2024 - Optimizing Prostate SBRT with an Empty Bladder Regimen: An In-Depth Analysis of Intrafractional Bladder Filling and Dosimetric Outcomes

Purpose/Objective(s): To evaluate the degree and rate of bladder filling with an empty bladder protocol during magnetic resonance imaging–guided linear accelerator (MRL) prostate stereotactic body radiotherapy (SBRT), and to determine the association of bladder filling with intra-fractional prostatic motion requiring positional shifts during therapy. The impact of bladder filling on post-treatment target and normal tissue dosimetry was also evaluated.

Materials/Methods: Sixty-two consecutive prostate SBRT patients treated on the MRL with a five-fraction regimen were evaluated. Bladder filling patterns during each treatment session and the frequency of required shifts to address intra-fractional prostate motion were studied. During each fraction, three MR image acquisitions were obtained: an immediate baseline T2-weighted sequence, a verification sequence after the plan was generated prior to treatment delivery, and a sequence post-treatment. Bladder filling rates were evaluated at these time points for each fraction and across the five treatment fractions. Multivariate analysis identified variables associated with increased bladder filling rates and the likelihood of positional target adjustments of the prostate during real-time adaptive planning. Post-treatment MR structures were used to recalculate plans for analysis of intra-fractional dosimetric variations in target and normal tissue doses.

Results: The median baseline bladder volume at fraction 1 was 88 cc (range 35–245), increasing to 138 cc (range 55–340) at verification MR and 156 cc (range 69–475) post-treatment. Bladder volume increases from baseline to verification MR and from verification MR to post-treatment MR were consistent across the cohort. Multivariate analysis identified the use of alpha receptor blockers during treatment (beta –17.36 mL; 95% CI –32.97, –1.74; p = .030) and lower baseline bladder volume (beta 11.62 mL; 95% CI 4.20, 19.05; p = .002) as significant factors in limiting both absolute bladder volume and the rate of bladder filling during adaptive SBRT fractions. Conversely, the need for a positional target shift at verification MR was associated with larger bladder volume (OR 1.20; 95% CI .98, 1.46; p = .075) and high International Prostate Symptom Score (OR 5.42; 95% CI 1.34, 21.89; p = .018). Post-treatment dosimetric analysis revealed no notable compromises to prostate target coverage (D95Gy median –0.19 Gy, IQR 0.49) or normal tissue constraints.

Conclusion: This analysis of bladder filling dynamics in patients undergoing prostate SBRT with real-time adaptive planning demonstrated predictable bladder filling patterns using an empty bladder regimen. Dose-volume constraints were consistently achieved for both target volumes and normal tissues. The finding that alpha receptor blockers reduced the rate of bladder filling during treatment fractions may have implications for improving treatment consistency and patient comfort in real-time adaptive planning workflows.