Evaluation of Bridging Radiation Therapy Before CAR T-Cell Infusion for the Treatment of Relapsed or Refractory Large B-Cell Lymphoma

Inclusion Criteria: Documented informed consent of the participant and/or legally authorized representative. Assent, when appropriate, will be obtained per institutional guidelines. Age: >= 18 years. Eastern Cooperative Oncology Group (ECOG) =< 2 or Karnofsky Performance Status (KPS) >= 60. Histologically confirmed large B-cell lymphoma. Relapsed/refractory disease. Planned to undergo commercial CAR T-cell infusion within 3 months of enrollment. 6 or fewer sites (treatable with a maximum of 3 isocenters) of FDG-PET avid disease, treatable with a a maximum of 3 isocenters. Measurable disease e.g., at least 1.5 cm on CT/MRI or by Response Evaluation Criteria in Solid Tumors 1.1 (RECIST 1.1). Fully recovered from the acute toxic effects (except alopecia) to =< grade 1 to prior anti-cancer therapy. Women of childbearing potential (WOCBP): negative urine or serum pregnancy test (performed within 30 days prior to day 1 of protocol therapy). If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required. Exclusion Criteria: Prior CD19-directed therapy. Radiation therapy within 21 days prior to day 1 of protocol therapy. Central nervous system (CNS) disease. History of allergic reactions attributed to compounds of similar chemical or biologic composition to study agent. Active diarrhea. Clinically significant uncontrolled illness. Active infection requiring antibiotics. Other active malignancy. Females only: Pregnant. Any other condition that would, in the investigator's judgment, contraindicate the patient's participation in the clinical study due to safety concerns with clinical study procedures. Prospective participants who, in the opinion of the investigator, may not be able to comply with all study procedures (including compliance issues related to feasibility/logistics).