Automatic contouring using deformable image registration for tandem-ring or tandem-ovoid brachytherapy

Introduction
To investigate the effectiveness of deformable image registration (DIR)-based automatic contouring for tandem-ring (T-R) or tandem-ovoid (T-O) 3-dimensional computed tomography (CT)-based image-guided brachytherapy (IGBT).

Material and methods
CT images of 28 patients with intact cervical cancer were retrospectively analyzed. Selected group had T-R or T-O insertion for IGBT. Hybrid DIR was performed between first fraction CT and subsequent CTs for IGBT. First IGBT CT images were reference images. All DIRs were performed based on these first IGBT CT scans. Contour similarities between manual and automated segmentations were evaluated with dice similarity coefficient (DSC) score. Mean volumes of the structures were delineated manually and automatically compared. Finally, dosimetric comparisons were performed in order to obtain how contour differences affect the doses to target and organs at risk (OARs).

Results
In general, mean volumes of the automatic contours were larger than manual contours for both T-R and T-O insertions. However, the difference in volume was statistically significant for the small bowel only (p < 0.05 and p < 0.01 for T-R and T-O, respectively). The DSC scores were small for the small bowel and the sigmoid in both applicator sets. When the two different applicator sets were compared, the performance of DIR-based contour propagation for the rectum was worse in T-O compared to T-R application. Dosimetric comparisons showed that volume differences between the manual and propagated contours did not affect dose-volume parameters. The treatment plans based on manually contoured targets also well-covered DIR contours. The average time for DIR was 2.0 ±0.1 minutes per fraction compared to 14.0 ±0.4 minutes in manual contouring (p < 0.001).

Conclusions
DIR-based automatic contouring of the structures seems successful for both the T-R and T-O applications in cervical IGBT. DIR significantly decreased the time for contouring. Our results indicate that automatic contouring in IGBT is safe and time-saving.