ESTRO 2025 - Abstract Book

S4148

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2025

Maintaining consistent bladder volume throughout the treatment can be challenging. This study aimed to assess bladder filling variability during VMAT radiotherapy for prostate cancer.

Material/Methods: Forty patients referred for definitive prostate cancer radiotherapy were prospectively enrolled in this study. Patients were planned for CBCT-guided VMAT with a prescription dose of 76 Gy in 38 fractions. Prior to planification and to each treatment session, patients were instructed to follow the following preparation protocol: Bladder and rectum emptying followed by 500 to 750 ml of water intake until full bladder sensation was reached. Daily CBCTs were performed for image guidance. Bladders were contoured on each CBCT. For each patient, CBCT bladder volumes were recorded and compared to baseline planning CT volumes. Bladder volume variability was assessed. ANOVA test was performed to investigate a potential trend in bladder volume variability throughout radiotherapy sessions. Results: A total of 1520 CBCTs were assessed. Mean planning CT bladder volume was 344.8 ± 146.9 ml. Changes in bladder volumes were substantial with mean bladder volume variability evaluated at -120.7 ml (-30%) ranging from -355.3 ml (-58.6%) to +18.3 ml (+10.2%). ANOVA analysis revealed a distinct trend in bladder volume variability over the course of radiotherapy sessions. An increase in volume variability was observed in the first 10 sessions. Between the 10 th and the 20 th session, volumes seemed to stabilize. A significant rise was noted around the 20 th session followed by a gradual decline.

Conclusion: Our study showed high bladder volume variability over the course of prostate radiotherapy. The volume changes observed followed a distinct pattern throughout treatment sessions. A better understanding of the factors influencing these volume fluctuations is needed.

Keywords: Prostate cancer, VMAT, CBCT, bladder volume