Hyperfractionated Dual Equivalent Fractionated Radiation Therapy

Inclusion Criteria: 1. Provision of signed and dated informed consent form. 2. Stated willingness to comply with all study procedures and availability for the duration of the study. 3. Adult aged 18 years or older. 4. Histologically confirmed diagnosis of R/R DLBCL with plan for CAR T or BsAb therapy at Yale New Haven Hospital. 5. ECOG performance status 0 to 3. 6. Ability to present for once or twice daily (M-F) fractionated radiation therapy, without contraindications for radiotherapy as determined by the treating radiation oncologist. 7. Women of childbearing potential must have a negative serum or urine pregnancy test at screening and at time of radiation treatment planning, per standard of care and departmental standard operating procedure. Participants must agree to take appropriate precautions to avoid pregnancy (with at least 99% certainty) from screening through safety follow-up. Permitted methods that are at least 99% effective in preventing pregnancy should be communicated to the participants and their understanding confirmed. Exclusion Criteria: Participants who meet any of the following criteria will be disqualified from entering the study: 1. Participants who are pregnant or currently breastfeeding. a. Females who have undergone surgical sterilization or who have been postmenopausal for at least 12 months are not considered to be of childbearing potential. 2. Participants with history of prior radiation exposure for research purposes within the past year, such that participation in this study would place them over the FDA limits for annual radiation exposure. 3. Participants who are unable to safely receive FDG PET tracer. 4. Any condition that would, in the investigator's judgment, interfere with full participation in the study and attending required study visits (if outpatient); pose a significant risk to the participant; or interfere with interpretation of study data. 5. Participants who would not be anticipated to derive any clinical benefit from bridging radiotherapy, are unable to participate in twice daily radiotherapy, or have clinical contraindications to radiation therapy per treating investigator.