Comparison of cytogenetic damage in cultured cells from cobalt-60 gamma-radiation and the Auger emitter zinc-65

Abstract

Purpose : To assess the ability of the Auger-emitting nuclide, zinc-65 (65 Zn), relative to γ-irradiation, to cause chromosomal aberrations in cultured rat prostate cells. Materials and methods : Rat prostate adenocarcinoma cells in culture were exposed to doses of 1, 2, 3 or 5 Gy of external gamma-irradiation for 24h or incubated with 0.7, 1.5, 1.8 or 2.8MBq of 65 Zn for 24h. The uptake by and clearance from cells of 65Zn was measured. Metaphase spreads prepared from washed cells were scored for chromatid- and chromosome-type aberrations. Results : Following exposure to 65 Zn or γ-irradiation, chromatidtype damage was more commonly observed than chromosometype aberrations. The relationship between induced chromatid damage and gamma dose (to 3 Gy) was best fitted by a secondorder polynomial function, while the activity-response relationship for chromatid damage caused by 65 Zn appeared to be best fitted by a straight line. Measurements of the uptake of 65 Zn by cells showed that average concentrations within cells were about 100 times the concentration in the culture medium. Assuming uniform distribution of 65 Zn within cells, with 36% in the nucleus, the dose was estimated as 0.70 Gy per MBq added 65 Zn, with Auger electrons contributing most (93%) of the dose. Assuming that 20% of cellular zinc was localized in the nucleus, based on previous measurements, the dose to the nucleus was calculated as 0.44Gy per MBq added 65 Zn. RBE values for chromatid damage induced by 65 Zn compared to gamma-radiation range from about 1 to 3 based on a uniform dose throughout the cell and from about 2 to 5 based on 20% of 65 Zn in the cell nucleus. Conclusion : The observed radiotoxicity of 65 Zn is consistent with its behaviour as an Auger-emitting radionuclide that is localized to some extent in the nucleus.