This early phase I clinical trial evaluates bridging radiation therapy given before chimeric antigen receptor (CAR) T-cell infusion to treat large B-cell lymphoma (LBCL) that has come back (relapsed) or has not responded to previous treatment (refractory). Patients with relapsed or refractory disease have historically poor prognosis. CAR T-cell therapy is a type of treatment in which a patient's T-cells (a type of immune system cell) are changed in the laboratory so they will attack cancer cells. T-cells are taken from a patient's blood (leukapheresis). Then the gene for a special receptor that binds to a certain protein on the patient's cancer cells is added to the T-cells in the laboratory. The special receptor is called a chimeric antigen receptor (CAR). Large numbers of the CAR T-cells are grown in the laboratory and given to the patient by infusion for treatment of certain cancers. While the outcomes from CAR T-cell therapy appear favorable, in the time between leukapheresis and CAR T-cell infusion many patients have symptomatic or life-threatening disease which often requires bridging therapy. Bridging therapy aims to slow disease progression and control symptoms during this critical period prior to CAR T-cell infusion. Radiation therapy uses high energy x-rays, particles, or radioactive seeds to kill cancer cells. Giving bridging radiation therapy to patients with relapsed or refractory LBCL prior to CAR T-cell infusion may improve treatment outcomes with minimal toxicity.