Purpose: Radiation therapy can both enhance and suppress cancer immunotherapy for cancer treatment. There is an unmet need to find the best synergy between the two therapies to advance cancer care. We investigated whether microbeam radiation therapy (MRT) has advantage over seamless conventional radiation therapy (BRT) to enhance Î±-PD-L1 immunotherapy as expressed by the abscopal effect.
Methods: TNBC mouse tumor model EO771 cells were subcutaneously implanted into both left and right rodent flanks. At the target tumor size of ~200mm^3, one tumor is irradiated with or without immunotherapy drug. The MRT beam width is 310Î¼m, beam separation 1.27mm with a peak dose of 50Gy (integral dose of 11Gy). Treatment groups are (1) 50Gy MRT alone, (2) 10Gy BRT alone, (3) Î±-PD-L1 alone, (4) 50Gy MRT + Î±-PD-L1, (5) 10Gy BRT + Î±-PD-L1, and (6) Sham-treated controls. Mice are given 200Î¼g Î±-PD-L1 peritoneal injections beginning on the day of radiotherapy treatment and then every 3 days for a total of 4 injections. Animals are euthanized when the maximum combined tumor burden of 3000mm^3 has been met, or after 30 days, whichever comes first. We also harvest tissues for 6 animals from each treatment group at 10 days post-radiotherapy for flow-cytometry studies of immunofluorescence.
Results: Microbeam radiation is superior to the seamless broad beam radiation in abscopal effect (tumor control of the un-irradiated tumor) when immunotherapy drug was used. Conventional seamless radiation is superior to MRT in controlling treated tumor growth independent of the immunotherapy.
Conclusion: Our study indicates that compared to conventional seamless radiation of a similar integral dose, microbeam radiation therapy is not effective for tumor control of the treated tumor but it is superior in enhancing immunotherapy as demonstrated by the abscopal effect.
Not Applicable / None Entered.