45. Salvage re-irradiation for local failure of prostate cancer after curative radiotherapy: Dosimetric and radiobiological modelling of rectal toxicity

Abstract

Introduction To assess the impact of radiation dose and clinical parameters on rectal toxicity following salvage external beam radiation therapy (EBRT) with or without a brachytherapy (BT) boost for exclusive local failure after primary-EBRT for prostate cancer. Methods Fourteen patients with no residual toxicity after primary EBRT ( ± BT) were re-irradiated after a median time interval of 6.1 years (4.7–10.2). The median NTD2Gy was 74 Gy (66–98.4) at primary-RT and 85.1 Gy (70–93.4) at re-irradiation. Rectal dose volume histograms of both treatments (converted to NTD2Gy) and the corresponding normal-tissue-complication-probability (NTCP) values for gastro-intestinal (GI) toxicity were calculated. Results The 5-year Grade ⩾3 GI toxicity-free survival rate was 57.1 ± 13.1%. Five patients developed Grade 4 GI toxicity. Rectal V70Gy and the maximum dose to 1 cc of rectum at primary EBRT were both predictive for Grade ⩾3 GI toxicity; 12.2% vs. 3.8%, p = .042 and 72.2 Gy vs. 66.8 Gy, p = .0027, respectively. When adding primary-RT and re-irradiation plans, the median maximum dose to 1 cc of rectum was 139.8 Gy (126.7–147.8) vs. 125.9 Gy (99.1–133.1) (p = .0063) for Grade ⩾3 and Grade ⩽2 GI toxicity groups. Higher NTCP values at primary-RT were predictive for Grade ⩾3 toxicity (p Conclusions A higher rectal NTCP value, even in the absence of high-grade late-toxicity after primary-RT, is correlated with an increased risk of severe rectal side-effects after salvage re-irradiation. Rectum doses greater than 70 Gy at primary-RT, and NTCP values of more than 10%, might predict for grade ⩾3 rectal toxicity at re-irradiation, with a possible threshold for total rectum dose of around 130 Gy.