This is a clinical trial to evaluate the safety of combining CAR T-cell therapy with low-dose total body irradiation (LD-TBI) in patients with previously treated large B-cell lymphoma (LBCL) or multiple myeloma (MM). The investigators' hypothesis is that the combination will make the immune system more active in fighting cancer by increasing the display of antigens and activating pathways that lead to cell death, including death receptors like FAS and TRAIL2. This approach is expected to help the CAR T cells grow and last longer, leading to stronger anti-tumor effects and more cancer cell deaths. Participants will receive LDTBI treatment before their CAR T cell therapy and will be followed up for 2 years.