Tracing echoes from the past: a district-level analysis of thyroid cancer incidence and radiation exposure in post-Chornobyl Ukraine

Introduction Increases in thyroid cancer have been reported as one of the major public health consequences of the Chornobyl accident. Using an excess relative risk (ERR) model and detailed district-level data across Ukraine, we aimed to estimate the association between iodine-131 exposure and thyroid cancer risk while accounting for spatial confounding.

Methods This ecological study used data on all diagnosed thyroid cancer cases in 2001 among individuals aged 15 years or older and settlement-level iodine-131 thyroid doses from 1986, covering the entire territory of Ukraine and encompassing 627 spatial units (districts and cities). District doses were estimated by sex and age using areal-weighted interpolation. District-level standardised incidence ratios (SIRs) were calculated and smoothed with negative binomial regression and spatial P-splines. The ERR was assessed using an intrinsic conditional autoregressive negative binomial (ICAR-NB) model adjusting for sex, age at exposure and district-specific effects.

Results Excess thyroid cancer incidence was observed not only in northern districts near the Chornobyl plant but also in several central districts. The ICAR-NB model produced lower ERR estimates than the corresponding model without district-level adjustment, suggesting that risks may be overestimated when spatial dependencies are ignored (ages at exposure 0–4: 24.85 (95% CI 7.78 to 41.78) vs 48.09 (95% CI 17.05 to 79.11)). ERRs were significant for both sexes although sex differences were not statistically significant. Risk decreased strongly with age at exposure.

Conclusions This study underscores the value of small-area data analysis and spatial modelling for assessing radiation-related health risks and offers methodological guidance for future studies following nuclear accidents.