Marieke van Son

125 TOXICITY AFTER ULTRAFOCAL SALVAGE HDR-BRACHYTHERAPY regression was performed to model development of grade ≥2 toxicity over time, with potential risk factors included as fixed effects and a random effect per patient and per follow-up time point. In this multivariable model, odds ratios and their 95% confidence intervals (CI) were calculated to assess the independent effect of each risk factor on the outcome, with p-values <0.05 considered statistically significant. Statistical analyseswere performedwith R statistical software (version 3.5.1; the R foun- dation for Statistical Computing, Vienna, Austria) and IBM SPSS statistics (version 23.0). RESULTS Table 1 summarizes baseline characteristics. Most patients were primarily treated with EBRT or low-dose-rate (LDR)-BT, with 20% receiving (neo)-adjuvant ADT in the primary setting. A small group (<5%) had already received a previous salvage treat- ment. Median interval between primary treatment and current salvage treatment was 8 years. Seven patients presented with a solitary lymph node or bone metastasis for which they received upfront stereotactic radiotherapy. Baseline GU and GI toxicity was limited to 12% and <2% grade 2 toxicity, respectively. Approximately half of all patients had grade ≥2 ED at baseline. Dosimetry constraints were adhered to in 83% of patients, with maximum outliers to 18.5 Gy (urethra D10%), 14.5 Gy (bladder D1cc) and 12.6 Gy (rectum D1cc). Median follow-up time was 20 months (IQR 12–31). Table 1 – Patient and treatment characteristics (n=150) Median (IQR) or number (%) Missing (%) Age (years) 72 (68 – 75) Primary treatment EBRT LDR-BT Whole-gland HDR-BT Ultrafocal HDR-BT 80 (53.3%) 67 (44.7%) 2 (1.3%) 1 (0.7%) History of ADT* No Neo-adjuvant Adjuvant 120 (80%) 8 (5.3%) 22 (14.7%) Previous salvage treatment No Whole-gland LDR-BT Ultrafocal HDR-BT 143 (95.3%) 4 (2.7%) 3 (2%) History of TURP 10 (6.7%) Interval primary–salvage treatment (years) 8 (5.3 – 10.7) TNM-stage on imaging T T2 T3 T4 85 (56.7%) 63 (42%) 2 (1.3%) N N0 N1 146 (97.3%) 4 (2.7%) M M0 M1 147 (98%) 3 (2%) 7