TO THE EDITOR: I read with interest the Invited Perspective by Duncan et al. (1), which was in response to the article by Siegel et al. (2) criticizing the Biologic Effects of Ionizing Radiation (BEIR) VII report’s endorsement of the linear no-threshold (LNT) model for radiation-induced cancer. Duncan et al. stated that the BEIR VII report provided a detailed case against a low-dose threshold, referring to the linearity of dose response in the atomic bomb survivor cancer data. However, the Ozasa et al. update (3) to the atomic bomb survivor cancer mortality data has shown significant curvature in the dose–response relationship in the 0–2 Gy range. This curvature is inconsistent with the LNT model but consistent with radiation hormesis (4). The Grant et al. update (5) to the cancer incidence data of the atomic bomb survivors has also shown significant curvature in the dose–response relationship that would not be consistent with the LNT model. Thus, the main epidemiologic evidence quoted in the BEIR VII report no longer supports the LNT model.
Duncan et al. also stated that a threshold dose could exist only if the repair mechanisms after exposure to low-dose radiation leave no cells harboring DNA mutations. In making such a statement, the authors did not consider the larger amount of DNA damage that occurs due to endogenous causes. Because low-dose radiation enhances defenses such as DNA repair enzymes, there would be reduced endogenous DNA damage in the period after exposure to low-dose radiation (6), and the ultimate result would be reduced overall DNA damage and mutations, as has been observed in mice, for example (7). Therefore, even though DNA repair mechanism is imperfect, there would be reduction of overall DNA damage after low radiation exposures.
In addition, Duncan et al. did not address the point raised by Siegel et al. (2) of the importance of the immune system deficiency as the cause of cancer. For example, with the suppression of the immune system, cancer mortality rate increased nearly 80-fold in young organ-transplant patients (8), and cancer incidence rate increased nearly 40-fold in young AIDS patients (9). Such data demonstrate the extreme importance of the immune system in preventing cancers. Therefore, low-dose radiation, which has an immune-enhancing effect, would reduce cancers (10).
In summary, the atomic bomb survivor data, with the updates, are inconsistent with the LNT model. The arguments presented by Duncan et al. (1) for the LNT model are invalid due to incomplete consideration of biologic response to low radiation exposures and the neglect of the importance of the immune system in preventing cancers.
Footnotes
Published online Aug. 2, 2018.
- © 2018 by the Society of Nuclear Medicine and Molecular Imaging.
REFERENCES
- Received for publication July 11, 2018.
- Accepted for publication July 16, 2018.
