The present study was undertaken to characterize carbon and iron ion radiation-induced adverse biological effects in terms of toxicity and transformation in vitro. HTori-3 human thyroid epithelial cells were irradiated with 0.3-GeV/n (13.6 KeV/µm) carbon ions and 1-GeV/n (150 KeV/µm) iron ions, both of which represent high-mass, high atomic number (Z) and high-energy particles known as HZE particles, as well as γ-rays. The survival of the irradiated cells was determined by a clonogenic survival assay. The yield of colonies growing in soft agar was used as a surrogate endpoint biomarker for transformation in vitro. The results showed that HZE particles and γ-ray radiations are effective in increasing the yield of anchorage-independent colonies. Based on the relative biological effectiveness (RBE) values in the clonogenic survival assays, 0.3-GeV/n carbon ions and 1-GeV/n iron ions were 2.9 and 2.4 times, respectively, as effective as γ-rays at killing the irradiated HTori-3 cells. At a dose of 200 cGy, 0.3-GeV/n carbon ions and 1-GeV/n iron ions were found to be 3.5 and 7.3 times, respectively, as effective as γ-rays at inducing anchorage-independent growth. These results suggest that the carcinogenic potential of 0.3-GeV/n carbon ions, as represented by the ability to induce anchorage-independent growth, may be lower than that of 1-GeV/n iron ions.

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