Statement on the Health of Children Born to Radiation Workers
Introduction
In its Second [1] and Third [2] Reports, the Committee on the Medical Aspects of Radiation in the Environment (COMARE) recommended that studies should be set up to consider any possible effects on the health of the offspring of parents occupationally exposed to radiation. The Department of Health (DH) and the Health and Safety Executive (HSE) provided funding for two major epidemiological studies. Prior to the setting up of these two studies, Gardner et al [3] published a study suggesting that occupational exposure of fathers to ionising radiation prior to the conception of any offspring, raised the risk of the subsequent development of leukaemia or non-Hodgkin's lymphoma (LNHL) in those children and that this raised risk was dose dependent. Gardner's conclusions rested mainly on the observation that there was an increase in the incidence of LNHL in the children of those fathers who received the highest radiation dose (greater than 100 mSv). This association and the possibility that it could be the cause of some cases of childhood leukaemia has come to be known as the "Gardner hypothesis". Following publication of this study COMARE made further recommendations for biological research to address the Gardner hypothesis. COMARE also stated that both of the epidemiological studies already recommended should be designed to test this hypothesis. In 1990, DH and HSE jointly established the Co-ordinating Committee on Health Aspects of Radiation Research (CCHARR) to manage Government-sponsored research to investigate the association suggested by Professor Gardner and his colleagues. The epidemiological studies recommended by COMARE were included in the CCHARR programme.
Studies of health outcomes in children of radiation workers
The first of these two epidemiological studies to be completed is that of Draper et al [4] and is referred to here as the "Linkage study". This has been considered by COMARE prior to publication. The second is known as the Nuclear Industry Family Study (NIFS), which is due to be published in 1998. A further study by Roman et al [5] , which was funded by HSE and published in 1996, examined the reproductive outcome and health of the children of medical radiographers, who may also be exposed to radiation because of their occupation.
COMARE has considered the results of these two recently published studies [4,5], which have both examined the health of children whose parents were exposed to ionising radiation in the course of their work. As the Nuclear Industry Family Study has yet to be completed, this statement is the interim view of COMARE. A full statement will be provided when COMARE has seen the results of NIFS and the biological research funded by CCHARR.
Findings of the studies
(1) The Linkage study
The objective of this case-control study 4 was to determine whether or not parents who are occupationally exposed to radiation before the conception of their children, have an increased chance of having children who develop cancer and to investigate the radiation dose to which those parents were exposed prior to the conception of their children. The cases were children aged 0-14 years, who were born and diagnosed with cancer between 1952 and 1986 in Britain. Scottish cases of LNHL diagnosed in the period 1987-1990 were also included. The study used the database held by the National Radiological Protection Board (NRPB), on more than 120,000 workers who were registered as being monitored for their potential exposure to radiation. This database, which is known as the National Registry for Radiation Workers (NRRW) and which includes occupational data on doses of radiation, was linked to other databases holding information on childhood cancers.
These childhood cancer databases are held by the National Registry of Childhood Tumours, the Oxford Survey of Childhood Cancers and a database of Scottish childhood LNHL compiled by Professor Kinlen. The aim was to identify children of radiation workers and details of workers preconception radiation doses. The NRRW database was also linked to data on control children matched to the cases by area and time of birth. Information was not available on the children of non-radiation workers employed at nuclear installations.
The main finding of the study was that fathers of children with LNHL were significantly more likely to be radiation workers than were fathers of control children born in the same part of the country. The relative risk for children of fathers who had been radiation workers prior to their conception was 1.77 with 95% confidence limits of 1.05-3.03. However, the Linkage study found no relationship with dose in the preconception period. Indeed, the association was most marked in those with doses below the limit of detection, that is below 0.1 mSv and there was no association with preconception doses greater than 100 mSv, the level at which Gardner et al 3 found a high relative risk. There was no increased risk for any other childhood cancers associated with paternal radiation exposure (relative risk 0.94 with 95% confidence limits of 0.56-1.58).
The relative risk for all childhood cancers in the children whose mothers were radiation workers was higher at 5.0 with 95% confidence limits of 1.42-26.94. However, the latter figures are based on very small numbers (15 cases and 3 controls) and must be interpreted with great caution.
The authors discussed possible explanations for their results including population mixing (sometimes known as the "Kinlen hypothesis" [6] ). Kinlen and his colleagues and other workers have carried out a number of analyses relating to areas in which there have been high levels of population mixing, particularly between persons from urban and rural backgrounds such as occurs in the construction of remote industrial plants and have found a significantly increased incidence of childhood leukaemia in such areas. This effect has been attributed to the bringing together of infectious and susceptible individuals in abnormal numbers.
The authors of the Linkage study have concluded that their results do not support the Gardner hypothesis. They have suggested that other factors relating to radiation workers such as exposure to infective or other agents, or chance, may explain the findings.
(2) The medical radiographers study
The study of medical radiographers 5 used a questionnaire technique to investigate the incidence of adverse reproductive outcomes and the subsequent health, including cancer, of the children of medical radiographers, a predominantly female workforce. The frequencies of adverse reproductive outcomes (eg, miscarriage and stillbirth) were similar to those reported in other studies. There were no substantial differences in the risks of major congenital malformations, chromosomal anomaly or cancer in the children of radiographers as compared to the general population. In the age group 0-14 years, 3 cancers were observed in children of male radiographers compared to 1.1 expected. For the children of female workers, the figures were 7 cancers observed to 7.7 expected. Because of the small numbers involved the authors of the study themselves stress that caution needs to be exercised in the interpretation of the results. It should be noted that long-term dose records are not routinely held for medical radiographers. Thus, dose-response analyses could not be performed.
COMARE Interim Statement on the Studies
COMARE has noted the results of the Linkage study and agrees with the authors that there is no evidence for a dose-related effect between preconception radiation dose to parents and the incidence of LNHL in their children. There was no association with preconception doses greater than 100 mSv and the association was most marked in the children of those workers with occupational doses below the limit of detection. Hence, an increase in LNHL with radiation dose, implicit in the Gardner hypothesis, is not supported by this study and this is in line with the findings of other recently published data [7,8,9].
One aspect of the Linkage study, however, needs further consideration. That is the finding that male radiation workers were 1.77 times more likely to have children who develop LNHL than the general population. This finding must be interpreted with some caution, but if it represents a real risk and is not due to chance, then the risk must be small in absolute terms. The outcome of the radiographers study also supports this conclusion. This finding is consistent with the possibility that any effect could be due either to being a radiation worker or being employed at an establishment where radiation work is undertaken, whether or not by the individual concerned. The latter interpretation is not unreasonable as an hypothesis because it has been shown that both radiation and non-radiation workers in the nuclear industry have a cancer and mortality profile that differs significantly from that of the general population. Overall mortality and the incidence of all cancers taken together are lower in nuclear workers, but with an increase in one specific group of cancers, namely LNHL. This may possibly be due to the differences in the lifestyle of nuclear workers [10] . There is also some suggestion in the Linkage study that female radiation workers have an increased risk of having children with cancer. However, as the number of female workers included in the study was small, no firm conclusion can be drawn at present.
The authors of the Linkage study suggest that population mixing may play a role in explaining their findings, a possibility that COMARE discussed in its Fourth Report [11] in regard to the rate of LNHL in young people resident in the village of Seascale close to the Sellafield reprocessing plant. If this were to be the explanation for the results of the Linkage study, increased risks due to these factors should also apply to non-radiation workers at nuclear sites, but neither the Linkage study nor other studies as yet published have been specifically designed to address this issue.
We may get a much clearer picture when the results from the Nuclear Industry Family Study (NIFS) become available. We are advised that this study should be completed early in 1998. NIFS will contain information about the health of families of non-radiation workers as well as radiation workers. Consequently, COMARE proposes to reserve further comment until the results of NIFS can be considered in conjunction with the studies described above and the results of the biological research commissioned by CCHARR.
References
1. Committee on Medical Aspects of Radiation in the Environment (COMARE) (1988). Second Report. Investigation of the possible increased incidence of leukaemia in young people near the Dounreay Nuclear Establishment, Caithness, Scotland. HMSO, London.
2. Committee on Medical Aspects of Radiation in the Environment (COMARE) (1989). Third Report. Report on the incidence of childhood cancer in the West Berkshire and North Hampshire area, in which are situated the Atomic Weapons Research Establishment, Aldermaston and the Royal Ordnance Factory, Burghfield. HMSO, London.
3. Gardner M J, Snee M P, Hall A J, Powell C A, Downes S and Terrell J D (1990). Results of case-control study of leukaemia and lymphoma among young people near Sellafield nuclear plant in West Cumbria. Br Med J , 300 , 423-429.
4. Draper G J, Little M P, Sorahan T, Kinlen L J, Bunch K J, Conquest A J, Kendall G M, Kneale G W, Lancashire R J, Muirhead C R, O'Connor C M and Vincent T J (1997). Cancer in the offspring of radiation workers: a record linkage study. Br Med J , 315 , 1181-1188.
5. Roman E, Doyle P, Ansell P, Bull D and Beral V (1996). Health of children born to medical radiographers. Occup Environ Med, 3 , 73-79.
6. Kinlen L J (1995). Epidemiological evidence for an infective basis in childhood leukaemia. Br J Cancer , 71 , 1-5.
7. Little M P, Wakeford R and Charles M W (1994). A comparison of the risks of leukaemia in the offspring of the Sellafield workforce born in Seascale and those born elsewhere in West Cumbria with the risks in the offspring of the Ontario and Scottish workforces and the Japanese bomb survivors. J Radiol Prot , 14 , 187-201.
8. Little M P (1993). A comparison of the risks of leukaemia in the offspring of the Japanese bomb survivors and those of the Sellafield workforce with those in the offspring of the Ontario and Scottish workforces. J Radiol Prot , 13 , 161-175.
9. Little M P, Wakeford R, Charles M W and Anderson M (1996). A comparison of the risks of leukaemia and NHL in the first generation offspring (F1) of the Danish thorotrast patients with those observed in other studies of parental pre-conception irradiation. J Radiol Prot , 16 , 25-26.
10. Carpenter L, Higgins C, Douglas A, Fraser P, Beral V and Smith D (1994). Combined analysis of mortality in three United Kingdom nuclear industry workforces 1946-1988. Radiat Res, 138 , 224-238.
11. Committee on Medical Aspects of Radiation in the Environment (COMARE) (1996). Fourth Report. The incidence of cancer and leukaemia in young people in the vicinity of the Sellafield site, West Cumbria: further studies and an update of the situation since the publication of the report of the Black Advisory Group in 1984. HMSO, London.
7 November 1997
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