SU‐GG‐T‐494: Clinically Derived Dose‐Response Relations for Rectum and Penile Bulb from Combined Photon and Proton Prostate Cancer Radiotherapy

Purpose The purpose of this study was the determination of the parameters D₅₀ and γ for the rectum and penile bulb using different radiobiological models, when proton and photon beams are combined. Material and Methods In this study, 98 patients were included with localized adenocarcinoma of the prostate. The patients were treated with a conventional photon beam configuration to a dose of 50 Gy in 2 Gy fractions. Additionally, a proton boost of 20 Gy in 5 Gy fractions was applied. In some patients, the distance between prostate and rectum was increased during proton therapy by retraction of the rectum in the dorsal direction by the use of a cylindrical rod of Perspex inside the rectum. In the calculation of the proton dose, we considered RBE=1.1. The best estimates of the parameters D₅₀ and γ of the Binomial, Poisson and Probit models were determined by applying a maximum likelihood fitting. Results 33 of the patients produced rectal dysfunction (33.7%). The analysis shows that the treatment plans with rectal retraction significantly reduces the dose in the rectum and as a consequence the responders of the treatment (27.3% vs. 36.9% with and without the rod, respectively). The estimated parameters values for the Poisson and Binomial models are D₅₀=61.2Gy (68%CI=57.5–65.2Gy) and γ =0.8 (68%CI=0.6–1.0) for the rectum. The estimated values for the Probit model are D₅₀=60.5Gy (68%CI=56.9–64.1Gy) and γ=1.0 (68%CI=0.7–1.3) for the rectum. Of the 89 patients used in the penile bulb analysis, 25 had sexual dysfunction (28.1%) with mean doses for the responders and non‐responders of 55.5 Gy and 54.2 Gy, respectively. Conclusions The additional use of the cylindrical rod in the proton treatment appears to reduce rectal side effects. The determined parameters can be useful in estimating the expected clinical outcome during treatment planning and perform optimization of the delivered treatment.