Background: Despite improvements in radiation techniques, pelvic radiotherapy is responsible for acute and delayed bladder adverse events, defined as radiation cystitis (CysR). About 50% of irradiated patients present acute symptoms and experience a significant decrease in their quality of life (QOL). The pathophysiology of CysR is not well understood, which limits our ability to study this process and develop treatments. The RABBIO study evaluates the correlation of urinary biomarkers with the intensity of acute CysR and the QOL of patients, assessed with the digital telemonitoring platform Cureety. Methods: Patients with intermediate-risk localized prostate cancer eligible for localized radiotherapy were included. Urinary biomarkers were analyzed before the start of radiotherapy, and at weeks (W) 4, 12 and 48 of irradiation. Between sample collection visits, patients completed various questionnaires related to radiation cystitis symptoms and QOL using a digital remote monitoring platform. Upon receipt of the questionnaires, an algorithm processed the information and classify patients according to the severity of symptoms and adverse events reported according to CTCAE. We also collected FACT-P questionnaires at baseline, W4 and W12. We correlated the levels of urinary biomarkers with the severity of acute CysR symptoms and patient-reported QOL. Results: The study started in March 2022, with 15 patients included to date. The median age was 76 (range 65-99). 80% (15/17) presented a localized prostate cancer de novo and 20% presented a biochemical recurrence. The compliance was 100% at baseline, 93% at W4 and 100% at W12. We collected a total of 172 AE questionnaires over the duration of the study so far, of which 61.0% (105/172) indicated a correct health status and 31.4% (54/172) a compromised status. The mean (range) FACT-P score at baseline for all patients was 33.9 (24 – 40), which changed to 29.8 (20 – 35) at W4, and 38.7 (37 – 42) at W12. Elevated SHBG (p=0.019, r=-0.93) and decreased IL8 (p=0.034, r=0.91) at baseline correlated with worse FACT-P score at W4. Conclusions: This prospective study is the first to explore the overexpression of inflammatory proteins in fluid biopsies from patients with symptoms of acute CysR. Our first results revealed that an elevated SHBG and a decreased IL8 in urine at baseline were a prognostic factor in the deterioration of QOL for localized prostate cancer patients during radiotherapy. The results of this study will allow us to develop strategies to limit radiation damage to the bladder and improve the QOL of patients. Clinical trial information: NCT05246774.