2070 - Toward Online Intrafraction Adaptation Using Sequential Beam on Volumetric Imaging for MR-Guided Online Adaptive Prostate SBRT

Purpose/Objective(s): Radiotherapy plan quality can be degraded by intrafraction motion, which may have a significant dosimetric impact for hypofractionated regimens. This study evaluates the feasibility of online intrafraction adaptation for MR-guided prostate SBRT and explores the role of beam angle delivery sequencing in enhancing the efficacy of intrafraction adaptation.

Materials/Methods: A retrospectively study was performed on a prostate SBRT patient treated on an MR linac with adapt-to-shape workflow followed by adapt-to-position for baseline shift correction. Clinical beam sequencing ran consecutively from -160 to 168 degrees. Sequential 3D-MRIs acquired during beam-on (BO-MRIs) were associated with the radiation delivery using a custom script that split the daily plan into partial plans based on the MRL logs. Daily structures were deformably propagated from the pre-beam MRI (PB-MRI) to the BO-MRIs in a research version of treatment planning system and used for Monte Carlo calculations of the partial plans on the BO-MRIs. These partial doses were subsequently deformably mapped and accumulated onto the last acquired BO-MRI. Two strategies for online intrafraction adaptation were studied: (1) using the cumulative dose as a bias to generate an adaptive plan with the three remaining clinically sequenced beams left to be delivered during the last BO-MRI acquisition, and (2) simulating the treatment using a new beam sequence in which radiation is delivered every fourth angle in the original beam sequence, with the calculated and mapped dose used as a bias to generate the online adaptive plan with the remaining three beams. Dose-volume histograms (DVH) metrics were compared for no intrafraction adaptation and scenarios 1 and 2.

Results: A comparison of DVH metrics is shown in the table. Without adaptation, a loss in PTV coverage was observed due to intrafractional motion. In adaptive approach 1, PTV coverage was not fully restored without overdosing organs at risk, mainly due to the limited separation of the three remaining beams in the original delivery sequence. More efficient adaptation was achieved with approach 2, utilizing the proposed delivery sequence.

Conclusion: Dose reconstruction for MR-guided prostate SBRT highlighted the importance of incorporating adaptive strategies to mitigate the impact of intrafractional motion. Simulation of online intrafraction adaptation based on the delivered dose was feasible with the proposed workflow. Optimizing beam angle sequencing is crucial for efficiently restoring PTV coverage while sparing organs at risk.