DNA Dosimeter Measurement of Relative Biological Effectiveness for 160 kVp and 6 MV X Rays

In this work, we developed a DNA dosimeter, consisting of 4-kb DNA strands attached to magnetic streptavidin beads and labeled with fluorescein, to detect double-strand breaks (DSBs). The purpose here was to evaluate whether the DNA dosimeter readings reflect the relative biological effects of 160 kVp and 6 MV X rays. AVarian 600 C/D linac (6 MV) and a Faxitron cabinet X-ray system (160 kVp), both calibrated using traceable methods, were used to deliver high- and low-energy photons, respectively, to DNA dosimeters and multiple cell lines (mNs-5, HT-22 and Daoy). The responses were fit versus dose, and were used to quantify the dose of low-energy photons that produced the same response as that of the high-energy photons, at doses of 3, 6 and 9 Gy. The equivalent doses were utilized to calculate the relative biological effectiveness (RBEDSB and RBEcell survival). Additionally, a neutral comet assay was performed to measure the amount of intracellular DNA DSB, and ultimately the RBEcomet assay. The results of this work showed 160-kVp photon RBE values and 95% confidence intervals of 1.12 ± 0.04 (mNS-5), 1.16 ± 0.06 (HT-22), 1.25 ± 0.09 (Daoy) and 1.21 ± 0.24 (DNA dosimeter) at 9 Gy and 1.32 ± 0.16 (comet assay) at 3 Gy. Within the current error, the DNA dosimeter measured RBEDSB values in agreement with the RBEcell survival and assay from the cell survival and comet assay RBEcomet measurements. These results suggest that the DNA dosimeter can measure the changes in the radiobiological effects from different energy photons.