Abstract
ObjectiveTo 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 (CTVHR) in image-guided adaptive brachytherapy (IGABT) for locally advanced cervical cancer (LACC).MethodsFive 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 CTVHR (based on the GEC-ESTRO recommendations) was contoured by five investigators on the three modalities (CTVHR_CT, CTVHR_TRUS-CT, and CTVHR_MRI). A consensus reference CTVHR_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.ResultsThe 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 CTVHR_CT volume of all observers was 71% larger than the MRIref volume, whereas the mean CTVHR_TRUS-CT volume was 15% larger.ConclusionHybrid TRUS-CT as an imaging modality for contouring the CTVHR in IGABT for LACC is feasible and reproducible among multiple observers. TRUS-CT substantially reduces overestimation of the CTVHR volume of CT alone while maintaining similar interobserver variability.
Highlights
In image-guided adaptive brachytherapy (IGABT) of locally advanced cervical cancer (LACC), magnetic resonance imaging (MRI) with applicator in place is currently considered as the gold standard for tumor visualization and dose optimization [1–9]
As transrectal ultrasound (TRUS) and computed tomography (CT) are widely available in radiotherapy departments, combining their assets could possibly provide an equivalent for MRI in contouring target volumes in IGABT for LACC
The proposed workflow of hybrid TRUS-CT with optical tracking of the applicator was successfully applied. 3DTRUS imaging, automatic applicator reconstruction, volumetric contouring on TRUS, and image fusion with CT were feasible in all patients
Summary
In image-guided adaptive brachytherapy (IGABT) of locally advanced cervical cancer (LACC), magnetic resonance imaging (MRI) with applicator in place is currently considered as the gold standard for tumor visualization and dose optimization [1–9]. MRI is, not always available in centers with fewer resources, which is why computed tomography (CT) has been evaluated as alternative, as most radiotherapy departments are equipped with a CT scanner [10]. Contouring on CT enables excellent visualization of the applicator and organs at risk (OAR), and generally leads to an overestimation of the target volume compared to MRI [10]. First studies investigating TRUS for target definition in LACC brachytherapy indicated that TRUS appears non-inferior to MRI for assessing the highrisk clinical target volume (CTVHR) dimensions [12, 13]. As TRUS and CT are widely available in radiotherapy departments, combining their assets could possibly provide an equivalent for MRI in contouring target volumes in IGABT for LACC. A clinical workflow for combined TRUS/CT treatment planning in LACC brachytherapy has already been successfully simulated in a patient [14]
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