Advances in radiation oncology · 2026
Optical Flow-Guided Analysis of Intrafractional Anatomical Variations in Pancreatic Tumors and Organs at Risk During Magnetic Resonance-Guided Adaptive Radiation Therapy Under Abdominal Compression.
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Adachi Takanori, Mukumoto Nobutaka, Inokuchi Haruo, Mukumoto Naoki, Hamaura Nobunari, Itoyama Hiroshige, Hata Yuki, Yamagishi Mutsumi, Sakagami Mai, Hayashi Kenji, Ogino Ryo, Nakamura Mitsuhiro, Shibuya Keiko
Аннотация
PURPOSE: This study aimed to investigate intrafractional anatomic variations in pancreatic tumors and organs at risk (OARs) during magnetic resonance-guided adaptive radiation therapy (MRgART) using optical flow calculations.
METHODS AND MATERIALS: This study included 732,612 magnetic resonance (MR) slices from 345 fractions obtained from 25 consecutive patients with pancreatic cancer. All patients underwent MRgART using the Elekta Unity MR-Linac under abdominal compression. For each fraction, single-slice 2-dimensional cine MR images were acquired every 600 ms at the tumor center. On the first frames in the coronal and sagittal planes, the OARs (duodenum, stomach, small bowel, and colon) were delineated. Displacements were measured at 30-second intervals using the Farnebäck optical flow in the superior-inferior (SI) direction in the coronal and sagittal planes within the overlapping regions between each delineated structure and the rectangular region, with a side length of 10 cm centered on the tumor. To exclude outliers, range of motion in each direction was defined as the 2.5th-97.5th percentile of displacement, and statistical differences were assessed using the Wilcoxon signed-rank test with Holm-Bonferroni correction (P < .05).
RESULTS: The median SI range of motion of the tumor was 5.8 mm in both the coronal and sagittal planes, which was significantly smaller than that of all OARs (P < .05). Among the OARs, the colon and small bowel (median, 8.9-10.2 mm) exhibited larger motion ranges than the duodenum and stomach (median, 6.3-7.6 mm; P < .05). The calculated margins in the SI direction were 3.1 to 3.4 mm for the tumor and 2.1 to 2.9 mm for the OARs.
CONCLUSIONS: Optical flow-guided analysis of intrafractional variations revealed that pancreatic tumors exhibited smaller ranges of motion than the adjacent OARs. These findings warrant direction-specific margins that account for intrafractional anatomic variations in both the tumors and surrounding OARs during pancreatic MRgART under abdominal compression.
METHODS AND MATERIALS: This study included 732,612 magnetic resonance (MR) slices from 345 fractions obtained from 25 consecutive patients with pancreatic cancer. All patients underwent MRgART using the Elekta Unity MR-Linac under abdominal compression. For each fraction, single-slice 2-dimensional cine MR images were acquired every 600 ms at the tumor center. On the first frames in the coronal and sagittal planes, the OARs (duodenum, stomach, small bowel, and colon) were delineated. Displacements were measured at 30-second intervals using the Farnebäck optical flow in the superior-inferior (SI) direction in the coronal and sagittal planes within the overlapping regions between each delineated structure and the rectangular region, with a side length of 10 cm centered on the tumor. To exclude outliers, range of motion in each direction was defined as the 2.5th-97.5th percentile of displacement, and statistical differences were assessed using the Wilcoxon signed-rank test with Holm-Bonferroni correction (P < .05).
RESULTS: The median SI range of motion of the tumor was 5.8 mm in both the coronal and sagittal planes, which was significantly smaller than that of all OARs (P < .05). Among the OARs, the colon and small bowel (median, 8.9-10.2 mm) exhibited larger motion ranges than the duodenum and stomach (median, 6.3-7.6 mm; P < .05). The calculated margins in the SI direction were 3.1 to 3.4 mm for the tumor and 2.1 to 2.9 mm for the OARs.
CONCLUSIONS: Optical flow-guided analysis of intrafractional variations revealed that pancreatic tumors exhibited smaller ranges of motion than the adjacent OARs. These findings warrant direction-specific margins that account for intrafractional anatomic variations in both the tumors and surrounding OARs during pancreatic MRgART under abdominal compression.