Enhancement of radiation sensitivity in iododeoxyuridine labelled cells exposed to low energy x-rays

In order to investigate if low-energy x-rays induce Auger cascades by photoelectric absorption in iodine present in DNA, CHO cells were labelled with iododeoxyuridine (IUdR) for 72 hours. Following labelling, the cells were either irradiated with low-energy x-rays (75 kV, 4 mm Al) or 137Cs-gamma-rays. The radiation response was measured using clonogenic survival, and the survival parameters were analyzed according to the linear quadratic model. The dose modifying factors were determined as the ratios of the alpha-coefficients. The IUdR labelled cells were found to be about 3.2 times as sensitive as the control cells when irradiated with low-energy x-rays. For 137Cs-gamma the ratio was about 1.5. The standard deviations were estimated by Gauss' approximation to be about 0.5 for both irradiation conditions.