1993年 · 第13卷第06期

Excess risk of female breast cancer is among the most comprehensively documented late effects of exposure to substantial doses of ionizing radiation, based on studies of medically irradiated populations and the survivors of the atomic bombings of Hiroshima and Nagasaki, Japan. Relatively little is known about the joint effects of radiation dose with other breast cancer risk factors. A case-control interview study of 201 living breast cancer cases and 580 matched controls, from a defined cohort of Japanese A-bomb survivors, was conducted to identify epidemiological variables strongly related to risk and to investigate their interactive relationship with radiation dose. A non-standard design was used, in which radiation dose was one of the matching variables; information about radiation dose effects was provided by a concurrent incidence study of the entire cohort. Age at first full-term pregnancy, number of births, and total months of lactation were identified as strong, and partially independent, risk factors. Each of these variables interacted with radiation dose in a non-additive, and roughly multiplicative, relationship. The above variables appeared to increase or decrease the excess breast cancer risk associated with radiation exposure in about the same ratios as they affected risk in nonexposed women. This finding is consistent with an initiation-promotion model in which promotion by factors related to reproductive history depends very little on how the cells were initiated.

The nominal risk coefficients for radiation induced cancer are largely based on the follow-up of the mortality from solid cancers among the atomic bomb survivors. For those who have been exposed as adults, the observations are essentially complete, and the risk estimates are, therefore, firmly based on observations. Those who have been exposed as children, have still not reached the age of high cancer incidence. Their observation is, therefore, still incomplete, and the risk estimates are correspondingly uncertain. The modelling of risk has predominantly been based on the postulate, that the relative risk （i.e. the actual cancer rate divided by the age specific normal rate） depend on dose and on age at exposure, and that it does not decline with time since exposure. The high relative risks observed at young ages lead, therefore, with this type of model, to high estimates of life time attributable risk. The ICRP recommendations contain these high risk estimates for young ages at exposure; the high sensitivity of children and juveniles has, indeed, become one of the basic tenets of radiation protection. It is here shown that these conclusions are still hypothetical, because they are merely a matter of the choice of the model. An alternative model assumes a dependence of the excess relative risk on age attained, rather than age at exposure. This model fits the data equally well, and predicts no increased risk for young ages at exposure. A decision between the two models is not possible at present, it will have to await the continued follow-up of those who survived the atomic bombs as children. The ICRP has been criticised for postulating a dose reduction factor （DDREF） in their nominal risk coefficients. If they abandoned this factor, and used the age attained model, rather than their present model, their numerical risk coefficients would remain unchanged.