Low-dose radiation-induced protective process and implications for risk assessment, cancer prevention, and cancer therapy.

A low-dose protective apoptosis-mediated (PAM) process is discussed that appears to be turned on by low-dose gamma and X rays but not by low-dose alpha radiation. PAM is a bystander effect that involves cross-talk between genomically compromised [e.g., mutants, neoplastically transformed, micronucleated] cells and nongenomically compromised cells. A novel neoplastic cell transformation model, NEOTRANS(3), is discussed that includes PAM. With NEOTRANS(3), PAM is activated by low doses and inhibited by moderate or high doses and is, therefore, a hormetic process. A low-dose region of suppression of the transformation frequency below the spontaneous frequency relates to the hormetic zone over which PAM is presumed to operate. The magnitude of suppression relates to what is called the hormetic intensity. Both the hormetic intensity and width of the hormetic zone are expected to depend on dose rate, being more pronounced after low dose rates. It is expected that PAM likely had a significant role in the following observations after chronic irradiation: (1) what appears to be a tremendous reduction in the cancer incidence below the spontaneous level for Taiwanese citizens residing for years in cobalt-60 contaminated apartments; and (2) the published reductions in the lung cancer incidence below the spontaneous level in humans after protracted X irradiation and after chronic gamma plus alpha irradiation. Implications of PAM for cancer prevention and low-dose cancer therapy are briefly discussed.