Quantitative ESR-studies of decay-produced radicals in 5-iododeoxyuridine labeled with 125I, 131I or tritium: role of the Auger effect.

In view of the enhanced biological damage caused by the "Auger nuclide" iodine-125, we have carried out quantitative ESR-studies of the radical formation in polycrystalline 5-iododoeoxyuridine (IUdR) resulting from the following internal or external radiation sources: (1) Decay of 3H, 125I or 131I in labeled IUdR; (2) Lanthanum K-photons corresponding to the K-edge of iodine; (3) 60Co gamma-rays. The results clearly indicate that inner shell ionization with its accompanying Auger effect as caused by the lanthanum K X-ray produces about 30% more free radicals per unit dose absorbed than 60Co gamma-rays, when considering the long-lived secondary radicals. Similarly, the concentration of free radicals is by about 30% higher in 125I- than in 131I-labeled IUdR at comparable doses. In the case of 3H-labeled IUdR the dose curve is almost identical with that observed for 125I-labeled IUdR. The results are discussed in terms of a localized radiation damage from low energy electrons.