Epidemiology
Thyroid cancer is the most common cancer of the endocrine system but is a relatively uncommon malignancy with fewer than 1200 patients registered in the UK every year. The incidence of thyroid nodules in the general population is approximately 5%, with nodules being more common in females than males (2.5:1). The prevalence of thyroid cancer in a solitary nodule or in a multinodular thyroid is 10–20%, increasing with irradiation of the neck.
Papillary, follicular and medullary types are more common in young adults, while the elderly are more likely to develop anaplastic cancer. Geographical regions that were exposed to the Chernobyl disaster or Japanese populations following the atomic bomb detonations have a higher incidence.
Aetiology
Benign thyroid conditions such as enlargement of the thyroid (goitre), inflammation (thyroiditis) and thyroid nodules (adenoma) all increase the risk of thyroid cancer. Radiation exposure is a common predisposing factor and this may be following treatment for childhood cancer or high levels after environmental incidents such as the Chernobyl disaster. Individuals with low levels of iodine who are exposed to radiation are at higher risk of developing thyroid cancer than those with normal levels.
Pathophysiology
Thyroid cancer is classified by morphology from the classical papillary, follicular and anaplastic cancers to Hürthle cell and medullary cell carcinoma and lymphoma. Differentiated (papillary and follicular) thyroid cancers account for more than 90% of cases.
Papillary carcinoma is associated with mutations in BRAF (V599E) (40% of cases) and overexpression of cyclin D1 (50%). Furthermore, disruption of the pRb signalling pathway can be seen in papillary and anaplastic cancers due to upregulation of E2F1. Papillary tumours arise from thyroid follicular cells, are unilateral in most cases and are often multifocal within a single thyroid lobe. They vary in size, from microscopic cancers to large cancers that may invade the thyroid capsule and infiltrate into contiguous structures. Papillary tumours tend to invade the lymphatics, but vascular invasion (and haematogenous spread) is uncommon.
Follicular carcinoma, although mostly encapsulated, commonly has microscopic vascular and capsular invasion. There is usually no lymph node involvement. Follicular carcinoma cancer can be difficult to distinguish from its benign counterpart, follicular adenoma, as the distinction is based on the presence or absence of capsular or vascular invasion, which cannot be evaluated by FNA.
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