Part I | Chapter 4 66 Abstract Background and purpose To evaluate seminal vesicle (SV) intrafraction motion using cinematic magnetic resonance (cine-MR) imaging during the delivery of online adaptive MR-Linac radiotherapy fractions, in preparation of MRI-guided extremely hypofractionated radiotherapy for intermediate to high-risk prostate cancer patients. Materials and methods Fifty prostate cancer patients were treated with 5 x 7.25 Gy on a 1.5 Tesla MR-Linac. 3D Cine-MR imaging was started simultaneously and acquired over the full beam-on period. Intrafractionmotion in this cine-MR was determined for each SV separately with a previously validated soft-tissue contrast-based tracking algorithm. Motion statistics and coverage probability for the SVs and prostate were determined based on the obtained results. Results SV motion was automatically determined during the beam-on period (approximately 10 min) for 247 fractions. SV intrafraction motion shows larger spread than prostate intrafraction motion and increases over time. This difference is especially evident in the anterior and cranial translation directions. Significant difference in rotation about the left-right axis was found, with larger rotation for the SVs than the prostate. Intrafraction coverage probability of 99% can be achieved when using 5 mm isometric expansion for the left and right SV and 3 mm for the prostate. Conclusion This is the first study to investigate SV intrafraction motion during MRI-guided RT sessions on an MRLinac. We have shown that high-quality 3D cine-MR imaging and SV tracking during RT is feasible with beam-on. The tracking method as described may be used as input for a fast re-planning algorithm, which allows for intrafraction plan adaptation.
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