Hybrid TRUS/CT with optical tracking for target delineation in image-guided adaptive brachytherapy for cervical cancer
Stéphanie Smet, N. Nesvacil, Johannes Knoth, Alina Sturdza, Dina Najjari-Jamal, Filip Jelínek, Gernot Kronreif, Richard Pötter, Joachim Widder, Christian Kirisits, Maximilian Schmid
Abstract
Abstract Objective To prospectively compare the interobserver variability of combined transrectal ultrasound (TRUS)/computed tomography (CT)- vs. CT only- vs. magnetic resonance imaging (MRI) only-based contouring of the high-risk clinical target volume (CTV HR ) in image-guided adaptive brachytherapy (IGABT) for locally advanced cervical cancer (LACC). Methods Five patients with LACC (FIGO stages IIb–IVa) treated with radiochemotherapy and IGABT were included. CT, TRUS, and T2-weighted MRI images were performed after brachytherapy applicator insertion. 3D-TRUS image acquisition was performed with a customized ultrasound stepper device and software. Automatic applicator reconstruction using optical tracking was performed in the TRUS dataset and TRUS and CT images were fused with rigid image registration with the applicator as reference structure. The CTV HR (based on the GEC-ESTRO recommendations) was contoured by five investigators on the three modalities (CTV HR _CT, CTV HR _TRUS-CT, and CTV HR _MRI). A consensus reference CTV HR _MRI (MRIref) was defined for each patient. Descriptive statistics and overlap measures were calculated using RTslicer (SlicerRT Community and Percutaneous Surgery Laboratory, Queen’s University, Canada), comparing contours of every observer with one another and with the MRIref. Results The interobserver coefficient of variation was 0.18 ± 0.05 for CT, 0.10 ± 0.04 for TRUS-CT, and 0.07 ± 0.03 for MRI. Interobserver concordance in relation to the MRIref expressed by the generalized conformity index was 0.75 ± 0.04 for MRI, 0.51 ± 0.10 for TRUS-CT, and 0.48 ± 0.06 for CT. The mean CTV HR _CT volume of all observers was 71% larger than the MRIref volume, whereas the mean CTV HR _TRUS-CT volume was 15% larger. Conclusion Hybrid TRUS-CT as an imaging modality for contouring the CTV HR in IGABT for LACC is feasible and reproducible among multiple observers. TRUS-CT substantially reduces overestimation of the CTV HR volume of CT alone while maintaining similar interobserver variability.