Relative Biological Effectiveness and Non-Poissonian Distribution of Dicentric Chromosome Aberrations following Californium-252 Neutron Exposures of Human Peripheral Blood Lymphocytes.

Cells exposed to fast neutrons often exhibit a non-Poisson distribution of chromosome aberrations due to the high ionization density of the secondary reaction products. However, it is unknown whether lymphocytes exposed to californium-252 (252Cf) spectrum neutrons, of mean energy 2.1 MeV, demonstrate this same dispersion effect at low doses. Furthermore, there is no consensus regarding the relative biological effectiveness (RBE) of 252Cf neutrons. Dicentric and ring chromosome formations were assessed in human peripheral blood lymphocytes irradiated at doses of 12-135 mGy. The number of aberrations observed were tested for adherence to a Poisson distribution and the maximum low-dose relative biological effectiveness (RBEM) was also assessed. When 252Cf-irradiated lymphocytes were examined along with previously published cesium-137 (137Cs) data, RBEM values of 15.0 ± 2.2 and 25.7 ± 3.8 were found for the neutron-plus-photon and neutron-only dose components, respectively. Four of the five dose points were found to exhibit the expected, or close to the expected non-Poisson over-dispersion of aberrations. Thus, even at low doses of 252Cf fast neutrons, when sufficient lymphocyte nuclei are scored, chromosome aberration clustering can be observed.

[Effects of dose rate on carcinogenesis after exposure of rats to low doses of neutron irradiation]. / Effets du débit de dose sur la cancérogenèse après exposition de rats à de faibles doses d'irradiation neutronique.

In this experiment we compared the efficiency of fission neutrons of californium 252 at doses of 25 or 53 mGy in function of the dose rate. Two groups of male Sprague-Dawley rats were exposed at 100 or 420 days of age to fission neutrons with dose rates of 950 or 760 microGy/h for 26 or 33 h, and 2 other groups were irradiated over a year from 3 months to 15 months of age with dose rates of 3.58 or 7.72 microGy/h. The 4 groups of animals were compared with a control group of 501 rats. The reduction of effectiveness on cancer induction that we have previously shown at low dose rate with rats exposed to gamma rays or to alpha particles was not observed for low dose rate exposure with fission neutrons.

[Biological action of the radiation from high-activity 252Cf sources]. / Biologicheskoe deistvie izlucheniia istochnikov 252Cf vysokoi aktivnosti.

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.

Survival of cultured human cells exposed to californium 252Cf radiation at low dose rates.

Human T-1 cells were used to determine the rbe of low dose rate 252Cf (n + gamma) radiation, compared to low dose rate gamma radiation. The T-1 cells were irradiated hypoxically at room temperature, and the dose rates were as follows: a) 252Cf (n + gamma), 0.045, 0.090, and 0.270 Gy/hour; and b) 137Cs gamma radiation, 0.117, 0.246, and 0.765 Gy/hour. The RBE was obtained as the ratio of the initial slope of the 137Cs survival curves (approximately the same for all three dose rates), and the initial slope of the 252Cf survival curves (same for all three dose rates). The RBE was 5.0 +/- 1.0 for all components of the 252Cf radiation and 7.1 +/- 1.7 for the neutron components. As the dose increased, an inverse dose rate effect (more sensitive at lower dose rates) for the 137Cs survival curves was observed.

Life shortening in BALB/c mice following brief, protracted, or fractionated exposures to neutrons.

The effects of acute, protracted, or fractionated exposures to fission neutrons on survival times of female BALB/c mice were examined and compared. Mice were given single, brief exposures or exposures given in equal fractions at either 1- or 30-day intervals to doses of 0, 2.5, 5, 10, 20, 50, and 200 rad at the Health Physics Research Reactor (HPRR) or protracted exposures at rates ranging from 0.1 to 10 rad/day using a moderated 252Cf source to doses of 0, 2.5, 5, 10, 20, and 40 rad. The 252Cf source was moderated to have a similar spectron to that of the HPRR facility. After single or fractionated exposures the extent of life shortening increased rapidly over the 0-50 rad range and then began the plateau. No simple model adequately described the dose response over this entire dose range. Over the 0-50 rad dose range for exposures at the HPRR and over the 0-40 rad dose range for protracted exposures the dose response could be adequately described by either a linear model or a square root of the dose regression model except when the dose was fractionated using a 30-day interval. In this instance a linear model provided an adequate fit while a square root of the dose model could be rejected. No increase in effectiveness after fractionation or protraction was observed for neutron-induced life shortening at doses below 50 rad, while at 50 and 200 rad an increase in effectiveness was observed in this and in previous studies. These data were interpreted to suggest that in the dose range below 20-40 rad the dose-effect curve for life shortening may be linear and begins to flatten at higher doses rather than continuously bending at low doses.