Revisiting radiation dose-response for tissue reactions: with emphasis on diseases of the circulatory system.

In the system of radiological protection, tissue reactions are characterized by the presence of a threshold dose. While the threshold dose is defined as the estimated dose for 1% incidence (ED1), it may be complicated by substantial baseline levels for diseases that develop with aging in the absence of radiation exposure. ICRP Publication 118 states that ED1 refers to effects just starting to rise above the baseline levels in unirradiated, age-matched individuals and in the case of circulatory diseases, to a dose which would increase the already high natural incidence or mortality by only 1%. This implicitly assumes that radiation has a unique mechanism of action in inducing circulatory disease. However, circumstantial evidence suggests that radiation is likely to act jointly with other risk factors to promote atherosclerosis. If radiation-induced subclinical damage is added to that resulting from other risk factors, the threshold dose could be decreased depending on the extent of the joint effect. This phenomenon might be observed in age-related diseases that result from the accumulation of subclinical damage, and it is inferred that radiation accelerates the development of these diseases in a dose-dependent manner.