Hyperthyroidism Following Radiation Therapy for Childhood Cancer: A Report from the Childhood Cancer Survivor Study.

BACKGROUND: The association of hyperthyroidism with exposure to ionizing radiation is poorly understood. This study addresses the risk of hyperthyroidism in relation to incidental therapeutic radiation dose to the thyroid and pituitary glands in a large cohort of survivors of childhood cancer.

METHODS: Utilizing the Childhood Cancer Survivor Study, a cohort of five-year survivors of childhood cancer diagnosed at hospitals in the United States and Canada between 1970 and 1986, the occurrence of hyperthyroidism through 2009 was ascertained among 12,183 survivors based on serial questionnaires. Radiation doses to the thyroid and pituitary glands were estimated from radiotherapy records, and chemotherapy exposures were abstracted from medical records. Binary outcome regression was used to estimate prevalence odds ratios (ORs) for hyperthyroidism at five years from diagnosis of childhood cancer and Poisson regression to estimate incidence rate ratios (RRs) after the first five years.

RESULTS: Survivors reported 179 cases of hyperthyroidism, of which 148 were diagnosed five or more years after their cancer diagnosis. The cumulative proportion of survivors diagnosed with hyperthyroidism by 30 years after the cancer diagnosis was 2.5% (95% CI: 2.0-2.9) among those who received radiotherapy. A linear relation adequately described the thyroid radiation dose-response for prevalence of self-reported hyperthyroidism five years after cancer diagnosis (excess OR/Gy=0.24; 95% CI: 0.06-0.95) and incidence rate thereafter (excess RR/Gy = 0.06; 95% CI: 0.03-0.14) over the dose range of 0-63 Gy. Neither radiation dose to the pituitary gland nor chemotherapy was associated significantly with hyperthyroidism. Radiation-associated risk remained elevated > 25 years after exposure.

CONCLUSIONS: Risk of hyperthyroidism following radiotherapy during childhood is positively associated with external radiation dose to the thyroid gland, with radiation-related excess risks persisting for > 25 years. Neither radiation dose to the pituitary gland nor chemotherapy exposures were associated with hyperthyroidism among childhood cancer survivors through early adulthood.