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

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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: = 0.258). Rectum volume showed no strong correlation with motion. Baseline bladder volume below 332 mL increased the required SV treatment margins to 5.8 mm, compared to 3.5 mm for larger volumes. 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.

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