Study of the PD-L1 Inhibitor Atezolizumab With or Without Low-dose, Local Radiation in Patients With Relapsed or Refractory Advanced Stage Follicular Lymphoma

1200mg IV on day 1 of a 21-day cycle for a planned total of 17 cycles of therapy (1 year).

4Gy will be administered in 2 fractions to a single nodal site amenable to radiation as identified by the radiation oncologist on day 2 and day 3 of Cycle 1 (over the two days following to the first dose of atezolizumab ).