Hybrid TRUS/CT with optical tracking for target delineation in image-guided adaptive brachytherapy for cervical cancer

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.