Quantitative evaluation of respiratory motion-induced blurring in expiratory-phase CBCT for pancreatic SBRT

Cone-beam computed tomography (CBCT) is widely used in image-guided radiotherapy because it reflects the patient's anatomy immediately before treatment. However, CBCT images of the abdominal organs, including the pancreas, are often degraded by respiratory motion.

To address this limitation, this study quantitatively evaluated expiratory-phase CBCT (EP-CBCT), an acquisition method designed to reduce respiratory motion-induced blurring during stereotactic body radiotherapy (SBRT) for pancreatic cancer.

This retrospective analysis included 10 patients with pancreatic cancer undergoing SBRT at our institution. Daily CT and EP-CBCT scans were acquired on each treatment day. EP-CBCT images were obtained with repeated end-exhalation breath-holds to collect projection data during the expiratory phase. Because of feasibility limitations in routine clinical practice, a direct intra-patient comparison with conventional free-breathing CBCT was not performed. Image quality was evaluated using edge response width (ERW), structural similarity index (SSIM), and a blinded subjective assessment based on mean opinion scores (MOS). A supplementary retrospective comparison with a historical four-dimensional CBCT (4D-CBCT) dataset was also performed.

Differences in ERW between daily CT and EP-CBCT (ΔERW) were evaluated using equivalence testing with a predefined equivalence margin of ± 1 mm. The mean ERW (98% confidence interval) was 2.04 mm (1.95-2.13) for EP-CBCT and 1.73 mm (1.66-1.79) for daily CT. The mean ΔERW was -0.31 mm (-0.40 to -0.23 mm). Equivalence testing demonstrated that EP-CBCT was statistically equivalent to daily CT (p < 0.01). The mean SSIM value was 0.83, indicating high structural similarity between EP-CBCT and daily CT. Subjective image quality assessment showed significantly higher scores for daily CT than for EP-CBCT, whereas EP-CBCT consistently outperformed historical 4D-CBCT, with good interobserver agreement.

These findings suggest that EP-CBCT can reduce respiratory motion-induced blurring to a degree comparable to that of daily CT, supporting its potential utility for anatomical assessment immediately before irradiation in patients undergoing pancreatic SBRT.