The clinical presentation of thyroid cancer is usually as a solitary thyroid nodule or increasing goitre size. Although thyroid nodules are common, thyroid cancers are rare. The four major histological types are papillary, follicular, medullary, and anaplastic, and each displays a different epidemiology. The annual incidence of all thy roid cancers ranges between 1 and 10/ 100 000 population in most countries and is two to four times more frequent in women than men. Papillary and follicular tumours, which comprise 60– 90% of the total, are rare in children and adolescents but their incidence increases with age in adults.
Papillary thyroid carcinoma (PTC) is the most common thy roid malignancy and worldwide constitutes 50– 90% of differentiated follicular cell- derived thyroid cancers. Papillary thyroid microcarcinomas (diameter less than 1 cm) are found in 4– 36% of adults post- mortem in population- based studies. Most diagnoses of PTC occur in patients 30– 50 years old (median age 44 years), and the majority (60– 80%) occur in women. The reported increase in incidence of these carcinomas in recent years has been attrib uted to an improvement in pathological techniques and earlier and increased detection of small (subclinical) papillary cancers secondary to more widespread use of neck ultrasonography and fine- needle aspiration of very small thyroid nodules. However, a recent analysis of the National Cancer Institute’s Surveillance, Epidemiology, and End Results database found an increase in the rates of differentiated thyroid cancer of all sizes, including tumours more than 4 cm and an increase in incidence- based mortality between 1974 and 2013, from 0.40 to 0.46/ 100 000 person- years. External radiation exposure, particularly in childhood, is a major risk factor for papillary cancer. Four years after the nuclear accident at Chernobyl, a significant increase in the incidence of childhood thyroid cancer (almost exclusively papillary tumours in which a translocation of the RET gene occurs) was recorded in the regions most exposed. There is no association between radioiodine therapy for thyrotoxicosis and subsequent development of thyroid cancer in adults.
Follicular thyroid cancer occurs relatively infrequently compared to papillary cancer and accounts for approximately 15% of all thy roid cancers. There is an increased frequency of follicular to papillary carcinoma (5:1) in iodine- deficient endemic goitre areas. It tends to be a malignancy of older persons, with a peak incidence of between ages 40 and 60 years and is approximately three times more common in women than in men. Medullary thyroid cancer (MTC) occurs in both sporadic and hereditary forms. The highest incidence of sporadic disease occurs in the fifth decade. Hereditary MTC can be inherited as an autosomal dominant trait with a high degree of penetrance associated with multiple endocrine neoplasia (MEN) type 2 syndrome or as familial MTC without any other endocrinopathies. It can be diagnosed before clinical presentation by genetic and biochemical screening. Anaplastic thyroid cancer is very rare and is more frequent in populations with endemic goitre.
Thyroid lymphoma is also uncommon, constituting approximately 2% of extranodal lymphomas and occurring predominantly in older women. Up to one- third of patients have a history of goitre, whereas some have established autoimmune thyroiditis and may be taking L- T4 therapy.
