Characterization and radiosensitivity at high or low dose rate of four cell lines derived from human thyroid tumors

Purpose: This study attempted to establish cell lines derived from human differentiated thyroid tumors, and to characterize and evaluate the radiation dose-rate effect. Methods and Materials: Two cell lines (K1 and K2) were derived from papillary carcinomas, one (K7) from a follicular less- differentiated carcinoma and one (A14) from a follicular adenoma. Cell- survival curves after irradiation were established by the in vitro colony method. Radiation doses were delivered either at a high (45.9 Gy/h) or low dose rate (0.6 Gy/h) by a ⁶⁰Co source. The data were analyzed according to the linear quadratic and multitarget model of radiation action. Results: Doubling times were around 24 h. All cell lines were positive for intracellular thyroglobulin. Cyclic adenosine monophosphate (cAMP) response to thyrotropin (TSH) was significant for the cell lines derived from the follicular adenoma and follicular carcinoma. The two cell lines derived from papillary carcinomas were tumorigenic after inoculation into nude mice. After high-dose-rate irradiation, the surviving fraction at 2 Gy (SF2) was not significantly different for the three malignant cell lines K1, K2, and K7, and ranged from 0.39 to 0.42. For the adenoma cell line A14, we found a higher radiosensitivity with a lower SF2 value and a higher α parameter. After low-dose-rate irradiation, only one cell line (K2) showed a significant low-dose-rate sparing with a dose reduction factor of 1.35. Conclusion: The major result of our study is the weak effect of the dose rate on the survival of thyroid cell lines.