Seminal vesicle intrafraction motion during the delivery of radiotherapy sessions on a 1.5 T MR-Linac

Highlights: • First study of seminal vesicle intrafraction motion during MR-Linac RT sessions. • High quality 3D cine-MRI and seminal vesicle tracking is feasible during RT beam-on. • Seminal vesicles show larger intrafraction rotational spread than prostate. • 5 mm isometric expansion provides seminal vesicle coverage probability of 99%. To evaluate seminal vesicle (SV) intrafraction motion using cinematic magnetic resonance imaging (cine-MR) during the delivery of online adaptive MR-Linac radiotherapy fractions, in preparation of MR-guided extremely hypofractionated radiotherapy for intermediate to high-risk prostate cancer patients.