Quantification and Dosimetric Impact of Normal Organ Motion During Adaptive Radiation Therapy Plan Verification Using a 1.5 Tesla Magnetic Resonance Equipped Linear Accelerator (MR LINAC). Int J Radiat Oncol Biol Phys 2021 Nov 01;111(3S):S48-S49
Date
10/28/2021Pubmed ID
34700566DOI
10.1016/j.ijrobp.2021.07.130Scopus ID
2-s2.0-85120924793 (requires institutional sign-in at Scopus site)Abstract
PURPOSE/OBJECTIVE(S): Patients receiving adaptive radiation therapy (RT) undergo contour and plan modification prior to treatment delivery. This process takes time to complete, ranging from 15 to 60 minutes or longer. We hypothesize that during the time required to create an adapted RT plan, normal organ movement will result in unanticipated doses delivered to local organs at risk. This study quantifies the dosimetric impact of abdominal organ motion during the time required to create an adaptive RT plan.
MATERIALS/METHODS: Patients receiving 5-fraction treatments with MR guided adaptive RT using a 1.5 Tesla MR LINAC were prospectively enrolled in an ongoing imaging study (NCT03500081). Two sets of MRIs were acquired: the first (pre-beam MRI) used to contour organs at the start of treatment, and the second (verification MRI) used to verify organ position immediately before beam-on. Both MRIs were collected in commercially available software. Each normal organ within 1cm of radiation targets was contoured on the verification MRI. The differences in organ position between the two MRIs represents the normal organ movement during the time required for plan adaptation. Max dose (Dmax), volumetric 0.5 cubic centimeter (cc) dose (D0.5cc), volumetric 3000cGy (V30), and volumetric 2000cGy (V20) were calculated. Mixed model tests analyzing differences between the MRI sets were performed using a data management and decision management software.
RESULTS: Thirty adapt-to-shape patients were included. Recontoured normal organs included colon, duodenum, small bowel, and stomach, for a total of 120 new individually created contour volumes. The table below summarizes the mean differences of Dmax in the normal organs between the MRI sets for various organs at risk. Note that only small bowel demonstrated a statistically significant difference in Dmax doses. The D0.5cc, and V30 were also statistically significant for small bowel.
CONCLUSION: Statistically significant differences in small bowel doses are demonstrated when doses are calculated using the verification MRI. This is likely due to the small bowel's increased mobility compared to other organs analyzed. There is an absence of statistically significant differences in all other abdominal organ doses between these scans. These results reflect that highly mobile structures, such as small bowel, benefit from acquisition of a verification MRI following daily adaptive plan generation. Future research should focus on methods to shorten the adaption duration and/or account for these unanticipated doses, as they represent a significant limitation of adaptive MR guidance.
