The investigators hypothesized that local radiation therapy can enhance the effect of anti-PD-L1 monoclonal antibody through priming T-cell effector function against cancer cells. Described as above, The investigators concluded that modest dose of radiation to local site prior to immunotherapy is the best to enhance T-cell-mediated immunity. Accordingly, The investigators will investigate the combining effect of hypofractionated-sublethal dose of radiation therapy followed by anti-PD-L1 monoclonal antibody, atezolizumab, for SCLC patients who are recurrent or refractory for initial platinum-based chemotherapy