Purpose: To improve the utility of limited field-of-view (LFOV) images acquired for radiotherapy purposes with on-board CT systems Materials and Methods: On-board CT systems can benefit radiotherapy through improved set-up, verification, and dose calculation. However, these benefits are reduced when the CT is LFOV, as peripheral structures are not shown and artifacts are introduced (Fig 1), while dose calculations are impaired. Three methods of compensating for LFOV problems have been identified, and were applied to 17 days of CT data for each of 3 patients: 1) Automatic fusion: The possibility of aligning LFOV images to complete FOV reference images was investigated using new and existing fusion algorithms. 2) Fusion-aligned reprojection (FAR): A new method was developed to improve LFOV treatment images using full FOV references images. 3) Deformable fusion: A method was developed to automatically map regions of treatment images to a planning image for dose accumulation over multiple fractions in light of inter-fraction organ motion. It was tested for complete FOV and FAR images. Results: 1) Automatic fusion by mutual information and extracted feature fusion was found to be generally reliable for patient set-up when the imaged FOV was at least 50% of the patient size. 2) FAR significantly reduced LFOV bowl artifacts and improved ROI conspicuity (Fig 2). Dose calculations based on FAR images were nearly identical to those for full images. 3) Deformable fusion tracked anatomical changes in multi-day prostate CT. Most prominent were changes in the patient outer contour, bladder, and rectum (Fig 2). Deformations were characterized for complete FOV and FAR images. Conclusion: A limited field-of-view can diminish the benefits of on-board CT. However, through 1) appropriate fusion techniques for patient alignment, 2) the FAR method for image correction and improved dose calculation, and 3) deformable fusion for inter-fraction dose accumulation, the utility of LFOV radiotherapy images can be enhanced. View Large Image Figure ViewerDownload (PPT)