Impact of Interobserver Variability in Volume Delineation on Deformable Image Registration for Prostate

The performance of CT-CBCT (Cone Beam CT) Deformable Image Registration (DIR) for prostate propagation for dose tracking purposes can be improved by including bladder and rectum as guiding structures. Currently, these volumes are manually delineated on CBCT. This study quantified the interobserver variability in delineating bladder and rectum on CBCT and to assess its impact on the propagated prostate. Bladder and rectum were delineated as solid structures by 3 observers on the reference CT and 13 CBCTs from each of 10 prostate patients. Intensity-based DIR was performed 3 times on each CBCT, using the 3 different sets of volumes as additional guiding structures. ProstateDIR was propagated from the reference CT onto CBCTs using the deformation vector fields of the corresponding DIR from each observer for comparison. Mean Distance between Surfaces (DSS) was calculated to assess interobserver variability and its impact on ProstateDIR. A DSS < 2mm was considered as having good agreement among the volumes. A total of 390 ProstateDIR was included for analysis. Interobserver variability in bladder delineation was low, with 96% of DSS < 2mm. However, high variability was noted in the delineation of rectum, with only 35% of DSS < 2mm for one observer. Despite a low agreement in one of the guiding structures for DIR, there was good agreement in ProstateDIR among observers, with ≥ 91% of DSS < 2mm. Upon image review, a DSS of > 2mm for ProstateDIR was attributed to a significant difference in defining the rectum among observers in the anterior direction, where the rectum-prostate interface is. Variation in defining rectum in other directions has no impact on ProstateDIR. Interobserver variability in defining the rectum in the anterior direction has an impact on the geometry of the propagated prostate when rectum is used as a guiding structure for CT-CBCT DIR. Further investigation is needed to assess its impact on estimating the accumulated delivered dose to the prostate.