Thyroid cancer incidence in cohorts exposed in childhood to 131 I released during the Windscale nuclear reactor accident at Sellafield, England, in 1957.

A fire in one of the Windscale nuclear reactors at Sellafield (Cumbria, England) in October 1957 released 1,800 TBq of 131 I (half-life, 8 days) to atmosphere. Measurements of 131 I activity in thyroids of exposed children showed typical thyroid doses of tens of milligray, but with some exceeding 100 mGy. Radiation exposure in childhood is known to increase the risk of thyroid cancer. Consequently, an investigation was conducted into whether raised numbers of thyroid cancer cases occurred in those exposed to 131 I as young children in Cumbria. A database of Cumbrian births from 1950 onwards allowed cohorts of 56,086 births during 1950-1958 and 137,444 births during 1959-1980 to be constructed, periods including children potentially exposed and unexposed, respectively, to 131 I. Three areas of Cumbria with different 131 I contamination levels were identified from monitoring data, and births assigned to these three areas for the two periods of birth. Members of these six sub-cohorts were linked to incident thyroid cancer cases in Great Britain during 1981-2020 using national cancer registration databases, providing thyroid cancer incidence rates. Incidence rate ratios (IRRs), with the lowest contamination area as a reference, were computed. No IRR differed discernibly from unity. For births during 1950-1958, the IRR for the combined highest and intermediate 131 I contamination areas was 0.68 (95% confidence interval: 0.24, 1.56), and no case of thyroid cancer was found in the small cohort born in the highest contamination area. In conclusion, no increased risk of thyroid cancer in those exposed to 131 I as young children in Cumbria in 1957 was detected. This study adds to the evidence on the long-term risk of thyroid cancer following childhood exposure to low and moderate levels of 131 I, such as occurred following the Fukushima nuclear accident in 2011.