Evidence of a neutron RBE of 70 (+/- 50) for solid-tumor induction at Hiroshima and Nagasaki and its implications for assessing the effective neutron quality factor.

Carcinogenic and genetic effects in the cohort of A-bomb survivors are the main source of information on radiation effects on humans. A detailed analysis of the age-specific mortality rates for solid tumors as a group using the widely accepted linear-quadratic cum cell-killing dose-effect model reveals that the Hiroshima-Nagasaki data are statistically consistent with a limiting, low-dose neutron RBE of about 70. This value, although consistent with a wide range of neutron RBEs, is about 3.5 times higher than the current effective quality factor for neutrons. The data on leukemia mortality do not support the notion of a difference in effectiveness between neutron and gamma radiation; however, these two radiation sources appear to act synergistically. For both types of tumors, the commonly used concepts of constant, dose-independent RBE and "gamma effective dose" cannot be justified other than at very low neutron doses.