Optimal Bladder Volume for Hypofractionated Proton Therapy in Each Localized Prostate Cancer Risk Group

Background This study aimed to determine the optimal bladder volume (BV) for hypofractionated proton therapy in prostate cancer (PC). Materials and methods Two hundred patients with PC were enrolled in this study and classified into low-, intermediate-, and high-risk groups. Treatment planning was performed by assuming a hypofractionated schedule of 63 Gy (relative biological effectiveness) in 21 fractions. The dose indices of the bladder (V₆₀ and V₅₀) were calculated and classified into four groups according to the BV. A cutoff value with a 95% confidence interval was calculated on the basis of the mean and standard deviation of the dose indices. These values were compared with the dose constraints (V₆₀ < 15 % and V₅₀ < 30 %). Results The dose indices were higher in the high-risk group than in the other risk groups. The cutoff value exceeded dose constraints in the low- and intermediate-risk groups with a BV of ≦ 149 cc. Additionally, the cutoff value exceeded the dose constraint in the high-risk group with a BV of ≦ 199 cc. In all the cases, the group with a BV of ≧ 200 cc was below the dose constraint. Conclusions In this study, the relationship between the dose and volume of the bladder in hypofractionated PT for PC was evaluated using a 95% CI to determine the optimal BV. The BV should be changed for each risk group, and a larger BV is required for a high-risk group than for other risk groups.