Dose/volume-based evaluation of the accuracy of deformable image registration for the rectum and bladder

Background and purpose. Deformable image registration (DIR) is a key component of image-guided and adaptive strategies in radiotherapy. DIR based on image intensities alone is promising for online applications, but is challenged in regions with low intensity gradients. In this study we have investigated the performance of intensity-based DIR applied to contour propagation of the rectum and bladder, focusing on the consequences in terms of dose/volume parameters. Material and methods. The rectum and bladder volumes were delineated in the planning computed tomography (pCT) scan and in 8-9 repeat CTs (Vmanual) for nine prostate cancer patients. The volumes from the pCT were propagated onto the repeat CTs using intensity-based DIR (Vprop). Dose/volume parameters for Vmanual and Vprop were derived by dose re-calculations following rigid registration on prostate fiducials. Linear regression was used to identify qualitative and quantitative volumetric measures of the DIR performance being associated with the differences in dose/volume parameters. Results. The median differences in dose/volume parameters assessed for Vprop and Vmanual were modest, but individual differences similar to 7 Gy were seen. The observed differences in dose/volume parameters showed strong correlations to the measures of the DIR performance as well as with the volume variations, most pronounced for the rectum (R-2 = 0.63-0.85; p <= 0.05). Conclusion. Limitations in the intensity-based DIR algorithm resulted in large individual differences in dose/volume parameters between propagated and manually segmented volumes, which were correlated with volumetric measures of the DIR performance.