Quantification and Dosimetric Impact of Normal Organ Motion During Adaptive Radiation Therapy Planning Using a 1.5 Tesla Magnetic Resonance-Equipped Linear Accelerator (MR-Linac).

Purpose: Patients receiving adaptive magnetic resonance guided radiation therapy (MRgRT) undergo contour modification prior to treatment delivery, which takes 15 to over 60 minutes. We hypothesized that during the time required to create an adaptive MRgRT plan, organ movement will result in dosimetric changes to regional organs at risk (OARs). This study quantifies the dosimetric impact of OAR motion during the time required to perform adaptive MRgRT.

Methods And Materials: Thirty-one patients with pancreatic adenocarcinoma, prostate adenocarcinoma, hepatocellular carcinoma, and oligo-metastases who received MRgRT using a 1.5 Tesla MR-Linac were prospectively enrolled in an open registry imaging trial (NCT03500081). Two magnetic resonance imaging (MRI) studies were acquired predelivery for each MRgRT treatment fraction: an initial "pretreatment" MRI (input to the adaptive evaluation with or without recontouring and replanning process), and a second "verification MRI" (acquired after the recontouring and adaption process and immediately before treatment delivery or "beam-on"). On the verification MRI, normal organs were recontoured offline. Recontoured normal organs included the colon, duodenum, small bowel, and stomach. Differences in OARs between organ positions represented the normal organ movement during the time required for plan adaption. Maximum dose (Dmax), volumetric (V) 0.5 cubic centimeter dose (D0.5cc), 3000 cGy (V30), and 2000 cGy (V20) were calculated from the recontoured verification MRI.

Results: Differences in Dmax, per fraction, for the listed normal organs were as follows: colon/rectum 239.50 cGy ( = .09), duodenum 136.40 cGy ( = .05), small bowel 488.27 cGy ( < .01), and stomach 95.92 ( = .17). Small bowel demonstrated a significant difference in Dmax, D0.5cc, and V30.

Conclusions: Statistically significant differences in small bowel doses are demonstrated as a result of motion during the timing required for adaptive MRgRT. These results reflect the importance of verifying MRI acquisition during adaptive MRgRT to confirm the location of OARs. They also identify the necessity of strategies to account for the dynamic nature of regional OARs.