Purpose: To investigate whether a plasmid DNA-based detector can be used to assess the DNA damaging effectiveness of a 100 kVp versus a 6 MV x-ray beam.
Methods: We constructed nearly tissue equivalent (ρ = 1.19 g/cm³) polycarbonate DNA-based detectors with nine 6.3 mm³ cylindrical cavities. We pipetted 3 μL of 18 ng/μL pBR322 plasmid DNA into each cavity and applied an aluminum sticker seal. Seven detectors were sent to Sunnybrook Hospital, Toronto, Canada. The detectors were irradiated to 10, 20, and 30 Gy in duplicate using a 100 kVp beam and a 6 MV beam. The remaining detector served as the shipment control, receiving no irradiation. An additional control detector was kept at San Diego State University, San Diego, USA to evaluate the effects of shipping and handling on the DNA. Plasmid DNA possesses three conformations (supercoiled, open circular, and linear corresponding to no damage, single strand break (SSB), and double strand break (DSB) damage, respectively). We used gel electrophoresis to separate and quantify the intensity of the DNA conformations and calculated the percent yield of each. From these yields, DNA damage type was modeled as a function of dose. This procedure was repeated post-irradiation using repair enzymes Fpg and Nth to quantify oxidized base damage.
Results: The 100 kVp beam caused 4.7±3.1 times more DSBs per SSB as compared to the 6 MV after irradiation, and 1.4±0.1 times more with enzymatic treatment. This is expected due to the greater linear energy transfer of the 100 kVp beam. The decrease in this ratio after enzymatic treatment implies that the 6 MV beam caused more oxidized base damage than the 100 kVp.
Conclusion: The plasmid DNA-based detector is sensitive enough to quantify relative DNA damage effectiveness of a 100 kVp beam versus a 6 MV beam.