Inclusion Criteria:

• * Diagnosis of relapsed or refractory B-cell lymphoma (including, but not limited to diffuse large B-cell lymphoma (DLBCL) not otherwise specified, primary mediastinal large B-cell lymphoma, high grade B-cell lymphoma, DLBCL arising from follicular lymphoma, follicular, and mantle cell lymphoma)
• * Being planned to undergo CAR T-cell therapy (patients can be enrolled prior to apheresis)
• * At least 1 measurable lesion according to the Lugano criteria35. Lesions that have been previously irradiated will be considered measurable only if progression has been documented following completion of radiation therapy. Re-irradiation is allowed for these patients. More than 1 lesion can be targeted as per radiation oncologist discretion. If disease is palpable, physical examination alone may be sufficient measurement for disease if a recent PET/CT or CT is not available as a radiologic measurement of the disease can be obtained during CT simulation.
• * Age ≥ 18 years
• * Eastern Cooperative Oncology Group (ECOG) performance status of less than or equal to 3.
• * Ability to understand and the willingness to sign a written informed consent document

Exclusion Criteria:

• * Any medical condition likely to interfere with assessment of safety or efficacy of RT.
• * CNS-only disease as the site for bridging radiation therapy (which cannot be reliably evaluated on CT). Patients with CNS disease with extra-axial involvement that can be evaluated on CT remain eligible.
• * Patient likely unable to lay supine for 45 minutes
• * Women of child-bearing potential who are pregnant because of the potentially dangerous effects of the preparative chemotherapy on the fetus or infant. Females who have undergone surgical sterilization or who have been postmenopausal for at least 2 years are not considered to be of childbearing potential
• * In the investigator's judgment, the subject is unlikely to complete all protocol-required study visits or procedures, including follow-up visits, or comply with the study requirements for participation.