Variability in Bladder Volumes of Full Bladders in Definitive Radiotherapy for Cases of Localized Prostate Cancer

Background and Purpose:To evaluate variation in bladder volume of full bladders in definitive radiotherapy for localized prostatecancer and to investigate potential predictors of increased bladder volume variations.Patients and Methods:In 40 patients, the bladder volume was measured with megavoltage computed tomography (MVCT)imaging performed just before irradiation during the administration of the 1st fraction (#1), the 10th fraction (#10), the 20thfraction (#20), and the 30th fraction (#30). Patients were instructed to avoid urinating for 60-90 minutes before the planningCT (pln-CT) scan and before daily irradiation. Patients were also encouraged to drink an unspecified volume of liquid that wouldresult in a clear but tolerable urge to urinate.Results:The population-mean bladder volume (±1SD) was 219 ml (±83 ml) at the planning CT scan (pln-CT), 186 ml (±96 ml)at #1, 149 ml (±73 ml) at #10, 137 ml (±59 ml) at #20, and 136 ml (±60 ml) at #30. The mean intrapatient variation in bladdervolume (1 SD relative to the mean bladder volume of each patient) was 38% (range: 10-84%). The bladder volume at the pln-CTwas correlated with the intrapatient variance in bladder volume with a correlation coefficient of 0.54 and p <0.001.Conclusion:We observed a significant decline in bladder volumes during the course of radiotherapy. The bladder volume at thepln-CT was a significant predictor of increased bladder volume variations.