抄録
This paper discusses the effect of dose rate on the strength of relationship between cancer risk and external exposure to low-LET ionizing radiation (such as X-rays and gamma rays), by reviewing important epidemiological studies. The study of atomic-bomb survivors, who had acute exposure mainly to low-LET ionizing radiation, has shown that the excess relative risk per Gray (ERR/Gy) of leukemia increases in a linear-quadratic manner with an increase of radiation dose. The estimate of ERR/Gy for medium-high dose ranges was shown to be approximately two-fold larger than that for a low-dose range in the atomic-bomb survivor study, and was larger than those obtained from the studies of low-dose-rate exposure. On the other hand, the risk of solid cancer (or all cancers excluding leukemia) showed a linear dose-response relationship. Whether chronic exposure has an ERR per dose lower than that of acute exposure is a topic of debate. In the case of high LET exposure such as exposure to alpha particles from radon, chronic exposure is suspected to be associated with a risk larger than acute exposure. In the case of low-LET exposure, however, the cancer incidence study of residents in high natural background radiation areas of Karunagappally Taluk in Kerala State, India suggests that the ERR per dose for solid cancer after chronic radiation exposure is significantly lower than that associated with acute exposure such as that experienced by atomic-bomb survivors.
