Inclusion Criteria
1. Written informed consent
2. Age 18 years and over
3. Histologically proven DLBCL, including transformed follicular- or marginal zone lymphoma
4. Measurable disease on cross-sectional imaging that is at least 1.5cm in the longest diameter and measurable in two perpendicular dimensions
5. Relapsed/refractory after 2 or more standard immuno-chemotherapies
6. Approved to receive tisagenlecleucel as per the licenced indication
7. Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1
8. Disease accessible for repeat biopsies (King's College Hospital patients only)
9. Disease amenable to RT as assessed by the treating clinical oncologist
10. Willing and able to comply with the requirements of the protocol, including contraceptive advice
Exclusion Criteria
1. Prior radiotherapy at a location or dose that would interfere with application of RT or outcome measures in this trial
2. Women who are pregnant or breast feeding
3. Previous therapy with any genetically modified autologous or allogeneic T-cell immunotherapy, unless treatment with doses of genetically modified autologous or allogeneic T-cell immunotherapy within an abandoned dosing cohort in a first in human dose-esclation phase I clinical trial