Host: The Japan Radiation Research Society
The LNT model was first proposed in a paper (Oliver, 1930) which described the results of the sex-linked recessive lethal test in Drosophila, using mature sperms. Today, however, it is known that mature sperms lack DNA repair function. We have previously reported that when immature spermatocytes with active DNA repair function were irradiated with low dose, low dose-rate X-rays, the mutation frequency became lower than in the non-irradiated control group and thus, there should be a threshold below which no increase in mutation frequency is observed. This phenomenon was not observed when a high dose rate irradiation was carried out, or when wild type flies were replaced by a mutant strain which is defective in DNA repair function. The background mutation frequency in this experiment was 10 to the –6th/gene which corresponds to that induced by a 5Gy irradiation. If a considerable fraction of the spontaneous damages are repaired without error, the background mutation frequency should be reduced, which compensates the increase in the induced mutation and thus would form a practical threshold. Here we present the results of further studies with various doses, which confirmed an U-shaped dose response relationship. The results suggests that the dose response in DNA repair defective cells is linear without thresholds, whereas in repair proficient cells, it is non-linear. It is possible that the LNT model is not suitable for estimation of human cancer risks.
