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  4. Dosimetric Advantage of MRI-Guided Online Adaptive Radiation Therapy in Abdomen: Initial Clinical Experience with a High-Field MR-Linac

Dosimetric Advantage of MRI-Guided Online Adaptive Radiation Therapy in Abdomen: Initial Clinical Experience with a High-Field MR-Linac

E Ahunbay1*, E Paulson2 , X Chen3 , W Hall4 , M Straza5 , X Li6 , (1) Medical College of Wisconsin, Milwaukee, WI, (2) Medical College of Wisconsin, Milwaukee, WI, (3) Medical College of Wisconsin, Milwaukee, WI, (4) Medical College of Wisconsin, Milwaukee, WI, (5) Medical College of Wisconsin, Milwaukee, WI,(6) Medical College of Wisconsin, Milwaukee, WI

Presentations

(Monday, 7/15/2019) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 2

Purpose: We have recently implemented MRI-guided online adaptive radiation therapy (MRgOART) for patient treatment. In this study, we retrospectively analyze the dosimetric advantages of the daily MRgOART for abdominal tumors.

Methods: A total 11 daily motion-average MRIs from 4DMRI acquired during daily MRgOART for a representative pancreas and a liver cancer patient treated with SBRT on a MR-Linac (Unity, Elekta) were utilized. The daily contours of targets and organs-at-risk (OARs) were delineated by the physicians in the online setting. Daily adaptive plans were generated online based on daily MRIs with bulk density overriding including the air and bone regions using Adapt-to-Shape (ATS) workflow available in the system. In addition, two plans were created on each daily MRI: (1) a plan using the Adapt-to-Position (ATP) workflow where the plan was optimized based on the reference image with the new isocenter location, and (2) repositioning plan without optimization, mimicking the common practice of IGRT. These plans were compared using clinically relevant dosimetric criteria.

Results: Average on-table time for the MRgOART with ATS workflow was 55 min. The quality of reference plans using the MR-Linac beams (unflat beam, fixed collimator angle, larger projected MLC width, magnetic field present) were practically equivalent to those on conventional Linacs when the same optimization criteria were used. For daily doses, the ATS adaptive plans provided the best target coverage, with average PTV100 of 95%, 89% and 86% for the ATS, IGRT and ATP plans, respectively. The doses to OARs were generally comparable between the three plans, e.g. the maximum dose to colon varied with 2% between the three plans.

Conclusion: The daily MRgOART using the ATS workflow allowing to optimize and deliver the plan based on the anatomy of the day provides the best target coverage and has been practiced in our clinic for abdominal tumors.

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