Purpose: To describe key commissioning and quality assurance methods for 3D-printed brachytherapy surface applicators.
Methods: Prior to clinical implementation of 3D-printed surface brachytherapy molds, a comprehensive commissioning plan was developed, including tests to determine channel tunnel radius, deliverable radius of curvature, and HU thresholds for accurate body contour creation. QA procedures for patient-specific molds were also established.
Results: Tunnel radius and curvature must be determined to ensure deliverability. Based on catheters and afterloader used, the proper tunnel radius and curvature radius were determined by iterative testing. For 4.7Fr catheters (~1.55mm radius) a tunnel radius of 1.8 mm was too narrow, while a 2mm tunnel radius was compatible. The radius of curvature recommended by the vendor (14 mm), plus larger and smaller radii, were tested for deliverability with the afterloader (Varisource iX, Varian). Plans with radii of curvature <14mm were undeliverable. Body contour directly affects mold fit and must be optimized. HU settings were iteratively adjusted and used to create the body contour for test prints on an anthropomorphic phantom. A window/level of -380HU to +300HU (with hand editing of the surface at this W/L setting) produced molds that fit the phantom with minimal air gaps. This method was utilized on several clinical cases, producing molds with excellent fit (<3mm gap). QA of 3D-printed molds is essential to ensure accurate plan creation and delivery. QA included visual inspection, deliverability of test plans with catheters fixed in place, post-processing of rough patches, and patient CTs with the mold to evaluate both fit and mold integrity.
Conclusion: This report describes commissioning and QA necessary for clinical implementation of 3D-printed surface brachytherapy applicators. These techniques have allowed clinical implementation of deliverable, highly customizable, and well-fitting molds for treatment of complex surface geometries with brachytherapy where commercial surface flaps or handmade molds are often problematic.