Despite the recent progress, current treatment modalities are not able to eradicate cancer. We show that Microbeam Radiotherapy (MRT), an innovative type of Spatially Fractionated Radiotherapy, can control murine melanoma by activating the host's own immune system. The beneficial effects are very pronounced in comparison to uniform radiotherapy, traditionally employed in the clinic. Our results displayed that MRT increased antigen presentation, activating Cytotoxic T Lymphocytes (CTLs) which are essential to MRT's treatment efficacy in melanoma. Depletion of CTLs abrogated treatment response. Multiplex nucleic acid hybridization technology revealed key features of lymphocyte populations such as proliferation, differentiation, and ligand-receptor interactions. In addition, CTLs were shown to be essential for locoregional metastatic control and systemic abscopal effects confirmed by activation of antigen presenting cells and CTL trafficking in the tumour-draining lymph nodes. MRT induces a robust antitumour immune response, matching the characteristics of in situ vaccination, that could be exploited to treat a variety of treatment-resistant malignancies.
