Session Item

The steep dose gradients in cervical cancer HDR brachytherapy (BT) necessitate a thorough understanding of the behaviour of the source cable in applicator channels. Whereas multiple studies have quantified source positioning accuracy, the influence of applicator design parameters on source cable behaviour has not been investigated. Moreover, it is also unknown how these design parameters impinge on the ease and accuracy of catheter insertions in hybrid intracavitary-interstitial (IC/IS) applicators. The purpose of this study is to develop and validate comprehensible computer models to simulate: (1) HDR BT source paths, and (2) insertion forces of catheters in curved applicator channels. These models can aid the development of novel (3D-printed) BT applicators and improve the accuracy of source path models in current applicator libraries.

Two types of interactions were modelled: (1) Flexitron source cable (Elekta, Stockholm, Sweden) positioning in CT/MR ring applicators (Elekta, diameters: Ø26, Ø30 and Ø34 mm, angles: 45° and 60°), and (2) ProGuide 6F catheter with obturator (Elekta) insertion in S-shaped channels with varying design parameters (curvature, geometric torsion, and clearance). Instruments were modelled as an interconnected series of flexible beam elements or rigid beam elements connected through revolute joints with springs. For evaluating the source cable models, the simulated source paths were compared with centreline data and the source paths provided by the manufacturer. Predicted catheter insertion forces were compared with force measurements in dedicated templates, produced with common 3D-printing methods for medical devices: digital light processing (DLP) and selective laser sintering (SLS).

The simulations illustrate curving and snaking of the BT source cable in applicator channels. Maximum differences between dwell positions of the simulated source path and centreline data were observed for the most distal dwell position and varied between 4.0-6.4 mm in ring applicators of different sizes. Simulated paths were in closer agreement with manufacturer-specified paths, with maximum differences of 0.7-1.4 mm in the distal dwell position (Figure 1). Insertion force simulation results for BT catheters were in close agreement with the experimental results for all channel design parameters (Figure 2), and predicted peak forces were within 25% accuracy. Accuracy of simulated force characteristics can be improved by incorporating friction coefficient measurements.

The developed models show promising results in predicting the behaviour of flexible instruments in BT applicators. Insights from these models can aid novel applicator design with improved motion and force transmission of BT instruments. Moreover, the presented methodology may be extended to study other applicator geometries, flexible instruments -including different source cables, marker wires or sensors-, and afterloading techniques.