The impact of bladder and rectal dynamics on prostate and seminal vesicles intrafraction motion and deformation in radiotherapy.

Introduction: Treatment uncertainties influenced by organ intrafraction motion complicate the widespread adoption of hypofractionated radiotherapy. This study aims to identify imaging features on pre-treatment magnetic resonance imaging (MRI) scans that describe prostate and seminal vesicle (SV) intrafraction motion, with the goal of informing and improving treatment planning. Materials/methods: Thirty prostate cancer participants treated on an Elekta Unity 1.5T MR-Linac were recruited, with a series of volumetric MR images acquired pre-, during and post- treatment over multiple fractions. nnU-Net was used to automatically contour the prostate, rectum, SV and bladder. These contours quantified prostate and SV intrafraction motion and enabled extraction of imaging features. A linear regression model assessed relationships between the organs intrafraction motion, treatment margins, and the extracted features. Results: Bladder filling during treatment influenced both SV and prostate intrafraction motion, especially, when baseline bladder volume was <190 mL for both prostate (R Conclusion: This study demonstrated that the baseline bladder volume at start of a treatment fraction predicts for both SV and prostate intrafraction motion, by mediating the effect of bladder filling, and that SV treatment margins could be reduced for a favourably sized bladder. These findings may support refining treatment protocols such as aiming for an initial bladder volume of at least 190 mL.