Mayak worker cohort: Characteristics and key results of epidemiological studies

Introduction. The medical registry of workers at the Mayak Production Association (PA) was initially established with the purpose of studying the long-term stochastic health effects of occupational radiation exposure at the first nuclear industry enterprise in the USSR. Objective. Assessment of radiogenic risk from prolonged occupational exposure among the Mayak PA worker cohort, including the subcohort of workers exposed to normal radiation conditions. Materials and methods . This study represents one phase of a lifelong retrospective epidemiological investigation of health indicators, including the incidence and mortality from malignant neoplasms (MN), conducted within the framework of the medical-dosimetric registry of Mayak PA workers. The available study cohort is limited to employees of three main production facilities and two auxiliary plants, hired between 1948 and 1982. Within the study cohort, two subcohorts are distinguished based on factual data on radiation exposure levels and assessed medical outcomes. These include the subcohort of 1948–1958, personnel hired during the technology development phase and characterized by high occupational radiation exposure levels and that of 1959–1982, hired during routine operational periods with radiation doses comparable to modern limits. At the current stage, the attained age of workers in the second subcohort and the volume of accumulated data have enabled an analysis focused on individuals having worked under standard conditions, excluding the effects of high doses and dose rates. This has expanded the scope of statistically significant direct estimates of radiogenic MN risk. All studies of radiogenic risk in the cohort of Mayak PA workers were conducted using the Epicure statistical software package. Results. The cohort comprised 25,755 workers. The vital status during the period of up to 31.12.2018 was known for 94% of subjects. In the 1948–1958 subcohort, the mean cumulative gamma radiation dose was 748 mGy, compared to 130 mGy in the 1959–1982 subcohort. Overall, 10,304 individuals (40.1% of the cohort) received low doses of gamma radiation. The mean cumulative lung dose from alpha radiation due to incorporated 239Pu was 179.4 mGy, with 329.2 mGy and 41.0 mGy for the 1948–1958 and 1959–1982 subcohorts, respectively. The estimated excess relative risk per 1 Gy of alpha radiation lung dose was 3.5–8 for 60-year-old males. No deviations from linearity were found. Radiogenic risk decreased with an increase in age. A nonlinear dose-response relationship was identified for liver MN. The primary long-term effect of external gamma radiation was leukemia development, where a nonlinear model incorporating effect modification by age at exposure, time since exposure, and attained age provided better approximation than a linear model. For solid MN, the risk coefficient from external gamma radiation ranged 0.1–0.4 per 1 Gy. Among workers employed under normal radiation conditions (1959–1982 hiring period), the attributable risk assessment suggests that 1–5% of MN (excluding tumors in plutonium primary deposition organs) were radiation-induced, solely due to external gamma exposure. Conclusions. The Mayak PA worker cohort, with its high-quality medical and dosimetric data, serves as a crucial source for direct epidemiological assessments of radiogenic risks from prolonged occupational radiation exposure. The identification of the routine production operation period not only validates the magnitude of carcinogenic risk but also highlights the need to extend both the follow-up period and the cohort itself to include more workers exposed to conditions comparable to modern standards.