The relaxation of supercoiled DNA molecules as a biophysical dosimeter for ionizing radiations: a feasibility study

In this paper we explore the feasibility of using DNA molecules as a biophysical radiation dosimeter. Supercoiled phi X174 bacteriophage DNA molecules were irradiated with different gamma radiation doses. The strand breakage produced by ionizing radiation within supercoiled double-stranded DNA molecules (RFI) yields relaxed circular DNA molecules (RFII) and linear DNA molecules (RFIII) as a result of single-strand breaks and double-strand breaks, respectively. The irradiated samples were subjected to electrophoresis on agarose gels to separate the three forms. A proprietary fluorescent dye was used to detect DNA bands within the gel, which was photographed under UV transillumination. The negative was scanned with a computerized imaging densitometric system for DNA band quantitation. The relative fractions of the three molecular forms are dose dependent, and can be modeled mathematically with five parameters. The values of the parameters were determined by optimizing the fit of the model to the data, using a nonlinear regression procedure of a commercial statistical analysis package. Once the parameters of DNA breakage have been determined, absorbed dose can be measured by this technique, which we have termed supercoil relaxation dosimetry. The average accuracy of dose determination for our system over the range of 1-40 Gy was about 5%. Supercoil relaxation dosimetry may be well suited to certain difficult dosimetric problems.