The tritium RBE at low-level exposure--variation with dose, dose rate, and exposure duration.

Differing values have been reported for the relative biological effectiveness (RBE) of 3H beta-radiation compared with X- and gamma-rays; but the significance of these differences has not been adequately clarified. Because large quantities of tritium are involved in nuclear energy operations, it is essential to have precise knowledge of its effectiveness compared with a "standard" radiation such as gamma-rays. This is especially true for chronic, low-level exposure since available information on protracted irradiation derives mostly from gamma-ray studies. We have approached this problem experimentally by measuring effects of low-level exposures to HTO and gamma-rays on a very sensitive mammalian system. Microscopic enumeration of oocytes in ovaries of exposed and control mice provided measurements of in vivo cell killing by constant HTO levels in body water and, for comparison, by continuous 60Co gamma-irradiation. For protracted exposure from conception to 14 days after birth, the RBE was found to be greater than 1 and to vary inversely with exposure level. At effective gamma-ray doses of 50 rad, delivered at 3.5 rad/day, the RBE was 1.6. But for 25 rad, at 1.8 rad/day, it increased to 1.9. It continued to rise as dose decreased, reaching a value of 2.5. Corrected for scattered radiation, the RBE was 2.8, close to 2.9 reported previously, but below the maximum value of 3.75 predicted by microdosimetric measurements and the theory of dual radiation action. An RBE close to 3 at low doses was not suprising. But decrease at higher doses, due to downward curvilinearity of the gamma-ray dose-response curve, was unexpected for protracted exposure. It suggested that primary oocytes in the prepubertal mouse (the cells used in this study) have limited capacity for recovery. Survival-curves for constant gamma-ray dose rates of 1 rad/min and 0.002 rad/min were therefore determined. Curvilinearity was seen in both, with greater steepness for the higher dose rate, indicating that oocytes do have some ability to recover, but that even with a dose rate only 3.2 rad/day recovery is incomplete. This in turn suggests that the observed gradient in RBE is due partly to a dose-rate effect and partly to inverse variation with dose itself, as predicted by the theory of dual radiation action for acute exposure (or incomplete recovery). Comparing protracted and shorter, 5-day exposures revealed further contrasts. At 30 rad of gamma-rays the RBE in short exposures was only 1.4, while it was 2 for protracted ones. This could be due at least in part to dose rate; it may to some degree, however, reflect difference in microdistribution of tritium atoms, greater radiobiological effect in protracted exposures resulting from 3H incorporation into especially effective sites. These systematic variations of RBE with dose, dose rate, and exposure duration help to explain some of the existing lack of agreement regarding tritium's RBE and provide a more secure basis for hazard evaluation of chronic, low-level exposure.