Technical Note: Commissioning of an ultrasound-compatible surrogate vaginal cylinder for transvaginal ultrasound-based gynecologic high-dose-rate brachytherapy

Purpose To provide a comprehensive set of commissioning tests for clinical implementation of three-dimensional transvaginal ultrasound (3D TVUS) as a replacement of computed tomography (CT) for applicator reconstruction in gynecologic (GYN) intracavitary high-dose-rate brachytherapy (HDRBT) with a multi-channel vaginal cylinder (MCVC). Methods We introduce an ultrasound-compatible "surrogate" vaginal cylinder (SVC) for reconstruction of Elekta's CT-MR Multi Channel Applicator (MCVC) in 3D TVUS. The MCVC is digitized over the SVC in 3DUS using digital library model overlay. Consulting guidelines from various sources (CPQR, GEC-ESTRO, AAPM), we identify and describe three tests specific to commissioning the SVC: (1) verification of SVC outer dimensions, (2) source position accuracy of MCVC digitization over the SVC in 3D TVUS, and (3) MRI/US registration error. Results The SVC outer dimensions (diameter and A-D marker locations) were well matched to the MCVC, however a 0.6 mm discrepancy in length between cylinder tips was observed. Source position accuracy was within 1 mm (tolerance recommended by CPQR) when reconstructing the MCVC in 3D TVUS. Dice similarity coefficients and target registration error for MRI/3D TVUS registration was similar to MRI/CT registration, which is the clinical standard. Conclusions These commissioning tests are performed using institutional equipment but provide the framework for any practitioner to repeat in their own setup, to demonstrate safe adoption of the 3D TVUS system for patient treatments. We demonstrate that MRI/US-based workflow achieves similar source position accuracy and image registration error as standard MRI/CT, which is consistent with standard tolerances. This is a critical step toward replacement of CT with US in GYN HDRBT treatments with the MCVC.