Background: The bladder is a mobile, deformable structure which makes radiotherapy (RT) delivery challenging. Plan of the day (POD) adaptive image guided RT and tumour boost dose escalation can optimise treatment. We aimed to define a feasible, safe schedule for muscle invasive bladder cancer (BC) using these techniques. Methods: RAIDER (ISRCTN 26779187) is a 3-arm, international phase II trial. Participants (pts) had unifocal T2-T4a urothelial BC and were randomized (1:1:2) to: standard whole bladder RT (WBRT), standard dose adaptive tumour focused RT (SART) or dose escalated adaptive tumour boost RT (DART). Two fractionation (f) schedules recruited independently. WBRT and SART dose was 64Gy/32f or 55Gy/20f and DART was 70Gy/32f or 60Gy/20f. For SART and DART, POD (small, medium, large) was chosen daily. Neoadjuvant chemotherapy (NAC) and concomitant radiosensitising therapy (CTh) were permitted. Primary endpoint is proportion of pts with RT related CTCAE grade≥3 (G≥3) toxicity 6-18 months (m) after RT. A non-comparative design to rule out >20% G≥3 toxicity in DART pts required 57 evaluable DART pts in each fractionation cohort (90% power, 5% 1-sided alpha). Adverse events (AE) are treatment emergent with RT relatedness assessed blind to treatment allocation. In each fractionation cohort, toxicity analysis is by treatment received in the evaluable population (pts with at ≥1 toxicity assessment between 6-18m). 3m local control was assessed by cystoscopic biopsy. Cancer outcomes are analysed by intention-to-treat with fractionation cohorts combined. Results: 345 pts were randomised (Oct 2015 - Apr 2020): 46/41 WBRT, 46/41 SART and 90/81 DART pts in 32f/20f cohorts respectively. Baseline characteristics for 32f/20f were median age 73 years (IQR 67, 79)/ 72 (67, 79); 83%/ 78% T2; 46%/ 52% had NAC and 71%/ 70% CTh. Median follow-up was 32f: 38.2m (IQR 26.2, 50.2), 20f: 42.1m (35.6, 50.1). 3588/6222 (58%) fractions delivered to SART and DART pts used small or large POD. Late toxicity outcomes are shown in table. RT related G≥3 in 20f DART was 1/58 (90% CI 0.1, 7.9) and in 32f DART was 0/57. 3m local control was achieved in 44/51 (86%) WBRT, 45/53 (85%) SART and 82/92(89%) DART. 2 year survival was 79% (95% CI: 69, 86) WBRT, 74% (63, 82) SART and 80% (73, 85) DART. Conclusions: Late G≥3 toxicity was low in all treatment groups. 20f and 32f DART was safe and feasible to deliver with toxicity rates below predefined thresholds. Local control for image guided (chemo)RT was good. CTCAE G≥3 and G≥2 treatment emergent toxicity 6-18m after RT. Clinical trial information: 26779187.