TPS601 Background: Radical cystectomy (RC) is the standard of care for patients with T2-T4a MIBC. Organ-preserving treatment is an alternative for patients who wish to attempt bladder preservation therapy or are ineligible for cystectomy. This includes maximal transurethral resection of bladder tumour, external beam radiation therapy (EBRT) and concurrent chemotherapy. The 5 and 10 year OS and disease specific survival (DSS) rates in contemporary RC series are comparable to that reported in bladder-preserving series. Due to EBRT can mediate inmunoestimulatory effects and that radiotherapy has been used in combination with different treatment regimens, a considerable interest has been attracted by combinatorial regimens involving EBRT plus checkpoint inhibitors. The purpose of the present study is to explore feasibility and activity of the combination of atezolizumab associated with EBRT after TURBT in the treatment of localized MIBC with preservation intent. Methods: The primary endpoint of this study is to assess the efficacy of atezolizumab concurrent with EBRT in terms of pathological complete response. The secondary endpoints include OS, DSS, disease free survival, bladder intact disease-free survival, number of patients (pts) with muscle invasive and non-muscle invasive local failure, the rate of distance metastases, the rate of pts with bladder preserved, the rate of immediate or late salvage cystectomy and safety profile and tolerability of the combination. Biomarkers will be evaluated as exploratory endpoints. Elegibility criteria include pts with histologically-confirmed diagnosis of MIBC in clinical stages T2-4a N0 M0, who are not candidates for radical cystectomy by medical reasons, refusal or patient’s choice, unfit for cisplatin and PS (ECOG) 0-2. Pts will receive atezolizumab 1.200 mg by IV infusion on Day1 of each cycle every 3 weeks for a total of 6 treatment courses, concurrent with EBRT 60 Gy in 30 fractions over 6 weeks at 2 Gy/day. With 35 evaluable pts and a pCR of 80%, the 95%CI of the estimation would be 0.62–0.92. The study will be stopped if we do not achieve at least 9 pCR among the first 13 evaluable pts. Otherwise, the study will continue until 39 pts. Clinical trial information: 2018-004348-47.