajectories were found to have mean values of 3.3 +/- 2.1 mm, 7.3 +/- 2.0 mm, 2.9 degrees +/- 2.3 degrees, and 7.0 degrees +/- 7.0 degrees, respectively. Conclusions: The clinical workflow for image-guided adaptive cervical cancer brachytherapy using the Montreal split ring applicator was validated.