Durvalumab, 1500 mg Q4W (equivalent to 20 mg/kg Q4W) intravenously for 13 doses and Tremelimumab 75 mg Q4W (equivalent to 1 mg/kg Q4W) intravenously for 7 doses then every 12 weeks for 2 doses. Weight-based dosing should be utilized for patients <30 kg: durvalumab 20 mg/kg and tremelimumab 1 mg/kg. Tremelimumab will be administered first and the infusion duration will be approximately 1 hour. The durvalumab infusion will start approximately 1 hour after the end of the tremelimumab infusion and the infusion will be administered over approximately 1 hour.