Optimal photon energies for IUdR K edge radiosensitization with filtered x-ray and radioisotope sources

The purpose of this work is to determine the most physically effective radi ation energy for K-edge absorption of x- or gamma-rays by iododeoxyuridine (IUdR) on Chinese hamster ovary (CHO) cells. Brachytherapy sources (Sm-145, I-125, Yb-169 and Am-241) and x-ray beams (3 0 kVp, 100 kVp and 100 kVp with gold, gadolinium, lead or tungsten filtrati on) were investigated for their preferential absorption qualities by IUdR s ensitized DNA. The 30 kVp, 100 kVp and 100 kVp with tungsten filtration wer e then used to irradiate CHO cells, with or without IUdR incorporation (i.e . 10(-5) M of IUdR for 3 days). Radiation absorption calculations were perf ormed to determine the increase in energy absorption in DNA with and withou t IUdR incorporated. In order to measure the in vitro biological effects of K-edge absorption, cell survival experiments were performed. The radiation physics calculations yielded an iodine dose enhancement ratio (DER) of 1.4 +/- 0.15, 1.8 +/- 0.15 and 2.7 +/- 0.15 for the 30 kVp, 100 k Vp and tungsten filtered 100 kVp respectively, for 18% IUdR replacement of thymidine in DNA. The corresponding cell sensitization enhancement ratios ( SER), determined from the cell survival assay, were determined to be 1.24 /- 0.2, 1.8 +/- 0.2 and 2.3 +/- 0.3 for the 30 kVp, 100 kVp and tungsten fi ltered 100 kVp respectively, for calls with 18 +/- 2% IUdR incorporation. T hese SER values are in reasonable agreement with the DER values of 1.4, 1.8 and 2.7. From these radiation calculations and radiobiology experiments we confirm t hat using x-radiation energies above the K-edge of iodine (33.2 keV) can ha ve a significant effect on cell survival. This effect is due mainly to the increase in the local dose to the DNA for IUdR-sensitized cells compared wi th the normal DNA which lacks the iodine contrast agent. Our results suppor t the clinical application of IUdR and low-energy brachytherapy, perhaps us ing new technologies such as the x-ray needle or new isotopes such as Yb-16 9.