Search

The dose-response relationship for radiation-induced leukemia was examined in a pooled analysis of three exposed populations: Japanese atomic bomb survivors, women treated for cervical cancer, and patients irradiated for ankylosing spondylitis. A total of 383 leukemias were observed among 283,139 study subjects. Considering all leukemias apart from chronic lymphocytic leukemia, the optimal relative risk model had a dose response with a purely quadratic term representing induction and an exponential term consistent with cell sterilization at high doses; the addition of a linear induction term did not improve the fit of the model. The relative risk decreased with increasing time since exposure and increasing attained age, and there were significant (P < 0.00001) differences in the parameters of the model between datasets. These differences were related in part to the significant differences (P = 0.003) between the models fitted to the three main radiogenic leukemia subtypes (acute myeloid leukemia, acute lymphocytic leukemia, chronic myeloid leukemia). When the three datasets were considered together but the analysis was repeated separately for the three leukemia subtypes, for each subtype the optimal model included quadratic and exponential terms in dose. For acute myeloid leukemia and chronic myeloid leukemia, there were reductions of relative risk with increasing time after exposure, whereas for acute lymphocytic leukemia the relative risk decreased with increasing attained age. For each leukemia subtype considered separately, there was no indication of a difference between the studies in the relative risk and its distribution as a function of dose, age and time (P > 0.10 for all three subtypes). The nonsignificant indications of differences between the three datasets when leukemia subtypes were considered separately may be explained by random variation, although a contribution from differences in exposure dose-rate regimens, inhomogeneous dose distribution within the bone marrow, inadequate adjustment forcell sterilization effects, or errors in dosimetry could have played a role.

The "Gardner hypothesis" suggests that an excess of childhood leukaemia near a nuclear reprocessing plant is caused by paternal exposure to ionizing radiation. But the evidence shows that this explanation is wrong.

Possible explanations for the recently reported increased incidence of childhood leukaemia around Dounreay were examined in the light of changes in the national incidence of leukaemia that occurred during the period of exposure to fallout from international testing of nuclear weapons in the atmosphere. It was concluded that the increase could not be accounted for by an underestimate of the risk of leukaemia per unit dose of radiation at low doses and low dose rates, nor by an underestimate of the relative biological efficiency of high as compared with low linear energy transfer radiation. One possible explanation was underestimation of doses to the red bone marrow due to the discharges at Dounreay relative to the dose from fallout, though investigation of ways in which this might have occurred did not suggest anything definite. Other possible explanations included a misconception of the site of origin of childhood leukaemia, outbreaks of an infectious disease, and exposure to some other, unidentified environmental agent. These findings weigh heavily against the hypothesis that the recent increase in childhood leukaemia near Dounreay might be accounted for by radioactive discharges from nuclear plants, unless the doses to the stem cells from which childhood leukaemia originates have been grossly underestimated.

The distribution of mortality from 11 causes of death (lymphoid leukaemia, other leukaemia, leukaemia of all types, Hodgkin's disease, other lymphomas, all lymphomas, multiple myeloma, lung cancer, other malignancies, all malignancies and all other causes) has been examined in three age groups throughout England and Wales over the period 1969-78. The reorganisation of local authority administration in 1974 meant that the smallest areas that could be examined were 400 county districts or (in some cases) approximate county districts formed by aggregating pre-1974 local authority areas. The variation in the numbers of deaths observed about the numbers expected was assessed using log-linear models to estimate the effect on the relative risk in each district associated with social class, rural status, population size, health authority region and proximity to one of 15 nuclear installations. Trends in risk with increasing proximity to an installation (as judged by the proportion of the population resident within 10 miles) were examined after adjustment for the other four variables. The results showed that in districts near to an installation there were significant excess mortalities in persons under 25 years of age from leukaemia (RR = 1.15, P = 0.01) and especially from lymphoid leukaemia (RR 1.21, P = 0.01) and from Hodgkin's disease (RR 1.24, P = 0.05) and a significant deficiency of mortality from lymphoid leukaemia in persons aged 25-64 years. No significant trends were observed with an increasing proportion of the population near to the installations and the greatest excess mortality from lymphoid leukaemia in young persons was observed in the districts with the intermediate proportion of the population (10.0-65.9%) near an installation.