The purpose of this study was to investigate the changes of bladder volume before therapy of proton beam in magnetic field, and to estimate the target volume (PTV) margin to be considered in treatment planning. Thirty
patients with prostate cancer were included in this study. Three gold fiducial markers were attached to the prostate to confirm the position of the prostate by pretreatment X-ray. The urine was completely emptied 1
hour and 30 minutes before the CT-Simulation, and the urine was confirmed by ultrasonography. The bladder was filled with 500 cc of water. Bladder volume was measured by CT scans after treatment. On the first day of treatment, 30 patients underwent the same pretreatment as in CT-Simulation, and their bladder volume was obtained using ultrasound during the 25th treatment, and compared with the treatment plan. We also investigated daytime volatility during the treatment period. The mean bladder volume ultrasonography mean (BV_USmean) was found to be 302 ± 89 ml (range: 168 ± 166 ml) −553 ml). BV_USmean was 84 % of BV_CTmean. Ultrasound- measured bladder volume was smaller than that measured by CT. The correlation coefficient between BV_USmean and BV_CTmean was 0.187 (p = 0.322). Bland-Altman analysis showed that the BV_CTmean and BV_USmean agreement ranged from −202.17 to 315.83, with an average value of 70. The mean (bias, bias) and standard deviation (precision) of the differences between the two measurements was 56.83 ± 132.14 ml. BV_CTmean and BV_USmean showed a large range of change. BV_USWeekly1-5, which was measured Weeklyly, showed a certain decrease with the treatment time, as compared with BV_CTmean. In conclusion, large changes and differences in bladder volume should always be treated with ultrasound before treatment, and the range of rectal area should coincide. Therefore, excessive radiography is unnecessary.