Synchrotron Microbeam Radiation Therapy as a New Approach for the Treatment of Radioresistant Melanoma: Potential Underlying Mechanisms.

Synchrotron microbeam radiation therapy (MRT) is a method that spatially distributes the x-ray beam into several microbeams of very high dose (peak dose), regularly separated by low-dose intervals (valley dose). MRT selectively spares normal tissues, relative to conventional (uniform broad beam [BB]) radiation therapy. To evaluate the effect of MRT on radioresistant melanoma, B16-F10 murine melanomas were implanted into mice ears. Tumors were either treated with MRT (407.6 Gy peak; 6.2 Gy valley dose) or uniform BB irradiation (6.2 Gy). MRT induced significantly longer tumor regrowth delay than did BB irradiation. A significant 24% reduction in blood vessel perfusion was observed 5 days after MRT, and the cell proliferation index was significantly lower in melanomas treated by MRT compared with BB. MRT provoked a greater induction of senescence in melanoma cells. Bio-Plex analyses revealed enhanced concentration of monocyte-attracting chemokines in the MRT group: MCP-1 at D5, MIP-1α, MIP-1β, IL12p40, and RANTES at D9. This was associated with leukocytic infiltration at D9 after MRT, attributed mainly to CD8 T cells, natural killer cells, and macrophages. In light of its potential to disrupt blood vessels that promote infiltration of the tumor by immune cells and its induction of senescence, MRT could be a new therapeutic approach for radioresistant melanoma.

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