Abstract
Clonogenic assay is regarded as the golden standard to evaluate the radiation sensitivity. However, there is controversial point about the evaluation of radio sensitivity by the clonogenic assay, because the results of the clonogenic assay are dependent on the plating efficiency and/or growth rate of the cells examined. In this study, we tried to establish the radio resistant cells that could proliferate under the condition of 2 Gy/day X-ray exposure as the model of radio resistant cells evoked during the cancer radiotherapy. In order to establish the radio resistant cells, HepG2 cells established from human hepatoblastoma were exposed to 1 Gy/day of X-rays for more than 4 years (total dose; 1,700 Gy) and we established 4 cell lines, HepG2-400, -89, -A and -8960. Those cells showed slightly radio-resistant compared with the HepG2 cells evaluated by clonogenic assay. Interestingly, no statistical significance was detected by the clonogenic assay with regard to radio resistance between HepG2 and newly established 4 cell lines, the growth rate of HepG2-8960 cells didn't change after the administration of 2 Gy/day of X-rays for 30 days. We think that HepG2-8960 cells are truly the radio resistant cells. In this study, we indicated that radio resistant cells would appear after the long-term X-ray exposure and we have to reconsider the validity of the clonogenic assay to evaluate the radiosensitivity.
