Measurement of radiation survival using the MTT assay

There is increasing interest in the development of rapid assays of radiosensitivity which can be used on clinical specimens. Unfortunately, the measurement of radiation survival using clonogenic assays, which are the established standard, can be difficult and time consuming. We have used the MTT assay to measure the radiation survival of four lymphoblastoid cell lines with low plating efficiencies. We measured surviving fractions both when the irradiated cells had regained exponential growth and when the non-irradiated cells had undergone four or more doublings. The results were compared to surviving fractions measured by clonogenic assay. We found both methods could be used successfully to rank the cell lines in order of radiosensitivity. However, cells exposed to the higher radiation doses in the MTT assay did not always regain exponential growth, limiting the dose range for which the assay was useful. We also found the best correlation between the two assays was sometimes obtained by using the MTT surviving fractions from different days for different radiation dose levels. Thus, although the MTT assay can be used to measure radiation survival in relation to other cell lines, its use can be complicated by restrictions on radiation dose ranges and difficulties with data interpretation.