[Biological action of the radiation from high-activity 252Cf sources]

The radiobiological properties of mixed gamma-neutron radiation from 252Cf sources of high activity have been studied on chinese sand hamster cell culture. The source used has contained 1500 micrograms of radionuclide. In the study of radioprotective effect of hypoxia on the cells it is proposed to use anoxic attenuation factor (AAF) instead of the widely used oxygen enhancement ratio (OER). The comparison of radiation doses leading to a 90% decrease in initial cell viability provided the value of relative biological effectiveness (RBE) of 1.8 +/- 0.2 for mixed radiation and 2.2 +/- 0.2 for the neutron component at the dose of 10 Gy/h. AAF of mixed radiation was estimated as a ratio of radiation doses resulting in the above decrease of the cells viability, defined for the cells irradiation under anoxic or normoxic conditions. Simultaneously, AAF has been defined for gamma-radiation of 60Co. AAF has been shown to be 1.7 +/- 0.2 for 252Cf at dose rate of mixed radiation 10 Gy/h and 3.3 for 60Co. The index of "play back" is proposed as a more objective criterion of any modifier capability to overcome the radioprotective effect of hypoxia on tumour cells. The use of 252Cf sources of high activity instead of 60Co for intracavitary radiotherapy will increase the damage to hypoxic cells of malignant tumours.