A Dosimetric Evaluation of Threshold Bladder Volumes for Prostate Cancer Radiotherapy.

Background: An interfraction variation in bladder filling results in uncertainties of dose received and also has workflow implications for busy departments. This study aims to examine the dosimetric impact of a reduced bladder volume while determining a suitable threshold for treatment.

Materials And Methods: A total of 15 definitive prostate patients were included for this retrospective dosimetry study. Each patient was planned to receive 80 Gy in 40 fractions using intensity-modulated radiation therapy. For each patient, a series of shrunken bladder volumes were created in 50-mL increments. The volume of bladder receiving 65 Gy (V65), 70 Gy, 75 Gy, and 80 Gy for each "shrunken" bladder volume were analyzed with paired samples t-tests. The effect of the shrunken volume relative to the established dose-volume constraint (DVC) was then assessed using single sample t-tests.

Results: The mean planning bladder volume was 345.01 ± 138.51 mL. Under maximum bladder shrinkage, mean difference between the percentage dose received and each DVC was seen to be statistically significant (P < .05). However, for the majority of patients, DVCs were only violated once the bladder volume shrunk to less than 150 mL. On average, the DVCs were violated once the bladder volume fell below 150 mL for the V75 and V80 constraints, with no violations noted for V65 and V70.

Conclusion: Even under exacerbated bladder shrinkage, bladder DVC violations were found to be rare. A bladder threshold of 150 mL would prove sufficient to meet bladder DVCs in over 90% of patients; however, case-by-case assessment is required to ensure patient suitability.