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TPS531 Background: Pembrolizumab leads to responses in ~20% of metastatic bladder cancer patients. Irradiation of bladder cancer cells in-vitro and in-vivo leads to upregulation of PD-L1, and in immunocompetent mouse models blockade of PD-L1 leads to delayed tumour growth following irradiation. Randomised data from PACIFIC trial in NSCLC shows the addition of PD-L1 inhibition to chemoradiation significantly prolongs PFS. A trial of chemoradiotherapy with pembrolizumab will assess safety and synergy of the combination in localised bladder cancer. Methods: This pilot study enrols patients with maximally resected non-metastatic muscle invasive bladder cancer, who either wish for bladder preservation or are ineligible for cystectomy. This study will assess the safety and feasibility of combining pembrolizumab with chemoradiotherapy in ECOG 0-1 patients without contraindications to pembrolizumab. The study has enrolled 4 of a planned 30 patients. All patients treated with 64Gy of radiation therapy in 32 fractions over 6 weeks, 2 days. Cisplatin 35mg/m2 IV concurrently weekly for 6 doses with radiation. Pembrolizumab commences concurrently with radiation and is given 200mg IV q21 days for 7 doses. Surveillance cystoscopy is performed 12 & 24 weeks after the commencement of chemoradiotherapy to assess response to therapy. Patients will enter follow up with clinical assessment, cystoscopy and CT staging performed at intervals until close of study. The primary endpoint assessed will be safety, as defined by a satisfactorily low rate of unacceptable toxicity (G3-4 adverse events or failure of completion of planned chemotherapy and radiotherapy according to defined parameters). The secondary endpoint will be efficacy, as assessed by the proportion of patients achieving a best response of complete response based on the first two 12 and 24 week post chemoradiotherapy cystoscopic assessments. Exploratory analysis will include assessment of tumour histopathological, molecular, genetic and immunological parameters. It is expected that it will take two years to accrue the 30 patients across 5 Australian centres. NCT02662062. Clinical trial information: NCT02662062.