Purpose: Use plasmid DNA to quantify and compare DNA damage caused by a proton and megavoltage (MV) photon beam delivering the same radiation dose.
Methods: A phantom with a single well, for DNA sample placement, was constructed using a tissue equivalent biocompatible plastic. A CT scan of the phantom was acquired, and treatment plans where created for both 102 MeV and 6 MV proton and photon beams, respectively. A 5 cm buildup was used to create a spread-out Bragg peak (SOBP) across the phantom wells for the proton beam, and to achieve a uniform (Â±1%) dose region for the photon beam. The same dose of 90.9 Gy was delivered with both modalities with no correction for the relative biological effect (RBE) of protons. The plasmid DNA, pBR322 (Thermo Fisher Scientific), exists in three possible conformations: supercoiled, open circular, and linear, corresponding to no damage, single strand break (SSB) damage, and double strand break (DSB) damage, respectively. Gel electrophoreses was used to separate the three DNA conformations into distinct bands, and ImageJ (National Institutes of Health) was used to quantify the relative intensity of each band to obtain relative yields of each conformation.
Results: The data groups being compared were found to satisfy normality (Shapiro-Wilk test) and equality of variances (F-test). Hence, the t-test was applied to assess statistical significance between group means. Relative yields of the linear DNA bands were 33.6Â±3.8% greater for proton versus photon irradiation (p<0.0006). However, relative yields of open circular bands were 5.06Â±0.05% less for proton versus photon irradiation (p<0.0002).
Conclusion: This study shows that for a given radiation dose, a proton beam causes significantly more DSBs than a photon beam, but the photon beam causes more SSBs. The next step in this study will be to measure changes in DNA damage across the SOBP.