Author: Hansong Bai, Peter Jermain, Xun Jia, Heng Li, Devin Miles, Daniel Sforza, Michael H. Shang, Daniel Robert Strauss, Keith Unger, Lingshu Yin 👨🔬
Affiliation: Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, MedStar Georgetown University Hospital, Johns Hopkins University, Department of Radiation Medicine, MedStar Georgetown University Hospital 🌍
Purpose: To investigate DNA Double Strand Break (DSB) induced by FLASH proton irradiation in a repair-free plasmid DNA model under different free radical scavenging conditions in plateau and Bragg peak regions.
Methods: pUC19 plasmid DNA in HEPES and MgCl2 buffer were irradiated to doses of 1, 3 and 5 kGy with conventional dose rate as well as FLASH proton beams at HEPES concentrations of 2 mM and 10 mM at the plateau and Bragg peak regions. A Hitachi PROBEAT system provided FLASH irradiation while a MEVION S250i system provided conventional dose rate irradiation. Atomic force microscopy was used to image the DNA and ImageJ software was used to measure the DNA fragments. Fragment size distributions were constructed under all irradiation conditions and analyzed to calculate the number DSBs per DNA molecule for each dose and radiation mode.
Results: In general, FLASH proton irradiation yielded less DNA DSB vs. conventional dose rate proton beams at lower concentration of HEPES, an efficient free radical scavenger, in both plateau and Bragg peak regions. During Bragg peak irradiation with 10 mM HEPES, the ratios of intact DNA to total initial DNA were 0.77 and 0.60 for FLASH and conventional irradiations, respectively. Corresponding numbers of DSB per DNA molecule were 2.51 for FLASH and 3.68 for conventional dose rate proton beam. At the same delivered dose, irradiation effects at the Bragg peak produced more DNA damage than the plateau, whereas higher concentrations of HEPES yielded less DNA fragmentation and smaller numbers of DSB per DNA molecule.
Conclusion: Proton FLASH irradiation reduced DNA damage compared to the conventional dose rate proton beam at both the plateau and Bragg peak and at all concentrations of free radical scavengers. These results provide support for understanding the biochemical mechanisms of the proton FLASH sparing effect.