Evaluation of Proton Minibeam Radiotherapy on Antitumor Immune Responses in a Rat Model of Glioblastoma

Treating radioresistant tumors like glioblastoma multiforme remains a challenge exacerbated by their immunosuppressive nature. Radiotherapy (RT) plays an immunomodulatory role, exerting both immunosuppressive and immunostimulatory effects. The nature of these effects depends on the total dose, dose per fraction, dose delivery method, and treatment length. Hypofractionation is observed to tip the balance toward immune stimulation. However, the use of hypofractionation is restricted in bulky tumors, such as gliomas, because of the high risk of toxicity. Therefore, finding new strategies leading to more favorable immune responses while reducing normal tissue toxicities could improve cancer treatment. In this study, we examine antitumoral immune responses to proton minibeam RT (pMBRT). Its immunomodulatory effects are not fully understood. To explore this, we conducted an in-depth characterization of the immune response to a curative dose of pMBRT in a preclinical orthotopic rat model of glioblastoma. Our findings revealed a close association between pMBRT and the immune response. pMBRT increased lymphocyte density in tumors more effectively than conventional proton therapy. Single-cell transcriptomics identified several immune cell types and unique transcriptional changes in tumor immune cells following pMBRT, including increased antibody production, chemotactic cytokine expression, and IFN responses. These results underscore the critical role of adaptive immunity, specifically T cells, in pMBRT's mechanism. The potential of pMBRT to trigger an antitumor immune response in a single RT session with minimal damage to healthy tissue makes it a promising candidate for future clinical trials and radioimmunotherapy combinations.