Dose Reconstruction for Epidemiological Studies among Ukrainian Chernobyl Cleanup Workers

Abstract

The present paper provides an overview of the methods and summarizes the results of estimating radiation doses and their uncertainties for Ukrainian-American epidemiological studies among the Chernobyl (Chornobyl) cleanup workers. After the Chernobyl accident occurred on April 26, 1986, more than 300,000 Ukrainian cleanup workers took part between 1986 and 1990 in decontamination and recovery activities at the site of the Chernobyl Nuclear Power Plant. The U.S. National Cancer Institute in collaboration with the Ukrainian National Research Center for Radiation Medicine conducted several epidemiological studies in this population. An important part of these studies was the reconstruction of the study participants’ radiation doses and the assessment of uncertainties in doses. A method called realistic analytical dose reconstruction with uncertainty estimation (RADRUE) was used to calculate the doses from external irradiation during cleanup missions, which was the main exposure pathway for most study participants. At the initial phase of the accident during the atmospheric releases of radioactivity from the destroyed reactor, the cleanup workers also received doses from inhalation of radionuclides. In addition, study participants received doses at their places of residence, especially those who lived in highly contaminated areas. The radiation doses estimated for 2,048 male cleanup workers included in the Ukrainian-American epidemiological studies varied widely: (i) bone-marrow doses from external irradiation in the case-control study of leukemia of 1,000 cleanup workers ranged from 3.7 × 10⁻⁵ mGy to 3.3 Gy (mean = 92 mGy); (ii) thyroid doses in the case-control study of thyroid cancer in 607 persons from all exposure pathways combined were from 0.15 mGy to 9.0 Gy (mean = 199 mGy); (iii) gonadal doses in 183 cleanup workers from all exposure pathways combined in the study of germline mutations in the offspring after parental irradiation (trio study) ranged from 0.58 mGy to 4.1 Gy (mean = 392 mGy); (iv) thyroid doses in the human factor uncertainties study among 47 persons were from 20 mGy to 2.1 Gy (mean = 295 mGy); and (v) lung doses in the study of germline genetic variants associated with host susceptibility to COVID-19 estimated for 211 cleanup workers were from 0.024 mGy to 2.5 Gy (mean = 249 mGy). Doses of female cleanup workers were much lower than those of male cleanup workers: the mean doses for female cleanup workers were 27 mGy for 34 women included in the trio study and 56 mGy for 48 women participated in the study of germline genetic variants associated with host susceptibility to COVID-19. Uncertainties in dose estimates included two components: (i) inherent uncertainties arising from the stochastic random variability of the parameters used in exposure assessment and from a lack of knowledge about the true values of the parameters; and (ii) human factor uncertainties due to poor memory recall resulting in incomplete, inaccurate, or missing responses during personal interviews with cleanup workers conducted long after exposure. This paper also discusses possible developments and improvements in the methods to assess the radiation doses and associated uncertainties for cleanup workers.

INTRODUCTION

After the Chernobyl (Chornobyl) accident occurred in Ukraine on April 26, 1986, more than 300,000 Ukrainian cleanup workers (also colloquially called ‘liquidators’) took part between 1986 and 1990 in the decontamination and recovery activities within the 70-km zone around the Chernobyl Nuclear Power Plant (ChNPP) (1). Population of cleanup workers was extremely heterogeneous and consisted of different occupational groups, including NPP personnel, nuclear workers and specialists, military, construction workers, and support staff. They were sent to their mission by different organizations to perform activities such as decontamination, repair and maintenance of ChNPP equipment, construction, supply, and logistics, at various locations from roofs of the reactor building to remote locations in the 70-km zone around NPP (2, 3). The U.S. National Cancer Institute in collaboration with the Ukrainian National Research Center for Radiation Medicine conducted several radiation epidemiological studies in this population.

An important component of these studies was the reconstruction of radiation doses for the cleanup workers along with an assessment of the uncertainties in doses (4–8). This paper provides an overview of methods and results of the reconstruction of radiation doses for cleanup workers along with an assessment of the uncertainties and discusses further developments in this area.

MATERIALS AND METHODS

Study Population

The Ukrainian-American radiation epidemiological studies among cleanup workers include:

• Case-control study of leukemia and related disorders conducted in two phases, 2001–2004 and 2008–2011 (9, 10);

• Case-control study of thyroid cancer conducted in 2009–2017 (11);

• The study of human factor uncertainties (HFU) in the radiation dosimetry of cleanup workers conducted in 2010–2015 (8);

• The American-Ukrainian trio study of effects of parental irradiation in cleanup workers and evacuees from Pripyat on germline mutations in their offspring conducted in 2015–2020 (12, 13); and

• The ongoing study of germline genetic variants associated with host susceptibility to COVID-19 (COVNET study).

The cleanup workers in these studies were divided into the following 11 categories depending on the period of beginning of work or their affiliation and the type of work performed (4):

1. Witnesses of the accident were at the ChNPP site when the accident happened or came there before May 1, 1986, and who were not diagnosed later with acute radiation syndrome (ARS).

2. Victims of the accident are the witnesses of the accident who had confirmed ARS diagnosis.

3. Early cleanup workers are all civilian cleanup workers (except for ChNPP personnel) who worked within the 70-km zone, including the industrial area of ChNPP, between April 27 and May 31, 1986.

4. ChNPP personnel are the plant staff members who worked between May 1986 and the end of 1990 to maintain and prepare other units of the ChNPP for regular operation.

5. Sent to assist the ChNNP staff are employees of other nuclear power plants who were sent to assist and temporarily substitute for the regular ChNPP staff at time of recovery and preparation for restart of Units 1–3.

6. Staff of AC-605 are persons from the organization, named Administration of Construction No. 605, involved in the construction of the shelter over the damaged reactor.

7. Staff of Kurchatov Institute are scientists and engineers from the Kurchatov Institute of Atomic Energy who studied the fuel mass inside the Unit 4 reactor building.

8. Military cleanup workers are either regular military or army reservists.

9. Sent on mission are civilians who were sent between June 1, 1986 and December 31, 1990 to perform various tasks in the 70-km zone.

10. Staff of ‘Combinat’ are individuals from that organization who performed a variety of tasks in the 70-km zone and coordinated and supervised the work of persons sent on mission to the 70-km zone.

11. Mixed refers to cleanup workers who worked at ChNPP site several times as members of different categories.

The majority (95%) of the cleanup workers registered in the State Chernobyl registry of Ukraine were male (14). However, two studies considered in the present paper included female cleanup workers: 34 women were participants of the study of germline mutations in the offspring after parental irradiation (trio study) and 48 women, of the study of germline genetic variants associated with host susceptibility to COVID-19.

Exposure Pathways

Table 1 summarizes the components of radiation doses reconstructed for epidemiological studies among Ukrainian cleanup workers. Exposure pathways considered in the studies were determined by the target organ and population groups included in the study after careful a priori evaluation of the significance of specific pathways and their contributions to target organ doses. The cleanup workers received doses from external irradiation during the cleanup mission (15, 16). However, during the 10-day lasting period, when most of the radioactivity was released into the atmosphere from destroyed Unit 4, the cleanup workers included in a case-control study of thyroid cancer also might receive doses to the thyroid gland from internal irradiation resulting from inhalation of ¹³¹I and short-lived radioiodine and radiotellurium isotopes (¹³²I,¹³³I, ¹³⁵I, ^131mTe and ¹³²Te). In addition to the dose received during the mission, these cleanup workers, who resided or visited heavily contaminated areas of Ukraine and Belarus [at that time also a part of the Soviet Union, where ¹³¹I ground deposition density reached up to 510 MBq m⁻² (7)], might have received thyroid doses at these locations due to consumption of locally produced food contaminated with ¹³¹I. Cleanup workers included in the trio study might have received gonadal doses at their places of residence from both external irradiation and internal irradiation due to consumption of locally produced food contaminated with ¹³⁴Cs and ¹³⁷Cs.

TABLE 1.

Summary of Dosimetry Support for the Ukrainian-American Radiation Epidemiological Studies

Study	Place of exposure	Component	Exposure pathway	Target organ
Case-control study of leukemia	Cleanup missiona	External	Gamma-emitting radionuclides	Red bone marrow
Case-control study of thyroid cancer	Cleanup missiona	External	Gamma-emitting radionuclides	Thyroid gland
	Cleanup missionb	Internal	Inhalation of 131I	Thyroid gland
	Cleanup missionb	Internal	Inhalation of 132I, 133I, 135I, 131mTe, and 132Te	Thyroid gland
	Residencec	Internal	Intake of 131I	Thyroid gland
Germline mutations in the offspring after parental irradiation (trio study)	Cleanup missiond	External	Gamma-emitting radionuclides	Gonads
	Residence in Pripyate	External	Gamma-emitting radionuclides	Gonads
	Residenced	External	Gamma-emitting radionuclides	Gonads
	Residenced	Internal	Ingestion of 134Cs and 137Cs	Gonads
Human factor uncertainties	Cleanup missiona	External	Gamma-emitting radionuclides	Thyroid gland
Germline genetic variants associated with host susceptibility to COVID-19	Cleanup missiona	External	Gamma-emitting radionuclides	Lungs
	Residence in Pripyate	External	Gamma-emitting radionuclides	Lungs
	Cleanup missionb,f	Internal	Inhalation of radionuclides mix	Lungs

^aBetween April 26, 1986 and December 31, 1990.

^bBetween April 26, 1986 and May 6, 1986.

^cBetween April 26, 1986 and June 30, 1986.

^dBetween April 26, 1986 and date of birth of the youngest child minus 38 weeks according to the study design (Bazyka et al. 2020).

^eBetween April 26, 1986 and date of evacuation, typically on April 27, 1986.

^fDose estimation for this exposure pathway is in progress and, therefore, is not presented in this paper.

Doses from External Irradiation During the Cleanup Mission

The time-and-motion method named realistic analytical dose reconstruction with uncertainty estimation (RADRUE) was used to estimate radiation doses from external irradiation during the cleanup mission in all studies considered in the present paper. The RADRUE technique calculates the organ-specific absorbed dose from external irradiation, $D_{\mathit{ext}}^{\mathit{mission}}$ (mGy), as shown by Kryuckov et al. (17):

$$D_{\mathit{ext}}^{\mathit{mission}}=C_{\mathit{org}}\cdot \sum _{i=1}^N\sum _{j=1}^{L_i}\mathit{AKR}\left(t_{i,j}\right)\cdot \Delta t_{i,j}\cdot {\mathit{LF}}_j$$ (1)

where C_org is the conversion coefficient from air kerma rate to the absorbed dose rate in the organ of interest, i.e., red bone marrow, thyroid or gonads (mGy h⁻¹ per mGy h⁻¹) (18); N is the number of days in cleanup mission; L_i is the number of cleanup activities (working, traveling, or resting) at different locations j on day i; AKR(t_i,j) is the air kerma rate in air at the location j of cleanup mission on day i (mGy h⁻¹); Δt_i,j is the time interval of cleanup activity at the location j on day i (h); LF_j is the location factor, i.e., the ratio of air kerma rate at given exposure point to air kerma rate at reference condition, i.e., 1 m above undisturbed lawn far from any buildings, at the place of cleanup activity j (unitless).

The RADRUE method has been validated through intercomparison exercises that compared RADRUE doses to the most reliable dose estimates available for various groups of cleanup workers: (i) 39 professional workers from the Ministry of Atomic Energy who wore calibrated personal thermoluminescence dosimeters; (ii) 20 early responders with dose estimates based on quantitative analysis of unstable chromosome aberrations (dicentrics); and (iii) 68 other cleanup workers with dose estimates based on measurement of electron paramagnetic resonance in tooth enamel (17). A pairwise comparison between RADRUE dose estimates and those reconstructed by the other three methods showed reasonable agreement within the range of uncertainty estimated by RADRUE.

A computer code, known as RADRUE, was developed to calculate doses from external irradiation during the cleanup mission. Dosimetry experts, who are experienced cleanup workers and are familiar with the cleanup activities in the 70-km zone, analyzed the work history collected for the study subjects during a personal interview (see section “Personal Dosimetry Interview” for details) and reconstructed the type, place and duration of activities and itineraries. To calculate organ-specific doses, this information was entered by the dosimetry expert into a RADRUE calculation data file and linked to a database of measured and interpolated exposure-rate values.

In some instances, because cleanup workers may have overestimated the amount of time they spent in specific high-exposure-rate areas or made mistakes in localizing their workplace, the doses calculated by RADRUE were well above the permissible dose levels. Therefore, a dose limitation procedure was introduced in RADRUE, and doses were estimated with limitations for the cleanup workers whose work met certain eligibility criteria for this procedure. Doses estimated with limitations, when applicable, were more realistic, and, therefore, were used in epidemiological analyses. A detailed description of the RADRUE method can be found elsewhere (17).

Doses from Inhalation of ¹³¹I and Short-Lived Radioiodine and Radiotellurium Isotopes During the Cleanup Mission

A model was developed to calculate the thyroid doses for cleanup workers from ¹³¹I inhalation (19). It considers several factors, including the ground-level outdoor air concentrations of ¹³¹I at the locations of residence and work of the cleanup worker, the reduction of ¹³¹I activity in inhaled air associated with indoor occupancy, the time spent indoors, the breathing rate, which depends on the type of physical activity, and intake of potassium iodine (KI) pills for iodine prophylaxis. Model was validated using the historical measurements of ¹³¹I thyroid activity conducted between April 30 and May 5, 1986, in a group of 594 cleanup workers. During the measurement, all cleanup workers were asked to provide brief information on their whereabouts and activities since April 26, 1986, as well as on intake of stable iodine (KI) pill to block the uptake of ¹³¹I by the thyroid gland; among them, a detailed of hour-by-hour whereabouts and work history was available for 60 measured cleanup workers. Thyroid dose from ¹³¹I inhalation to cleanup workers during cleanup mission, $D_{\mathit{inh}}^{I-131}$ (mGy), was calculated using the following equation (7):

$$D_{\mathit{inh}}^{I-131}=\frac{13.82\cdot E_{\mathit{th}}}{m_{\mathit{th}}}\cdot \sum _{i=1}^N\left[\left[\sum _{j=1}^{L_i}{I}_{\mathit{inh,j}}\cdot w_{\mathit{inh}}\cdot w_{\mathit{th}}\cdot {\mathit{CF}}_{\mathit{KI,i}}\right]+Q_{\mathit{thyr,inh,i}-1}\cdot e^{-\left({\lambda }_{\mathit{th}}+{\lambda }_{\mathit{r,I}-131}\right)\cdot \Delta t}\right],$$ (2)

where 13.82 is a unit conversion factor (Bq kBq⁻¹ g kg⁻¹ J MeV⁻¹ s d⁻¹ mGy Gy⁻¹); E_th = 0.2 MeV is the mean energy absorbed in the thyroid per decay of ¹³¹I; m_th = 20 g is the thyroid mass for adult male; N = 66 is the number of days counted since the accident until complete elimination of ¹³¹I from the thyroid after inhalation; I_inh,j is the intake function of ¹³¹I with contaminated air at the location of the cleanup activity j during the day i (kBq); w_inh = 0.66 is the fraction of inhaled iodine transferred to blood (20); w_th = 0.3 is the fraction of iodine uptake by the thyroid (21); CF_KI,i is the correction factor on day i of w_th that varies with time after intake of stable iodine for prophylactic reason (22) (unitless); Q_{thyr,inh,i−1} is the activity in the thyroid of the study subject on day i-1 from inhalation of ¹³¹I (kBq); λ_th = 0.0078 d⁻¹ is the rate of biological elimination of iodine from the thyroid of adult individual (21); λ_r,I−131 = 0.0862 d⁻¹ is the radioactive decay rate of ¹³¹I; Δt = 1 d.

Thyroid dose from inhalation of short-lived radiotellurium and radioiodine isotopes (^131mTe, ¹³²Te,¹³²I, ¹³³I, and ¹³⁵I) during the cleanup mission, was also estimated as a fraction of thyroid dose from inhalation of ¹³¹I using the approach suggested by Gavrilin et al. (23).

A special module to calculate doses from inhalation of ¹³¹I and short-lived radioiodine and radiotellurium isotopes during cleanup mission has been developed and added to the computer code initially developed to implement the RADRUE method, which was modified for the purposes of thyroid cancer study to a new computer code named Rockville. Another major modification was the creation of regular spatial grids of ¹³¹I concentration in air in the 70-km zone.

Doses from External Irradiation During Residence in Pripyat

A special method for assessing the exposure during residence in Pripyat, mainly between April 26 and April 30, 1986, has been developed and applied (24). The location factor-values (LF-values) were calculated for each apartment in each building in Pripyat (indoor LF-values) as well as for locations outside buildings (outdoor LF-values) for exposure from radionuclides deposited on the ground surface and from the passing radioactive cloud (24). The organ-specific absorbed dose from external irradiation during residence in Pripyat, $D_{\mathit{ext}}^{\mathit{Pripyat}}$ (mGy), was calculated using the following equation:

$$D_{\mathit{ext}}^{\mathit{Pripyat}}=C_{\mathit{org}}\cdot \sum _{i=1}^N\mathit{AKR}\left(t_i\right)\cdot \Delta t_i\cdot {\mathit{LF}}_i$$ (3)

where N is the number of different locations in Pripyat (apartment, store, outdoors, etc.) where individual spent time before the evacuation (unitless); AKR(t_i) is the air kerma rate in air (mGy h⁻¹) at location i in Pripyat at time t_i; Δt_i is the duration of staying in Pripyat at location i (h); LF_i is the location factor at location i (unitless).

The developed approach was implemented into the Rockville computer code as a special module to calculate doses from external irradiation for Pripyat residents. For cleanup workers who resided in Pripyat, the doses received during residence and during cleanup mission were calculated together, since it was inconvenient to separate these two exposure components. In addition, there was no need for such separation as the total dose from all components of exposure was required to be estimated in radiation epidemiological studies.

Doses Received During Residence in Locations Other than Pripyat

Dosimetry models developed originally for the general population of Ukraine and Belarus (25–31) were used to estimate doses from ¹³¹I intake, external irradiation, and ingestion of ¹³⁴Cs and ¹³⁷Cs with foodstuffs to cleanup workers during their residence outside the cleanup mission.

The thyroid dose from ¹³¹I intakes during residence, $D_{\mathit{ecol,res}}^{I-131}$ (mGy), was estimated by adjusting the ecological thyroid dose calculated using the following equation (30):

$$D_{\mathit{ecol,res}}^{I-131}=\frac{13.82\cdot E_{\mathit{th}}}{m_{\mathit{th}}}\cdot \sum _{k=1}^N\left[I_{\mathit{ing,k}}\cdot w_{\mathit{ing}}\cdot w_{\mathit{th}}\cdot {\mathit{CF}}_{\mathit{KI,k}}+Q_{\mathit{thyr,ing,k}-1}\cdot e^{-\left({\lambda }_{\mathit{th}}+{\lambda }_{\mathit{r,I}-131}\right)\cdot \Delta t}\right]$$ (4)

where N = 66 is the number of days counted since the accident until conventionally complete elimination of ¹³¹I from the thyroid after ¹³¹I intake during residence until June 30, 1986; I_ing,k is the intake function of iodine ¹³¹I with foodstuffs during the day k (kBq); w_ing = 1.0 is the fraction of ingested iodine transferred to blood (21) (unitless); Q_{thyr,ing,k−1} is activity in the thyroid of the study subject on day k-1 from ingestion of ¹³¹I (kBq); Δt = 1 d.

The area-specific scaling factors were then used to adjust the ecological thyroid dose from ¹³¹I intake. Their values were derived as ratios of measured ¹³¹I thyroid activity and ¹³¹I thyroid activity at the time of measurement calculated using an ecological model (29). A detail description of the dose reconstruction model and its parameter values is given elsewhere (7).

The organ-specific dose from external irradiation, $D_{\mathit{ext}}^{\mathit{resid}}$ (mGy), was calculated using the approach based on the integration of the time-dependent dose rate in air per unit deposition of radionuclides, considering the shielding properties of the residential environment and individual behavior of the person collected during the personal interview:

$$D_{\mathit{ext}}^{\mathit{resid}}={\mathit{BF}}_m\cdot \sum _i{\mathit{DC}}_i\cdot {\int }_{t_1}^{t_2}{\sigma }_{Cs137}\cdot R_{i/Cs137}\cdot e^{-{\lambda }_i^r\cdot t}\cdot p\left(t\right)\mathit{dt}$$ (5)

where ${\mathit{BF}}_m={\sum }_n{\mathit{LF}}_{\mathit{m,n}}·{\mathit{OF}}_{\mathit{m,n}}$ is the behavior factor that takes into account the location factor at place n indoor or outdoor (LF_m,n) and the typical fraction of time spent during the 24 h by a person at this place n(OF_m,n) in a rural or urban m-th type of settlement (unitless); DC_i is the organ-specific dose rate per unit activity of radionuclide i deposited on the ground surface (mGy d⁻¹ per kBq m⁻²); σ_Cs137 is the ¹³⁷Cs deposition density in settlement of residence (kBq m⁻²); R_i/Cs137 is the ratio of activity of radionuclide i in deposition to that of ¹³⁷Cs (unitless); ${\lambda }_i^r$ is the radioactive decay rate of radionuclide i (d⁻¹); p(t) is the attenuation function that reflects the decreasing dose rate due to radionuclides migration in soil; it was applied for long-lived ¹³⁴Cs and ¹³⁷Cs only (unitless); t₁, t₂ are times corresponding to the beginning and end of staying in settlement of residence (d).

The organ-specific dose due to ingestion of radiocesium isotopes, $D_{\mathit{ing}}^{\mathit{resid}}$ (mGy), was calculated using the approach based on the relationship between ¹³⁷Cs deposition density and ¹³⁷Cs soil-to-milk transfer and on the intake of radiocesium isotopes with foodstuffs that was derived from whole body counter measurements of radiocesium body burden carried out in population in Ukraine and Belarus in regions with different ¹³⁷Cs ground deposition densities. The organ-specific absorbed dose due to ingestion of radiocesium isotopes during residence in the settlement other than Pripyat was calculated as:

$$D_{\mathit{ing}}^{\mathit{resid}}={\sigma }_{Cs137}\cdot \sum _k{\mathit{DF}}_k\cdot {\int }_{t_1}^{t_2}{I}_k\left(t\right)\mathit{dt}$$ (6)

where k is the index to denote radiocesium isotopes, ¹³⁴Cs and ¹³⁷Cs; DF_k is the organ-specific absorbed dose due to intake via ingestion of unit activity of radiocesium isotope k (mGy Bq⁻¹) (21); I_k (t) is the variation with time of intake function of radiocesium isotope k normalized to ¹³⁷Cs deposition density (Bq d⁻¹ per kBq m⁻²).

A detail description of dose reconstruction models for external irradiation and ingestion of radiocaesium isotopes and its parameter values can be found elsewhere (6, 25).

Personal Dosimetry Interview

A personal interview with each study subject or, in the case of deceased or incapable subject, proxy-colleagues was conducted using the questionnaire that was specifically designed to estimate individual doses received during the cleanup mission(s). This questionnaire allowed the collection of detailed information on (i) cleanup worker’s routes to and from his/her³ work location(s) at the ChNPP site and in the 70-km zone, (ii) cleanup activities he/she performed, including duration and applicable shielding from radiation, and (iii) locations of residence and resting quarters during the mission. The questionnaire covers the entire period of participation in the cleanup activities between April 26, 1986 and December 31, 1990 (legally technical date of the official completion of cleanup work). Another questionnaire was used to collect information on residence in Pripyat, including (i) detailed hour-by-hour outdoor/indoor locations between the time of the accident and evacuation from Pripyat, (ii) exact address in Pripyat and floor of residence, and (iii) evacuation route within the 70-km zone. The questionnaire covers the time from the accident outbreak (1:23 AM on April 26, 1986) to the moment of eventual departure from the 70-km zone after evacuation. A detailed description of the questionnaire’s data processing by dosimetry expert, entering and validation that were applied in the RADRUE method can be found elsewhere (17).

For the studies, which included residential exposure (see Table 1), questionnaires were specifically designed to elicit information on (a) places and dates of residence(s) after the Chernobyl accident, (b) construction material of residential buildings in each place of residence, (c) consumption of locally produced foodstuff, namely cow’s milk, dairy products, meat (pork, poultry), potatoes, root vegetables and mushrooms during residence at each location. The questionnaire covers the period April 26–June 30, 1986 (for thyroid cancer study) or the entire period from the time of the accident to the date of birth of the youngest offspring included into trio study, except for participation in the cleanup mission and residence in Pripyat.

The dosimetry interviewers were trained by cleanup workers and former staff members of ChNPP, who are well-informed about the chronology of cleanup activities at the ChNPP site and within the 70-km zone. The senior interviewer, a former cleanup worker, provided coordination between the interviewers and ensured quality control of the completed study questionnaires.

Assessment of Uncertainties

Uncertainties in doses for epidemiological studies among cleanup workers included two components, the so-called “inherent” and “human factor” uncertainties (4).

Inherent uncertainties are associated with errors in radiation fields, shielding and other parameters of the dose calculation models. To assess the inherent uncertainties, a Monte Carlo method was used to calculate 10,000 individual stochastic doses for each study subject. For doses from external irradiation and inhalation of radioiodine and radiotellurium isotopes during the cleanup mission, all sources of errors were considered as independent and unshared. For the doses received by the cleanup workers during the residence, the approaches applied to the general population (6, 32, 33) were used, and 1,000 sets of the study population doses, considering categorizing of errors as shared or unshared, were calculated using the Monte Carlo method. The characteristics of the parameters’ distributions of dosimetry model as well as classification of parameters as sources of shared and unshared errors are given elsewhere (5–7, 17).

Human factor uncertainties arose from errors in answering questions during a personal interview being conducted 25–30 years later with the cleanup worker or proxy (for a deceased study subject). It is highly likely that due to poor memory recall, especially if proxies (colleagues) were interviewed instead of deceased subjects, the incomplete or inaccurate answers increased the uncertainties in doses. The HFU were assessed by comparison of two doses from external irradiation (8): (i) a “reference” dose, $D_{\mathit{ref}}$, calculated using the historical description of cleanup activities reported by 47 cleanup workers shortly after the completion of the cleanup mission (recollection time of 3 weeks and longer, medium = 1.7 years) that was considered to be ‘true’ (“gold” standard) dose; and (ii) a “current” dose, $D_{\mathit{cur}}$, calculated using information reported by 47 cleanup workers and respective 24 proxies (colleagues) nominated by cleanup workers during a personal interview conducted more recently (recollection time was 27 years in average). The same answers regarding the date, location, duration, and type of cleanup activity that were provided during historical and current interviews resulted in the same dose estimate referred to as “agreed” dose, $D_{agree}$. To assess the agreement between dose pairs, $D_{\mathit{ref}}$ and $D_{\mathit{cur}}$, the Jaccard similarity coefficient (34), $J_{\mathit{sim}}$, was calculated as:

$$J_{\mathit{sim}}=\frac{D_{\mathit{agree}}}{D_{\mathit{ref}}+D_{\mathit{cur}}-D_{\mathit{agree}}}$$ (7)

Values of J_sim vary between 0 and 1.0, when 0 means complete discrepancy and 1.0 means complete agreement.

RESULTS AND DISCUSSION

Doses Estimated for the Subjects of Epidemiological Studies

Table 2 provides the summary of doses from different exposure pathways reconstructed for male cleanup workers who were included in the Ukrainian-American radiation epidemiology studies. The radiation doses estimated for them varied widely: (i) red bone marrow doses from external irradiation in the case-control study of leukemia ranged from 3.7 × 10⁻⁵ mGy to 3.3 Gy (mean = 92 mGy); (ii) thyroid doses in the case-control study of thyroid cancer from all exposure pathways combined were from 0.15 mGy to 9.0 Gy (mean = 199 mGy); (iii) gonadal doses from all exposure pathways combined in trio study ranged from 0.58 mGy to 4.1 Gy (mean = 392 mGy); (iv) thyroid doses in the HFU study were from 20 mGy to 2.1 Gy (mean = 295 mGy); and (v) lung doses in COVNET study were from 0.024 mGy to 2.5 Gy (mean = 249 mGy).

TABLE 2.

Summary of Doses from Different Exposure Pathways Reconstructed for Male Cleanup Workers Who Were Participants of the Ukrainian-American Epidemiological Studies

Study	Target organ, exposure pathway	Number of subjectsa	Dose to target organ (mGy)			Ref.
			Mean	Median	Range
Case-control study of leukemia and related disorders	Bone marrow, external, missionb	1,000	92	17	3.7 × 10−5–3,260	(5)
Case-control study of thyroid cancer	Thyroid, external, mission	607	140	20	0.015–3,630	(7)
	Thyroid, 131I, mission	200	44	12	~0–1,680
	Thyroid, SL,c mission	198	11	1.6	~0–377
	Thyroid, 131I, residence	587	42	7.3	0.001–3,430
	Total thyroid dose	607	199	47	0.15–9,020
Germline mutations in the offspring after parental irradiation (trio study)	Gonads, external, mission	183	389	152	0.063–4,080	(6)
	Gonads, external, residence	183	1.9	0.68	0.007–43
	Gonads, Cs ingestion, residence	182	0.57	0.31	0.001–16
	Total gonadal dose	183	392	154	0.58–4,080
Human factor uncertainties	Thyroid, external, missiond	47	295	164	20–2,080	(8)
Germline genetic variants associated with host susceptibility to COVID-19 (COVNET study)	Lungs, external, mission	211e	249	100	0.024–2,510	—

^aNumber of subjects with given exposure pathway.

^bMission means the dose, received during the mission to the exclusion zone including exposure during residence in Pripyat for personnel of ChNPP. Possible doses received in other residence locations were neglected.

^cSL, short-lived radioiodine (¹³²I, ¹³³I, ¹³⁵I) and radiotellurium (^131mTe, ¹³²Te) isotopes.

^dDoses calculated using historical questionnaire are given in the table.

^eIncluding 91 cleanup workers for whom historical questionnaires from 1986 were used to develop a dose reconstruction model for internal lung exposure due to inhalation of radionuclides during the cleanup mission.

It should be noted that Table 2 provides doses to male cleanup workers only. The mean doses from external irradiation during the cleanup mission differ significantly between male and female cleanup workers, respectively, 389 vs. 27 mGy for subjects of trio study and 249 vs. 56 mGy for subjects of COVNET study. Doses of female cleanup workers were much lower than those of male cleanup workers due to differences in locations and type of work between female and male cleanup workers. The proportion of male cleanup workers who worked at the ChNPP site and within the 4-km zone was much higher than that among female cleanup workers (14).

Figure 1 shows the distribution of the thyroid doses from external irradiation during the cleanup mission (logarithm of value) calculated for 2,048 male cleanup workers included in the Ukrainian-American epidemiological studies. The thyroid gland was used as the target organ only as an example so that one figure could show doses from different studies calculated to different organs (bone marrow, thyroid, gonads, and lungs). Organ-specific doses for the subjects from the study of leukemia and related disorders (bone marrow), trio (gonads) and COVNET (lungs) studies were recalculated to the thyroid doses using a conversion factor of 1.1, 1.04, and 1.04 for bone marrow, gonadal, and lung doses, respectively. Conversion factors were obtained as a ratio of the conversion coefficient from air kerma rate to the thyroid gland to that to the red bone marrow, to that to the male gonads, and to that to the lungs for mean photon energy around 0.3 MeV, 1.1 = 0.739/0.669, 1.04 = 0.739/0.710, and 1.04 = 0.739/0.712, respectively (18). A rather wide range of thyroid doses was observed among cleanup workers: doses varied within eight orders of magnitude from 4.1 × 10⁻⁵ mGy to 4.24 Gy. Doses more than 0.5 Gy were estimated for 163 individuals (8.0% of the total) while 64 (3.1%) individuals received doses more than 1 Gy.

FIG. 1.

Distribution of the thyroid doses from external irradiation during the cleanup mission (logarithm of value) calculated for 2,048 male cleanup workers included in the Ukrainian-American epidemiological studies.

The dose distribution (Fig. 1) is not logarithmically normal, as it is typically observed in the general population, e.g., (33). The method of topographical classification of dose distributions (35) defines this distribution as a hybrid logarithmically normal, which is typical for occupational exposure when measures are applied to reduce exposure if doses approach some established dose limit (36). This kind of distribution could be expected given the wide variety of occupational groups and tasks performed by the cleanup workers, which led to extremely heterogeneous exposure. Application of the dose limitation procedure in the RADRUE method led to a cutoff of the right side of the distribution. This procedure was implemented to 270 (16.2% of the total) of male cleanup workers included in the Ukrainian-American epidemiological studies, mainly for the following categories: military (54.1% of the total), mixed (30.0%), sent to assist the ChNNP staff (5.9%), and ChNPP personnel (4.8%). This observation is supported by the practice of dose management at time of decontamination works, when daily dose constraints were introduced, and their implementation was controlled by setting the permissible duration for each element of the work, i.e., the work was stopped when preset time was expired.

Table 3 provides the distribution of the thyroid doses from external irradiation during the cleanup mission for 2,048 male cleanup workers included in the Ukrainian-American epidemiology studies and number of individuals from categories of cleanup workers in each dose range. More than half of the cleanup workers belonging to the category sent on mission (category no. 9) received doses less than 3 mGy. A rather uniform distribution over the dose ranges was observed for individuals from the categories of early cleanup workers (no. 3), military (no. 8), and staff of “Combinat” (no. 10). For more than 25% of individuals from the categories of witnesses of the accident (no. 1), victims of the accident (no. 2), sent to assist ChNPP staff (no. 5), and mixed (no. 11), estimated doses exceeded 300 mGy.

TABLE 3.

Distribution of the Thyroid Doses from External Irradiation During the Cleanup Mission for 2,048 Male Cleanup Workers Included in the Ukrainian-American Epidemiological Studies and Number of Individuals from Categories of Cleanup Workers in Each Dose Range

Category	Thyroid dose range (mGy)							Entire study
	<3.0	3.0–9.99	10–29.9	30–99.9	100–299.9	300–999.9	≥1,000
1. Witnesses of the accident	—	1 (8.3%)	3 (25.0%)	2 (16.8%)	1 (8,3%)	4 (33.3%)	1 (8,3%)	12 (100%)
2. Victims of the accident	—	—	—	—	—	—	3 (100%)	3 (100%)
3. Early cleanup workers	51 (22.5%)	56 (24.8%)	29 (12.8%)	27 (12.0%)	31 (13.7%)	28 (12.4%)	4 (1.8%)	226 (100%)
4. ChNPP personnel	—	—	1 (8.3%)	4 (33.4%)	5 (41.7%)	1 (8.3%)	1 (8.3%)	12 (100%)
5. Sent to assist the ChNPP staff	2 (18.2%)	—	2 (18.2%)	2 (18.2%)	2 (18.2%)	2 (18.2%)	1 (9.0%)	11 (100%)
6. Staff of AC-605	1 (5.6%)	2 (11.1%)	3 (16.6%)	—	5 (27.8%)	6 (33.3%)	1 (5.6%)	18 (100%)
7. Staff of Kurchatov Institute	—	—	1 (20.0%)	—	2 (40.0%)	1 (20.0%)	1 (20.0%)	5 (100%)
8. Military cleanup workers	102 (14.4%)	80 (11.3%)	94 (13.3%)	197 (27.9%)	178 (25.2%)	49 (6.9%)	7 (1.0%)	707 (100%)
9. Sent on mission	267 (52.7%)	79 (15.5%)	64 (12.6%)	50 (9.9%)	26 (5.1%)	18 (3.6%)	3 (0.6%)	507 (100%)
10. Staff of ‘Combinat’	3 (16.7%)	6 (33.3%)	4 (22.2%)	2 (11.1%)	3 (16.7%)	—	—	18 (100%)
11. Mixed	34 (6.5%)	53 (10.0%)	87 (16.4%)	84 (15.9%)	115 (21.7%)	114 (21.6%)	42 (7.9%)	529 (100%)
All categories	460 (22.5%)	277 (13.5%)	288 (14.0%)	368 (18.0%)	368 (18.0%)	223 (10.9%)	64 (3.1%)	2,048 (100%)

Table 4 provides the distribution by categories of cleanup workers of the thyroid doses from external irradiation during the cleanup mission. Major categories of cleanup workers were military (707 individuals, 34.5% of the total, mean dose = 113 mGy), sent on mission (507 individuals, 24.8%, 46 mGy), mixed (529 individuals, 25.8%, 320 mGy), and early cleanup workers (226 individuals, 11.0%, 121 mGy). The highest exposure categories of cleanup workers quite expectedly included victims of the accident (mean dose = 3,520 mGy), Staff of Kurchatov Institute (366 mGy), and witnesses of the accident (347 mGy). Mean thyroid dose due to external irradiation during the cleanup mission among the entire population of 2,048 male cleanup workers was 160 mGy. Representatives of the following categories received also thyroid doses from inhalation of ¹³¹I and short-lived radionuclides as they started cleanup mission between April 26 and May 6, 1986 (not shown in Table 4): early cleanup workers (43 individuals, mean dose = 25 mGy), military (n = 42, 38 mGy), and mixed (n = 114,72 mGy).

TABLE 4.

Categories of Male Cleanup Workers Included in the Ukrainian-American Epidemiological Studies and Their Thyroid Doses from External Irradiation During the Cleanup Mission [Based on (5–8)]

Category	N	Percent (%)	Thyroid dose from external irradiation during the cleanup mission (mGy)
			Mean	Median	Range
1. Witnesses of the accident	12	0.6	347	127	5.1–1,140
2. Victims of the accident	3	0.1	3,520	3,490	2,840–4,240
3. Early cleanup workers	226	11.0	121	14	0.16–1,350
4. ChNPP personnel	12	0.6	229	142	26–1,060
5. Sent to assist the ChNPP staff	11	0.6	263	48	0.32–1,110
6. Staff of AC-605	18	0.9	254	140	1.0–1,120
7. Staff of Kurchatov Institute	5	0.2	366	266	16–1,010
8. Military cleanup workers	707	34.5	113	55	0.0091–1,770
9. Sent on mission	507	24.8	46	2.2	4.1 × 10−5–2,080
10. Staff of ‘Combinat’	18	0.9	47	9.9	0.15–264
11. Mixed	529	25.8	320	107	0.28–3,760
All categories	2048	100.0	160	30	4.1 × 10−5–4,240

As mentioned above, the definition of categories of cleanup workers are based on the period of beginning of work or their affiliation and the type of work performed, and, therefore, cleanup workers of the same category can be expected to have fairly similar doses. However, huge variability, 3–5 orders of magnitude, was observed between the cleanup workers for the doses from external irradiation received during the mission (Tables 3 and 4). For some categories, victims of the accident, ChNPP personnel, and staff of Kurchatov Institute, the doses variability is much lower. However, due to the small number of individuals in these categories, it is difficult to conclude that the exposure of these categories was rather uniform.

To evaluate the dose rates of protracted exposure to the cleanup workers, their daily doses were analyzed. Figure 2 shows the variation with time of 163,925 daily thyroid doses from external irradiation during the mission (logarithm of value) calculated for the cleanup workers included in the Ukrainian-American epidemiological studies. There was a sharp decrease in daily doses in 1986 associated with a decrease of the exposure rates in air due to the decontamination of buildings and soil surfaces and radioactive decay as well as the improvement of the practice of radiation protection of cleanup workers. Most of the cleanup workers received their doses at low daily dose rates, however, 122 individuals received daily doses between 25 and 50 mGy, 91 individuals, between 50 and 100 mGy, 119 individuals, between 100 and 250 mGy, and 13 individuals had daily doses greater than 250 mGy at some point during their cleanup mission, mainly in 1986. It should be noted that the proportion of persons who received doses at a dose rate above 25 mGy among Ukrainian cleanup workers is significantly higher (345 out of 2,048 individuals, 16.8% of the total) than among cleanup workers from Belarus, Russia and Baltic countries included in the studies of hematological malignancies (7 out of 357, 2.0%) and of thyroid cancer (5 out of 530, 0.9%) conducted by International Agency for Research on Cancer (IARC) (37, 38).

FIG. 2.

Variation with time of 163,925 daily thyroid doses from external irradiation during the cleanup mission (logarithm of value) calculated for male cleanup workers included in the Ukrainian-American epidemiological studies.

Table 5 shows the mean, median and range of daily thyroid doses from external irradiation among 2,048 male cleanup workers during the cleanup mission by months and years of the mission. The mean daily dose was 0.96 mGy and the median was 30 μGy over the entire period of the cleanup activities between April 26, 1986 and December 31, 1990. As expected, the highest daily doses were realized in April 1986 with a maximum daily dose of 3,430 mGy for a cleanup worker who belongs to the category of victims of the accident. During 1986 the mean monthly- averaged daily doses dropped from 38 mGy in April to 0.51 mGy in December, and the median daily dose dropped sharply by almost 500 times from 8.9 mGy in April to 0.018 mGy in December. The mean year-averaged daily doses decreased by 45 times from 3.3 mGy in 1986 to 0.071 mGy in 1990. Again, huge variability, 4–6 orders of magnitude, of the daily doses from external irradiation was observed between the cleanup workers over the entire period of the cleanup activities (Fig. 2 and Table 5).

TABLE 5.

Arithmetic Mean and Range of Daily Thyroid Doses from External Irradiation among 2,048 Male Cleanup Workers During the Cleanup Mission by Months and Years of the Mission

Year	Month	Daily thyroid dose from external irradiation during the cleanup mission (mGy)
		Mean	Median	Range
1986	April	38	8.9	0.0024–3,430
	May	5.2	0.36	5.7 × 10−4–730
	June	2.2	0.14	0.0016–180
	July	1.4	0.11	0.0024–180
	August	1.2	0.10	0.0022–55
	September	0.82	0.074	0.0010–380
	October	0.84	0.060	0.0012–44
	November	0.74	0.030	7.2 × 10−4–44
	December	0.51	0.018	5.0 × 10−4–34
1986	All	3.3	0.14	5.7 × 10−4–3,430
1987	All	0.33	0.028	2.5 × 10−4–16
1988	All	0.16	0.019	2.1 × 10−4–31
1989	All	0.10	0.010	3.3 × 10−4–3.2
1990	All	0.071	0.069	4.3 × 10−4–1.7
April 26, 1986–December 31, 1990		0.96	0.030	2.1 × 10−4–3,430

Uncertainties in Doses

Sets of multiple individual stochastic doses were calculated for each study subject of the Ukrainian-American epidemiological studies among cleanup workers: 10,000 doses from external irradiation, inhalation of ¹³¹I and short-lived radionuclides during cleanup mission as well as 1,000 doses from ¹³¹I intake, external irradiation, and ingestion of radiocesium isotopes during residence, where appropriate. The fitted distribution of individual stochastic doses for each subject was found to be approximately lognormal and the geometric standard deviation (GSD) of that distribution was used to characterize the overall uncertainty for everyone. Figure 3 shows, as an example, normal probability plot of individual stochastic doses (logarithm of values) from different exposure pathways calculated for a representative subject of a case-control study of thyroid cancer.

FIG. 3.

Normal probability plot of individual stochastic doses (logarithms) from different exposure pathways calculated for the subject of a case-control study of thyroid cancer.

Table 6 summarizes inherent uncertainties in doses for male cleanup workers included in epidemiology studies. The study- and exposure pathway-mean GSDs attached to the individual stochastic doses varied from 1.3 to 2.6. The largest uncertainty was assessed for the thyroid doses from ¹³¹I intake during residence (GSD = 2.6) because, in the absence of measured ¹³¹I thyroid activity for study subjects, doses were estimated using ecological modeling only. The highest GSDs attached to the individual stochastic doses (up to GSD = 15) were found for thyroid doses from intake of short-lived radioiodine and radiotellurium isotopes during the cleanup mission from the high uncertainty in isotopes’ concentration in air.

TABLE 6.

Summary of Inherent Uncertainties in Doses for Radiation Epidemiology Studies among 2,048 Male Cleanup Workers

Study	Exposure pathway	Shared errors	Geometric standard deviation		Ref.
			Mean	Range
Case-control study of leukemia and related disorders	External, missiona	N	2.0	1.2–5.9	(5)
Case-control study of thyroid cancer	External, mission	N	2.0	1.2–6.9	(7)
	131I, mission	N	1.8	1.3–5.4
	SLb, mission	N	2.0	1.4–15
	131I, residence	Y	2.6	1.8–4.8
Germline mutations in the offspring after parental irradiation (trio study)	External, mission	N	1.8	1.3–4.6	(6)
	External, residence	Y	1.3	1.2–1.4
	Cs ingestion, residence	Y	1.4	1.2–1.6
Human factor uncertainties	External, mission	N	2.0	1.5–2.8	(8)
Germline genetic variants associated with host susceptibility to COVID-19 (COVNET study)	External, mission	N	2.0	1.2–5.0	—

^aMission means the dose, received during the mission to the exclusion zone including exposure during residence in Pripyat for personnel of ChNPP. Possible doses received in other residence locations were neglected.

^bSL, short-lived radioiodine (¹³²I, ¹³³I, ¹³⁵I) and radiotellurium (^131mTe, ¹³²Te) isotopes.

Figure 4 shows the distribution of the GSDs of the individual stochastic doses from external irradiation during the cleanup mission for 2,048 male cleanup workers from the Ukrainian-American epidemiological studies. GSD ranged from 1.17 to 6.9 with an overall arithmetic mean of 2.0 and a median of 1.8 across all studies.

FIG. 4.

Distribution of the GSDs of the individual stochastic doses from external irradiation during the cleanup mission for 2,048 male cleanup workers from the Ukrainian-American epidemiological studies.

The study-mean GSDs in doses from external irradiation during the cleanup mission varied between 1.8 and 2.0 (Table 6). A similar pattern of uncertainties was found in other case-control studies among cleanup workers conducted by IARC that also used the RADRUE method: the mean GSD was found to be 1.9 among 357 and 530 subjects of the studies of hematological malignancies (37) and of thyroid cancer (38), respectively.

With regards to HFU, the best agreement between the reference and current doses was observed for the early cleanup workers with an arithmetic mean of J_sim-values of 0.33 and a median of 0.34, followed by group of sent on mission cleanup workers (mean = 0.26, median = 0.21) and by the proxies (0.23, 0.22), while the worst agreement was for the military cleanup workers (0.19, 0.17) (8). As a result, the HFU due to poor memory recall in the distant past led to underestimation or overestimation of the “true” dose for most cleanup workers up to 3 times.

Further Possible Developments in the Dosimetry of Cleanup Workers

As mentioned above, the RADRUE method, which was implemented for studies among the cleanup workers, considers all sources of errors to be unshared. However, there are potential sources of shared errors related to the procedure of extrapolation and interpolation in space and time of expose rates measurements and concentration of radionuclides in air and these uncertainties were not assessed. Shared uncertainty has a much larger impact on epidemiological dose-response analysis than unshared uncertainty and leads to greater bias in the estimates of radiation-related risk coefficient (39, 40). Therefore, the introduction of shared errors in the RADRUE method is an important improvement. Consequently, this might require a reassessment of the radiation-related risk of leukemia and thyroid cancer among Ukrainian cleanup workers while adjusting for the structure of shared and unshared dose errors.

A method needs to be developed to incorporate in the dose estimates the sources of HFU associated with (i) poor memory recall resulting in incomplete, inaccurate, or missing responses during personal interviews with study subjects conducted long after exposure, and (ii) subjective interpretation of the information from the questionnaire by a dosimetry expert during entering the work history into the Rockville computer code that can influence the dose estimates. It is also important to assess how the HFU affects the epidemiological analysis.

To date, the RADRUE method has been used in several radiation epidemiological studies and has provided the best possible and reliable dose estimates for the cleanup workers, whether deceased or alive, since it includes (i) high-quality dosimetry interview data, which were verified by experienced interviewers, (ii) thorough analysis and interpretation of the questionnaire data, which was performed by highly competent expert-dosimetrists, and (iii) calculation of individual stochastic doses using a specialized computer code. However, implementation of the RADRUE method in epidemiological studies is rather time-consuming and expensive. For instance, the reconstruction of individual doses for 2,130 male and female cleanup workers considered in this paper took more than two decades of intensive work. Therefore, it is practically impossible to apply this method in a large cohort study of cleanup workers, e.g., more than 1,000 participants, and the development of an alternative approach needs to be considered. This alternative approach could be based on a short questionnaire that captures key information about the person and the cleanup mission, e.g., the date of mission beginning, affiliation, and the type of work performed.

In 1996–2000, a method called Soft Expert Assessment Dosimetry (SEAD) was developed that used interview data, dose distributions for each cleanup worker category, and the knowledge of a dosimetry expert to estimate an individual dose (41). However, because reliable dose distributions needed to implement the method to each cleanup worker category were not available at the time, the SEAD method was not considered applicable to all cleanup workers.

Since detailed information about the cleanup activities and individual dose estimates are available now for a sufficiently large number of cleanup workers (2,130 individuals) included in epidemiological studies, there is a possibility to improve the SEAD method. Machine learning algorithms based on neural networks can be applied to identify key parameters that define exposure during a cleanup mission, e.g., category of cleanup worker, start date of the cleanup mission, and location of the cleanup activities. The improved SEAD method and a short list of key parameters can be used to calculate external doses for cleanup workers for whom the RADRUE method has been already applied. A comparative analysis of two sets of doses estimated for the same individuals using two different methods will provide important information about the reliability of dose estimates and the possible uncertainties of the SEAD method versus uncertainties of the RADRUE method. However, it is unclear whether such a simplified method would be able to capture the significant dose variability that was observed across the dose estimates for different categories of cleanup workers performing different tasks (see Table 5) as well as within the same category of cleanup workers (see Table 4). The answers to such questions can be found in further studies to develop an updated version of the SEAD method. The success of these studies cannot be guaranteed at this time.

CONCLUSIONS

The present paper provides an overview of methods and summarizes the results of estimating radiation doses and their uncertainties conducted by the authors for 2,048 male and 82 female cleanup workers included in the Ukrainian-American epidemiological studies among the Chernobyl cleanup workers. All these studies used the RADRUE method to calculate the doses from external irradiation during cleanup missions, which was the main exposure pathway for most study participants. In addition, some Ukrainian-American studies (e.g., thyroid cancer study) considered other exposure pathways tailed to target organ and population groups (e.g., intake of ¹³¹I and short-lived radioiodine and radiotellurium isotopes during a cleanup mission and/or residence in contaminated areas).

It should be noted, that the RADRUE method provides the dose estimates for use in epidemiological studies with a reasonable degree of reliability, but it requires rather laborious and expensive efforts. The development of any alternative simplified method for use in a large epidemiological cohort study may face the problem of addressing the variability of doses across the study population and within a particular category of cleanup workers over the entire period of the cleanup activities between April 26, 1986 and December 31, 1990.