Study on cell survival, induction of apoptosis and micronucleus formation in SCL-II cells after exposure to the auger electron emitter 99mTc

Objective: To study the biological effectiveness of Auger electrons emitted by Tc-99m on cell survival, induction of apoptosis and micronucleus (MN) formation in the human squamous cell carcinoma cell line SCL-II and compare the effects observed to those observed after exposure to external Co-60 gamma radiation. Material and methods: Cells were either gamma(Co-60)-irradiated (0.67 Gy/min) or exposed to Tc-99m-pertechnetate (0.95-14.3 MBq/ml) for 24 h under cell culture conditions and assayed for cell survival (colony-forming assay), micronucleus formation (cytochalasin B assay) and the frequency of apoptotic cells (fluorescence microscopy). Monte Carlo based dosimetry has been applied to derive the absorbed dose corresponding to the accumulated decays of Tc-99m under the given geometry. Results: Absorbed doses up to 0.5 Gy could be achieved after Tc-99m-exposure leading to no substantial cell killing in this dose range except at one dose point (0.1 Gy) resulting in an relative biological effectiveness (RBE)(SF 0.9) of 0.64 when compared to the Co-60 reference radiation. MN formation was described best by a linear dose response and was consistently lower after Tc-99m exposure when compared to Co-60 irradiated cells resulting in an RBE of 0.37. Apoptosis induction was significantly increased after Tc-99m exposure at much lower doses (0.1 Gy) when compared to the reference radiation. The Tc-99m uptake experiments revealed an activity concentration ratio cells vs. medium of 0.07 after 24 h of exposure. Conclusion: No overall increased biological effectiveness due to the emitted Auger electrons of Tc-99m, applied as sodium-pertechnetate, could be observed in the investigated cell line when compared to acute external gamma radiation. The RBEs in the range of 0.37-0.64 might be well explained by dose rate effects. The significantly increased apoptotic response after Tc-99m-exposure at very low doses has to be further investigated.