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

Abstract

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 (R² = 0.142) and SV (R² = 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.

Keywords: Bladder; Image guided radiation therapy; Intrafraction motion; MRI; MRgRT; Magnetic resonance imaging; Prostate; Prostate cancer; Radiation therapy; Rectum; Seminal vesicles.

Graphical abstract

Fig. 1

Responsiveness of Prostate (Left) and SV (Right) Motion to Change in Bladder Volume (y-axis), against Baseline Bladder Volume (x-axis). The y-axis of this figure represents change of organ motion due to variations in bladder volume during a fraction, while the x-axis denotes the bladder volume at beginning of a fraction. The blue vertical line indicates the identified volume threshold, highlighting that the moderation effect is more pronounced when baseline bladder volume is below this threshold. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Responsiveness of Prostate (Left) and SV (Right) Motion to Change in Bladder Volume (y-axis), against Fat Thickness Between Prostate-Rectum (mm) at Start of Fraction (x-axis). The y-axis of this figure represents change of organ motion due to variations in bladder volume during a fraction, while the x-axis denotes the fat thickness between the prostate and rectum at beginning of a fraction. The blue vertical line indicates the identified threshold for this baseline features. The left panel of this plot suggests a potentially meaningful, but currently inconclusive, moderation effect on prostate motion responsivity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Prostate & seminal vesicles treatment margin being influenced by baseline bladder volume at start of fraction below and above the determined threshold (vertical dotted line). The horizontal line of this plot denotes the PTV margins required to achieve 95% coverage across all the treatment fractions.