Bladder Filling Kinetics and Intrafraction Prostate Motion: Implications for MR-Guided Adaptive SBRT Scan Timing
Abstract
Purpose
Maintaining a full bladder during prostate radiotherapy helps reduce bladder toxicity, but drinking before treatment leads to continuous bladder filling during adaptive planning, potentially displacing the prostate. This study characterizes bladder filling kinetics and associated intrafraction prostate motion to identify optimal MR-adaptive planning timing on the Unity MR-Linac.
Methods
Intrafraction variability was analyzed in three prostate SBRT patients. For each of their five fractions, two to four T2-weighted MRI scans were acquired. Prostate motion was quantified using translation-only rigid registration between the initial daily MRI and subsequent intra-fraction scans; rotation was not considered due to treatment constraints of the Unity platform. Pearson correlation analysis evaluated relationships between bladder filling and prostate displacement in superior-inferior, anterior-posterior, and left-right directions. Dosimetric impact was evaluated by recalculating the daily adaptive plan on each intrafraction scan in Monaco.
Results
Bladder filling increased linearly with time for individual patients, allowing prediction of intrafraction volume using the first fraction fill rate and starting daily bladder volume. Increased bladder volume produced a systematic inferior prostate shift, with shift magnitude correlating to the % of maximum bladder volume (per-patient range 0.54 to 0.68). Recalculated plans showed no meaningful degradation in prostate coverage; however, the maximum dose to the bladder increased by an average of 3.5% (0.9%, 6.1%) and by 5.3% (3.0%, 7.7%) when using only the worst intrafraction scenario for each fraction.
Conclusion
Bladder filling follows a predictable, patient-specific linear trajectory that drives an inferior prostate shift. These findings support a personalized workflow where scan timing is optimized based on initial daily bladder volume and MR simulation fill rate. Although the intrafraction motion did not significantly drop prostate coverage, patients with rapid bladder filling may benefit from a delay before the adaptive planning scan to allow the bladder to stabilize at near-full volume.