Radiation and Cancer Physics

SS 37 - Physics 11 - Online Imaging and Motion Management

270 - An End-to-End Verification of Online Adaptation Process on a High-Field MR-Linac

Wednesday, October 24
12:00 PM - 12:10 PM
Location: Room 302

An End-to-End Verification of Online Adaptation Process on a High-Field MR-Linac
E. E. Ahunbay1, X. Chen2, E. S. Paulson2, G. P. Chen2, and A. Li2; 1Medical College of Wisconsin, Department of Radiation Oncology, Milwaukee, WI, 2Medical College of Wisconsin, Milwaukee, WI

Purpose/Objective(s): Online adaptive replanning (OARP) is a major capacity with MR-Linac. OARP process can be complex and should be fully tested before its clinical use. We report a comprehensive end-to-end verification of the entire OARP workflow on a high-field MR-Linac.

Materials/Methods: Flexible OARP options based on the magnitude of interfraction changes are provided on a pre-clinical high-field MR-Linac prototype. Immediately after daily MRI acquisition, the following OARP options can be performed: (1) aperture shifting, shifting segment apertures of the reference plan to primarily correct for translational changes, (2) segment aperture morphing (SAM), adjusting segment shapes to primarily account for anatomic deformation, and (3) warm start optimization (WSO), optimizing segment shapes and weights starting with the reference plan to fully address interfraction variations. These options have been implemented in a pre-clinical planning system (TPS). The adaptive plan generated with one of these options is independently verified using an in-house software, and then transferred to a treatment services system (TSM) that controls plan delivery. We tested the entire process with all three OARP options using a phantom consisting of 3D diode array. The phantom was imaged on MR-Linac with a 0.5cm thick bolus surrounded, allowing it to be visible on MRI. For reference plan, hypothetical contours of target and organs at risk were created on the MRI and various IMRT plans were developed. To mimic the daily MRI, the phantom was imaged with off-center positions and the contours were manually deformed. The shifting, SAM and WSO plans were created and delivered following the OARP workflow. The calculated and delivered doses were compared. To further test the option of shifting, another phantom consisting of multiple ball bearings visible in both MV portal image and MRI was used. Based on the reference MRIs obtained with this MR-MV phantom at off-center positions, reference plans with 2 beams from 0 and 90 degree gantry angles and 6 segment shapes, each surrounding a ball bearing, were generated. The OARP shifting plans based on the “daily” MRI acquired with the phantom centered were then delivered. Another in-house software was developed to analyze the measured and calculated doses and to determine the shifting accuracies based on the alignments of ball bearings on MV images.

Results: All OARP plans were successfully delivered on the MR-Linac. The agreements between the measured and calculated dose distributions of the OARP plans were within the commonly-used acceptable criteria, i.e., with gamma passing rate > 95% for the diode array measurements. The process of using the MR-MV phantom for checking plan shifting was found to be robust and effective to identify shifting errors of sub millimeters.

Conclusion: The entire online adaptive replaning workflow on a MR-Linac was tested and validated successfully. The testing process developed with the MR-MV phantom is robust and may be used in routine QA.

Author Disclosure: E.E. Ahunbay: None. X. Chen: None. E.S. Paulson: Research Grant; Siemens Healthineers. G. Chen: President; North Central Chapter of AAPM. Clinical Director of Medical Physics; Medical College of Wisconsin. A. Li: None.

Ergun Ahunbay, PhD

Medical College of Wisconsin

Disclosure:
Employment
Medical College of Wisconsin: Associate Professor: Employee

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