Checkpoint Inhibitor Therapy, With And Without Radiation, In Diffuse Large B Cell Lymphoma: A Single-Center Analysis.

e19059 Background: Single agent checkpoint inhibitors (CPI) in NHLs have resulted in modest successes. Exceptions include RT, PCNS and testicular lymphomas, where higher activity is seen. As a single agent, CPI response rates range from 10-40% in relapsed refractory (RR) DLBCL, with few CRs and short response durations. In a cohort of RR DLBCL patients at our institution, we sought to identify clinical features that defined responders and non-responders. Methods: Between 9/2016 and 7/2018, 13 pts with DLBCL/RT, treated with a CPI, either on trial or as off-label therapy, with at minimum 1 infusion/cycle were included. Pathology specimens confirming DLBCL/RT were reviewed at FCCC. All pts had measurable disease by CT or PET/CT prior to CPI and had an evaluable response. Cell of origin was determined by Hans IHC. Results: Almost half (6/13) of pts achieved a response to CPI. Notably all responders had either concurrent or pre-treatment XRT. All 3 RT patients responded to CPI and continued to allo transplant. P3 and P6 both developed GVHD post allo, resulting in a demise in P3. No GC subtype pts responded nor had prior/ concurrent XRT with CPI. In 2 pts responses are ongoing, > 1 yr, and 1 RT pt remains in CR. Conclusions: XRT, prior or concurrent with CPI , was associated with durable responses in RR DLBCL. Patients with bulky ( > 7cm), rapidly progressive disease (8/13 cases) may require a 'debulking' strategy for CPI efficacy. An abscopal effect achieved with XRT/CPI combinations, may be impactful RR NHL. Analysis of PDL1/2 and MHC I/II, with other biomarkers, are underway. These clinical results warrant validation in a larger cohort, therefore a prospectively designed study is planned for 2019 in RR DLBCL/RT. [Table: see text]