The Effect of Changes in Cell Geometry Associated with Freezing on the Radiation Dose from Decay of Internal Isotopes

The biological effects of radioactive isotopes located within a cell can often be more easily interpreted if the cells are frozen. This is particularly true when these isotopes are present in small amounts. In this situation, freezing permits a significant amount of radiation damage to accumulate over an extended period without the complications which would arise at normal temperatures from repair of radiation damage and from cell proliferation. Such an approach has been applied to a number of different problems. However freezing also introduces an effect which alters the radiation dose. In order to freeze cells without killing them it is usually necessary to freeze them slowly in the presence of a cryoprotectant such as glycerol or dimethylsulphoxide. It is to be expected that under these conditions ice crystals will form in the extracellular fluid first. This increases extracellular glycerol concentration and cell water flows across the cell membrane into the extracellular medium in response to the osmotic pressure differential which develops. Such a loss of cell water concentrates intracellular radioactive isotopes bound within the cell and thus increases the radiation dose in the frozen cell relative to the unfrozen cell.