Thyroid Cancer after Exposure to External Radiation: A Pooled Analysis of Seven Studies
Level 3 - non-randomized controlled study
Pooled analysis of observational cohort and case-control studies
OpenAlex W2163592796 · doi:10.1667/rrav05.1
What was done
A pooled analysis was conducted using individual subject data from seven studies (five cohort studies including atomic bomb survivors and children irradiated for benign conditions, plus two case-control studies) evaluating external radiation exposure and thyroid cancer. The combined analysis included almost 120,000 individuals (approximately 58,000 exposed to a wide range of doses and 61,000 nonexposed subjects), nearly 700 thyroid cancer cases, and 3,000,000 person-years of follow-up.
What was found
For exposure before age 15, risk followed a linear dose-response down to 0.10 Gy, leveling off at doses above 10 Gy. Childhood exposure yielded a pooled excess relative risk per Gy (ERR/Gy) of 7.7 (95% CI: 2.1, 28.7) and an excess absolute risk per 10(4) PY Gy of 4.4 (95% CI: 1.9, 10.1). Attributable risk percent at 1 Gy was 88%. Risk declined significantly with older age at exposure, with little apparent risk after age 20. Only two cases occurred within 5 years of exposure; ERR started to decline around 30 years post-exposure but remained elevated at 40 years. ERR was non-significantly higher for females (P = 0.07), while EAR was higher in females reflecting background rates. Spreading dose over time showed suggestive evidence of lower risk.
Why it matters
This pooled study quantitatively demonstrates that the pediatric thyroid is exceptionally vulnerable to radiation-induced carcinogenesis down to low doses, with risk persisting for decades after exposure.
Limits
The analysis pooled heterogeneous exposure contexts (environmental atomic bomb radiation versus medical treatments) and mixed study designs. Data on dose fractionation over time were limited, and confidence intervals around the pooled relative risk estimates were wide.
Cited by
- supports Between the 1910s and 1940s, infants routinely underwent thymus irradiation to prevent sudden infant death syndrome (status thymicolymphaticus), causing thousands of subsequent cancer deaths.