SU‐GG‐T‐497: Dose‐Response Parameters for Urinary Bladder from Combined Photon and Proton Irradiations in Prostate Cancer Radiotherapy

Purpose: Proton's physical properties offer the maximum radiation dose to the target volume, which is an advantage in the radiotherapy development. The aim of this study is the clinical derivation of the dose‐response relations regarding urinary complications from combined photon and proton prostate radiotherapy. Material and Methods: 189 patients were treated for prostate cancer at Uppsala's Academic Hospital, Sweden from 2002 to 2006. The patients were treated with a proton boost of 20 Gy given in 5 Gy, followed by a X‐ray treatment of 50 Gy in 2 Gy fractions. The dose distribution delivered to the organs of interest and the clinical treatment outcomes (24 months follow‐up time) were available for 72 patients. The delineated regions of interest were the urinary bladder and the region of the lower 3 cm part of the bladder. The photon and proton doses were calculated with the BED (biologically effective dose) concept. Additionally, an RBE of 1.1 was used for protons. The radiobiological parameter acquisition of D₅₀ and γ was performed for the Poisson, Binomial and Probit models applying the Maximum Likelihood method. Results: Of the 72 patients, 15 (21%) showed urinary complications, whereas 57 (79%) didn't. The best estimated values of the parameters for the whole bladder and bladder 3 cm are D₅₀=104.0 Gy, γ =0.7 and D₅₀=88.4 Gy, γ =1.3 for Poisson, D₅₀=108.0 Gy, γ=0.6 and D₅₀=88.6 Gy, γ=1.3 for Binomial, whereas for Probit are D₅₀=97.0 Gy, γ=1.0 and D₅₀=85.6 Gy, γ=1.8, respectively. The analysis of the mean cumulative DVHs proves that the urinary complications were mainly due to the photon treatment. Conclusions: The dose‐response relations are described well by the estimated parameters of the Poisson, Binomial and Probit models. The results are necessary for a prospective estimation of the clinical effectiveness of radiation therapy using combinations of different radiation modalities.