Thyroid Nodules and Cancer: Evidence-Based Neuroimaging

Abstract

Thyroid nodules are very common. The prevalence of thyroid nodules largely depends on the method of screening and the population evaluated. Increasing age, female sex, iodine deficiency, and a history of head and neck radiation seem to increase the risk of thyroid nodules [1, 2]. The Danish investigation on iodine intake and thyroid disease monitors the iodine fortification program in Denmark [3]. They observed a large population before and after iodine fortification and reported that even small changes in iodine intake significantly influenced goiter prevalence, nodule incidence, and thyroid dysfunction [3]. Other factors, such as smoking, pregnancy, and alcohol use, also influenced goiter development. Imaizumi et al. [4] in their study on atomic bomb survivors found the prevalence of solid nodules, malignant tumors, benign nodules, and cysts of the thyroid was 14.6 %, 2.2 %, 4.9 %, and 7.7 %, respectively. A significant linear dose–response relationship was observed for the prevalence of all solid nodules, malignant tumors, benign nodules, and cysts (p < 0.001). The investigators estimated that about 28 % of all solid nodules, 37 % of malignant tumors, 31 % of benign nodules, and 25 % of cysts were associated with radiation exposure at a mean and median thyroid radiation dose of 0.449 sieverts (Sv) and 0.087 Sv, respectively [4]. The increasing use of imaging has increased the number of thyroid nodules detected incidentally. Autopsy and prospective US studies in North America have shown a prevalence of thyroid nodules in 50 % and 67 % of patients, respectively [5, 6]. A study comparing US and clinical examination showed that 46 % of the nodules detected by US would not be detected by clinical examination alone [7]. Most of these nodules are benign with a prevalence of thyroid cancer reported in the range of 1.2–2.6 per 100,000 men and 2.0–3.8 cases per 100,000 in women [8]. The incidence of thyroid cancer in the population has increased from 3.6 per 100,000 in 1,073 to 8.7 per 100,000 in 2002 in the USA [9]. However, most of this increase in thyroid cancer was due to an increased detection of small papillary cancers. In autopsy studies, clinically silent thyroid papillary microcarcinomas (<1 cm in diameter) have been reported in up to 36 % of patients [10]; however, a comparison with the incidence rates for clinically apparent papillary carcinomas suggests that most papillary microcarcinomas will not lead to clinically apparent thyroid carcinomas. A follow-up study of papillary microcarcinomas over a 9-year period also did not show any metastasis in patients with tumors <0.8 mm [11, 12]. The risk of cancer in thyroid nodules ranges from a 48 % likelihood of malignancy in a solitary solid nodule with punctuate calcifications in a man to a likelihood of <3 % in a noncalcified, predominantly cystic nodule in a woman [13].