Figure 4. Probability of developing hypothyroidism according to radiation dose in 5-year survivors of childhood cancer. Data from the Childhood Cancer Survivor Study. Sklar C, Whitton J, Mertens A, Stovall M, Green D, Marina N, Greffe B, Wolden S, Robison L: Abnormalities of the Thyroid in Survivors of Hodgkin's Disease: Data from the Childhood Cancer Survivor Study. The Journal of Clinical Endocrinology and Metabolism 85 (9): 3227-3232, September 1, 2000. Copyright 2000, The Endocrine Society.
As might be expected, children treated for head and neck malignancies are also at risk for primary hypothyroidism if the neck is irradiated. The German Group of Paediatric Radiation Oncology reported on 1,086 patients treated at 62 centers, including 404 patients (median age, 10.9 years) who had received radiation therapy to the thyroid gland and/or hypophysis. Follow-up information was available for 264 patients (60.9%; median follow-up, 40 months), with 60 patients (22.7%) showing pathologic values. In comparison to patients treated with prophylactic cranial irradiation (median dose, 12 Gy), patients with radiation doses of 15 Gy to 25 Gy to the thyroid gland had a hazard ratio (HR) of 3.072 (P = .002) for the development of pathologic thyroid blood values. Patients with greater than 25 Gy to the thyroid gland and patients who underwent craniospinal irradiation had HR of 3.768 (P = .009) and 5.674 (P < .001), respectively. The cumulative incidence of thyroid hormone substitution therapy did not differ between defined subgroups.
Any radiation field that includes the thyroid is associated with an excess risk of thyroid neoplasms, which may be benign (usually adenomas) or malignant (most often differentiated papillary carcinoma).[8,10,11,12,13] The clinical manifestation of thyroid neoplasia among childhood cancer survivors ranges from asymptomatic, small, solitary nodules to large, intrathoracic goiters that compress adjacent structures. CCSS investigators performed a nested case-control study to evaluate the magnitude of risk for thyroid cancer over the therapeutic radiation dose range of pediatric cancers. The risk of thyroid cancer increased with radiation doses up to 20 Gy to 29 Gy (odds ratio [OR], 9.8; 95% confidence interval [CI], 3.2–34.8), but declined at doses greater than 30 Gy, consistent with a cell-killing effect. The risk of thyroid nodule development increases with increasing time from radiation exposure. In a study of HL survivors, CCSS investigators identified time from diagnosis, female gender, and radiation dose of 25 Gy or more as significant risk factors for thyroid nodule development. Based on a cohort of 3,254 2-year childhood cancer survivors treated before 1986 and monitored for 25 years, the risk of thyroid adenoma increased with the size of the radiation dose to the thyroid during childhood cancer treatment and plateaued at doses exceeding 10 Gy. The risk of thyroid adenoma per unit of radiation dose to the thyroid was higher if radiation therapy had been delivered before age 5 years and before the attained age of 4 years. Younger age at radiation therapy has also been linked to an excess risk of thyroid carcinoma.[10,11,12,13] An increased risk of thyroid nodules/cancer has also been observed in association with chemotherapy, independent of radiation exposure.[10,11]