Risk of immunosuppression and hospitalization after checkpoint inhibitor therapy in patients with cancer and radiation therapy

2588 Background: Immune checkpoint inhibitors (ICIs) are potent new cancer therapies but can cause serious immune-related adverse events. Radiation therapy (RT) also induces systemic immunologic effects, and data on the interaction and safety of combining ICIs and RT are limited. Methods: In this retrospective cohort study using a large medical claims database from 2010 to 2017, we ascertained the risk of immunosuppressive steroid therapy as well as the risk of hospitalization within 180 days of treatment with an ICI in patients with diagnoses of malignant melanoma or lung cancer. Patients were stratified by use of RT within 30 days before and after ICI therapy. ICIs included pembrolizumab, nivolumab, and ipilimumab, while immunosuppressive agents included oral prednisone and intravenous methylprednisolone. Results: 2020 patients (218 with RT, 1802 without RT) met inclusion criteria for prednisone analysis, while 3519 patients (361 with RT, 3158 without RT) met inclusion criteria for all other analyses. On univariable analysis, RT was not associated with need for prednisone or methylprednisolone (RR 1.2, 95%CI 0.8-1.9 and RR 1.2, 95%CI 0.7-2.1 respectively). When assessing hospitalization, RT was significantly associated with hospitalization following ICI therapy for certain cancer/drug combinations (RR 1.4, 95%CI 1.2-1.6, p