Characteristics of follicular cell-derived thyroid carcinomas occurring after external radiation exposure: results of a case control study nested in a cohort.

Radiation exposure is the only well-established risk factor for follicular cell-derived thyroid carcinoma. To compare the clinical characteristics and outcome of thyroid carcinoma in patients with and without a history of radiation exposure, we performed a case control study nested in a cohort of 2,196 patients treated for a papillary or a follicular thyroid cancer at the Institut Gustave Roussy. The study was performed on 91 cases (71% females) and their 273 controls matched for gender, age at thyroid cancer diagnosis (+/-3 years), and period of initial thyroid cancer treatment (+/-6 years). More than 85% of the cases have been first exposed to external radiation before the age of 30 years. Thyroid cancers were more frequently multifocal in cases than in controls (odds ratio [OR] 2.5, 95% confidence interval [CI] 1.3-4.8), and local residual tumor was more frequently observed in cases, but the other clinical features did not differ overall. Female cases with a history of radiation exposure more frequently had a tumor of follicular histology than female controls (OR = 2.7, 95% CI: 1.1-6.5), and conversely the frequency of follicular histology was similar in male cases and in male controls (OR = 0.3, 95% CI: 0.1-1.4). The risks of recurrence and of thyroid cancer related death were similar in cases and in controls.