Lithuanian cohort of Chernobyl cleanup workers: Cancer incidence follow-up 1986–2012

1

Introduction

On the 26th of April 1986, an accident of the Chernobyl nuclear power plant (NPP) in Ukraine resulted in release of large quantities of radionuclides followed by exposure of millions of residents in nearby territories of the former Soviet Union, as well as over half a million of workers involved in emergency response, containment and cleanup at the Chernobyl site [ 1 ]. They performed different activities and were mainly exposed to external radiation from radioactive materials deposited on the ground and building surfaces [ 2 ]. In addition, individuals involved in recovery work during the first weeks after the accident may have received considerable thyroid doses due to intake of radioactive iodine (I-131), primarily by inhalation [ 1 ]. About 240,000 of cleanup workers worked in 1986 and 1987, when doses were the highest [ 1 ].

Cleanup workers’ doses were monitored with personal dosimeters, by group dosimetry (one personal dosimeter per group) or by prior assessment of dose based on the dose rate and duration of work before the implementation of the task [ 3 ]. The results, called ‘official doses’, were recorded in the official documents at the workers’ departure from Chernobyl. The average effective dose from external radiation received by cleanup workers between 1986 and 1990 was estimated to be about 120 milisievert (mSv) [ 1 ].

Up to the end of 1991, the main source of identification and information on cleanup workers was the All-Union Distributed Clinico-Dosimetric Register, which recorded information on 659,292 persons [ 1 ]. After the dissolution of the USSR into independent states, national Chernobyl registers continued to operate in Belarus, the Russian Federation and Ukraine.

About 17,000 men (mostly military reservists) were mobilised from Estonia, Latvia and Lithuania (almost 7000 men), then a part of the Soviet Union [ 4 ]. Within the framework of the collaborative study between the US National Cancer Institute, the Finnish Cancer Registry and researchers from the three Baltic countries, Lithuanian cleanup workers, similar to their Latvian and Estonian counterparts, were enrolled into a Baltic cohort [ 4 ].

Because of the relatively higher doses received, compared to general population, and because they are registered in national Chernobyl registries or established national cohorts and therefore feasible to follow-up, the cleanup workers comprise one the most suitable populations for epidemiological studies of long-term health effects, such as cancer, after exposure in adulthood [ 5 ]. So far, epidemiological studies of cleanup workers have reported dose-related increases of thyroid cancer [ 6 ], haematological malignancies [ 78910 ] and solid cancers [ 11 ].

Due to existing long-lasting population based cancer registries and high completeness of follow-up using unique PID, Baltic cohorts of cleanup workers, although smaller in their size than the cohorts from Belarus, the Russian Federation or Ukraine, provide a high quality data on cancer incidence and mortality. Several reports have been published on cancer incidence and mortality in these cohorts [ 4 , 121314 ]. In the most recent report of the Baltic cohort, due to incomplete data for death and emigration at that time, a proportional incidence ratio (PIR) – based on number of cancer cases by site, age group and year of diagnosis in the cohort, and in the respective male population – was used to estimate the site-specific cancer risk for the Lithuanian cohort [ 4 ]. Recent changes in the biomedical research legislation enabled us to obtain individual data on mortality and emigration which were used in the current report. In this study, for the first time, we independently report results of cancer incidence until 2012 in the Lithuanian cohort of Chernobyl cleanup workers, the largest cohort in the Baltics.