ARM III (SEQUENTIAL COHORT): Patients undergo SAR 2-3 times per week (with a minimum of 40 hours and a maximum of 96 hours between fractions) over 1.5-2 weeks for a total of 5 fractions beginning on day 1 of course 1. After completion of SAR (beginning on day 1 of course 2), patients receive anti-PD-L1 monoclonal antibody MPDL3280A IV over 30-60 minutes on day 1. Courses repeat every 3 weeks in the absence of disease progression or unacceptable toxicity.