ESTRO 2021 Abstract Book

S394

ESTRO 2021

Heide 7 , L.G. Kerkmeijer 8 1 UMC Utrecht, Radiation Oncology, Utrecht, The Netherlands; 2 Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands; 3 UMC Utrecht, Julius Center for Health Sciences and Primary Care, Utrecht University, Utrecht, The Netherlands; 4 Radboud University Medical Center, Radiation Oncology, Nijmegen, The Netherlands; 5 University Hospitals Leuven, Radiation Oncology, Leuven, Belgium; 6 UMC Utrecht, Imaging Division, Utrecht, The Netherlands; 7 Netherlands Cancer Institute, Radiation Oncology , Amsterdam, The Netherlands; 8 UMC Utrecht and Radboud University Medical Center, Radiation Oncology, Utrecht and Nijmegen, The Netherlands Purpose or Objective The primary results of our phase 3 multicenter randomized controlled FLAME trial showed that by adding a focal boost up to 95 Gy to conventional fractionated EBRT in the treatment of localized prostate cancer, the five-year biochemical disease-free survival improved significantly, without significantly increasing cumulative toxicity. The aim of the present study was to investigate the association between radiation dose to the bladder and urethra and late genitourinary (GU) toxicity grade ≥2 in the whole study cohort, irrespective of randomization arm. Materials and Methods The dose-effect relations of the urethra and bladder near maximum dose and GU toxicity grade ≥2 up to five years after treatment were assessed. Patients with a previous transurethral resection of the prostate were excluded from the analysis. While for the bladder and other organs at risk, dose constraints were used for treatment planning, a urethra dose constraint was not incorporated. A mixed model analysis for repeated measurements was used, adjusting for age, diabetes mellitus, T-stage, baseline GU toxicity grade ≥1 and institute. Additionally, the association between the dose and separate GU toxicity endpoints was investigated. Results With a median follow-up of 72 months (interquartile range (IQR) 58-86), acute and late cumulative GU toxicity grade ≥2 up to five years was seen in 45% and 31% of patients, respectively (Table 1). Urethral strictures occurred in 20 patients (4%). The median planned dose to the D2cm 3 of the bladder and the D0.1cm 3 of the urethra was 75 Gy (IQR 74-76) and 80 Gy (IQR 78-87), respectively. A total of 480 patients were included for further analysis. Dose-effect relations were observed for the bladder and urethra dose, with adjusted odds ratios of 1.14 (95% CI 1.12-1.16, p<0.0001) and 1.12 (95% CI 1.11-1.14, p<0.0001), respectively. For the subdomains urinary frequency, urinary retention and urinary incontinence, associations between both the dose to the urethra and bladder and the endpoints were observed (Table 1). Dose-effect curves are shown for both dose parameters (Figure 1).