BIOLOGICAL EFFECTIVENESS OF NEUTRON IRRADIATION ON ANIMALS AND MAN.

摘要

Neutron experiments on a highly radiosensitive in vivo system--oocytes in mice--provide new insight into the nature of the radiosensitive targets of these important cells. With the radiobiological literature as background, neutron data from animals and humans (including studies by the author using reevaluated Hiroshima-Nagasaki dosimetry) are integrated, and the controversial question of radiation protection standards for neutrons is addressed.; Oocyte killing in juvenile mice by 0.43-MeV, ('252)Cf-fission, and 15-MeV neutrons, compared with that by ('60)Co gamma rays, yields unusually low neutron RBEs (relative biological effectiveness). At 0.1 rad of 0.43-MeV neutrons the RBE is only 1.8, contrasting greatly with values of 100 or more reported at low-doses for other endpoints. In mice just prior to birth, however, when oocytes are less radiosensitive, the neutron RBE is much higher, similar to values for most other mammalian endpoints. This dramatic change in neutron RBE with mouse age (occurring within 2-3 days) can be explained as the result of a shift from a less radiosensitive target (presumably nuclear DNA) to a much more radiosensitive one (probably the oocyte plasma membrane).; Study of the neutron literature reveals that factors most important in influencing neutron RBE are dose and biological endpoint. For most (but not all) endpoints neutron RBE increases with decreasing dose. Effects of other factors (neutron energy, dose rate, etc.) must therefore be evaluated at constant dose (rather than constant level of effect as generally done). Critical analysis of published mutation data shows that the influence of neutron energy on RBE has frequently been overestimated.; Both animal and human data suggest the intriguing general pattern (albeit with exceptions) of low neutron RBEs for malignancies that have known (or suspected) viral association (e.g., lymphomas) and high RBEs (similar to those for chromosome aberrations) for malignancies not so associated (e.g., lung cancer).; Using various approaches, a value for the neutron Quality Factor (Q, a radiation protection standard) is estimated as 17 ((+OR-) 100%), much lower than 100 which has been suggested. With the large uncertainty, 17 is not markedly different from the value of 10 presently in general use.