Sigurdson AJ et al. (2005) Primary thyroid cancer after a first tumour in childhood (the Childhood Cancer Survivor Study): a nested case–control study. Lancet 365: 2014–2023
Patients who have undergone upper-body radiotherapy for childhood cancer have an increased risk of developing primary thyroid cancer in later life. A recent case–control study published by Sigurdson et al. in the Lancet demonstrates that this risk is reduced in those who received radiation doses exceeding 30 Gy compared with those treated with lower doses.
Study participants were selected from the Childhood Cancer Survivor Study cohort, and were all diagnosed with various malignancies between 1970 and 1986, before they were 21 years old. Cases (n = 69) had primary thyroid cancer diagnosed more than 5 years after their original childhood tumor, and matched controls (n = 265) had intact thyroid glands and no evidence of thyroid malignancy.
Children who had undergone radiotherapy were shown to have a significantly increased relative risk of developing primary thyroid cancer (P = 0.028) when compared with those who had not been irradiated. This risk increased with radiation doses up to 29 Gy, and decreased for doses greater than 30 Gy, an effect that was more apparent in those younger than 10 years at first cancer diagnosis. After adjusting for treatment, Hodgkin's lymphoma was also associated with an increased risk of developing thyroid cancer when compared with other first diagnoses. The authors postulate that the downturn in the dose–response relationship for thyroid cancer at high radiation doses is consistent with a cell-killing effect.
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King, A. Childhood radiotherapy and thyroid cancer: lower risk at very high radiation doses. Nat Rev Clin Oncol 2, 487 (2005). https://doi.org/10.1038/ncponc0279