Deformable image registration for contour propagation from CT to cone-beam CT scans in radiotherapy of prostate cancer

Background and purpose. Daily organ motion occurring during the course of radiotherapy in the pelvic region leads to uncertainties in the doses delivered to the tumour and the organs at risk. Motion patterns include both volume and shape changes, calling for deformable image registration (DIR), in approaches involving dose accumulation and adaptation. In this study, we tested the performance of a DIR application for contour propagation from the treatment planning computed tomography (pCT) to repeat cone-beam CTs (CBCTs) for a set of prostate cancer patients. Material and methods. The prostate, rectum and bladder were delineated in the pCT and in six to eight repeat CBCTs for each of five patients. The pCT contours were propagated onto the corresponding CBCT using the Multi-modality Image Registration and Segmentation application, resulting in 36 registrations. Prior to the DIR, a rigid registration was performed. The algorithm used for the DIR was based on a 'demons' algorithm and the performance of it was examined quantitatively using the Dice similarity coefficient (DSC) and qualitatively as visual slice-by-slice scoring by a radiation oncologist grading the deviations in shape and/or distance relative to the anatomy. Results. The average DSC (range) for the DIR over all scans and patients was 0.80 (0.65-0.87) for prostate, 0.77 (0.63-0.87) for rectum and 0.73 (0.34-0.91) for bladder, while the corresponding DSCs for the rigid registrations were 0.77 (0.65-0.86), 0.71 (0.55-0.82) and 0.64 (0.33-0.87). The percentage of propagated contours of good/acceptable quality was 45% for prostate; 20% for rectum and 33% for bladder. For the bladder, there was an association between the average DSC and the different scores of the qualitative evaluation. Conclusions. DIR improved the performance of pelvic organ contour propagation from the pCT to CBCTs as compared to rigid registration only. Still, a large fraction of the propagated rectum and bladder contours were unacceptable. The image quality of the CBCTs was sub-optimal and the usability of CBCTs for dose accumulation and adaptation purposes is therefore likely to benefit from improved image quality and improvements of the DIR algorithm.