SOLO (Epidemiological studies of exposed populations in the Southern Urals) was a five year integrated, multi-disciplinary project to investigate the risks to human health of low and protracted radiation exposures, considering exposures to radionuclides within the body as well as external radiation. The project continued and developed upon previous follow-up studies of the health of two key population cohorts from the Southern Urals of the Russian Federation: the Mayak Worker Cohort (MWC) and the Techa River Cohort (TRC), the latter being exposed as a result of Mayak discharges of radioactivity to the Techa River. It also makes use of the Sellafield worker cohort (SWC) in the UK and, for the first time, has pooled cohorts to assess risks from exposures in utero (MWC and TRC) and from plutonium in workers (MWC and SWC)*.
The reliability of risk estimates obtained from epidemiological studies is dependent on the quality of radiation dose information for the members of the cohorts being studied. Two important strands of work were undertaken to improve the reliability of external dose estimates for the Mayak and Techa cohorts. Personal dosemeter data for Mayak workers and dose estimates for Techa residents based on location were compared for validation with results obtained using EPR measurements on teeth and FISH chromosome aberration techniques on cultured white blood cells. In addition, detailed environmental measurements at a particular village on the Techa River were made to facilitate dose reconstruction for inhabitants. Important developments included improvements in the reliability of EPR measurements and insights into the use of FISH to measure internal doses from strontium-90 and plutonium-239 as well as external dose. The estimation of internal doses to the foetus were improved for plutonium-239 using measurements of urinary excretion in mothers who had been Mayak workers, and their offspring, and for strontium-90 by detailed modelling of data relating to Techa residents. Updated models and a common approach were applied to the calculation of plutonium doses to Mayak and UK Sellafield workers.
Cancer risks in adults from external radiation and internal plutonium
Analyses were completed of morbidity from lung, liver and bone cancer, all other solid cancers (as a single group) and from haematolymphatic cancers in the MWC. The results provided important support to previous observations of mortality, with risk estimates consistent with published values, and evidence that plutonium-239 intakes can cause lung, liver and bone cancer but that the radionuclide is ineffective as a leukaemogen. Analyses of lung cancer and leukaemia in a pooled cohort of Mayak and Sellafield plutonium workers showed consistency of results across the pooled cohort with raised risks of lung cancer but not leukaemia. The dose-response and risk estimates for plutoinium-239 induced lung cancer were consistent with comparable risk estimates for external gamma radiation when account is taken of the relative biological effectiveness of alpha particles emitted by plutonium-239.
Non-cancer risks in adults from external radiation and internal plutonium
Previous analyses of ischaemic heart disease (IHD) and cerebrovascular disease (CeVD) mortality and morbidity in the MWC were extended with longer follow-up and more cases to give greater statistical power. These analyses showed good statistical evidence for an association between external radiation exposure and morbidity but not mortality, an observation requiring further investigation. A linear-dose response relationship was observed for the incidence of IHD and CeVD in relation to exposure to external radiation but the dose-response for plutonium-239 exposure was more complex. Analysis of circulatory disease mortality in the pooled cohort of Mayak and Sellafield plutonium workers was complicated by differences in the background disease rates; risk estimates were mostly positive in relation to external exposure (although notably less so for MWC compared to SWC) but there was little evidence for a link with internal exposure. Analyses of risks of chronic respiratory disease incidence in the MWC recorded significant excess risk in relation to internal plutonium-239 exposure to the lung but no excess risk in relation to external gamma radiation exposure.
Cancer risks following in utero irradiation from external radiation and internal radionuclides
A pooled cohort of children was defined whose mothers either worked at Mayak or lived near the Techa River before and/or during pregnancy. External and internal radiation doses to these children were calculated based on dosimetry models developed during the project. The pooled cohort currently lacks the statistical power to show significant results, but the absence of a positive result at this stage is important in showing that risks of in utero exposure have not been dramatically underestimated, particularly for internal radionuclides.
*The research was coordinated with related research being carried out by others, particularly in the Russian Federation and the USA, so as to avoid duplication.
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