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Radiation cancer therapy with ultra-high dose rate (UHDR) exposure, so-called FLASH radiotherapy, appears to reduce normal tissue damage without compromising tumor response to therapy. The aim of this study was to clarify whether a 59.5 MeV proton beam at an UHDR of 48.6 Gy/s could effectively reduce the DNA damage of pBR322 plasmid DNA in solution compared to the conventional dose rate (CONV) of 0.057 Gy/s. A simple system, consisting of pBR322 plasmid DNA in 1× Tris-EDTA buffer, was initially employed for proton beam exposure. We then used formamidopyrimidine-DNA glycosylase (Fpg) enzymes. which convert oxidative base damages of oxidized purines to DNA strand breaks, to quantify DNA single strand breaks (SSBs) and double strand breaks (DSBs) by agarose gel electrophoresis. Our findings showed that the SSB induction rate (SSB per plasmid DNA/Gy) at UHDR and the induction of Fpg enzyme sensitive sites (ESS) were significantly reduced in UHDR compared to CONV. However, there was no significant difference in DSB induction and non-DSB cluster damages. UHDR of a 59.5 MeV proton beam could reduce non-clustered, non-DSB damages, such as SSB and sparsely distributed ESS. However, this effect may not be significant in reducing lethal DNA damage that becomes apparent only in acute radiation effects of mammalian cells and in vivo studies.

Induction of DNA strand breaks and oxidative base damages in plasmid DNA by ultra-high dose rate proton irradiation