Cancerous tumours of the thyroid

The most common types are papillary thyroid cancer (papillary carcinoma) and follicular thyroid cancer (follicular carcinoma). Papillary and follicular thyroid cancer are also known as differentiated thyroid cancer. A cancerous tumour of the thyroid can spread, or metastasize, to other parts of the body. Cancerous tumours are also called malignant tumours. Different types of cancerous tumours are treated in different ways.

More than 80% of all cancerous tumours that develop in the thyroid are papillary thyroid cancer. It may also be called papillary carcinoma. It is most often diagnosed in young women.

Papillary thyroid cancer starts in follicular cells in the thyroid. When seen under a microscope, the cancer cells often have small, finger-like projections (called papillae). It is common for papillary thyroid cancer to start in more than one place in the thyroid at the same time (called multifocal cancer). These tumours usually grow slowly and respond well to treatment.

There are several variants, or forms, of papillary thyroid cancer. When seen under a microscope, the cancer cells of each variant have unique features that make them look different from typical papillary thyroid cancer. The follicular variant happens most often. It has features of both papillary and follicular thyroid cancers, but it is treated the same way as typical papillary thyroid cancer. Less common forms are the tall cell variant, columnar cell variant, solid variant and diffuse sclerosing variant. These variants tend to be more aggressive, which means that they grow and spread faster than typical papillary thyroid cancer or the follicular variant.

Follicular thyroid cancer is the second most common type of thyroid cancer. But less than 10% of all thyroid cancers are follicular thyroid cancer. It is also called follicular carcinoma.

Like papillary, follicular thyroid cancer starts in follicular cells. It usually grows slowly and has a very good prognosis, or outcome, in most cases.

Hurthle cell thyroid cancer is a variant of follicular thyroid cancer. Some studies have shown that it can have a poorer prognosis than typical follicular thyroid cancer.

Anaplastic thyroid cancer is also called anaplastic carcinoma. It makes up less than 2% of all thyroid cancers. Anaplastic thyroid cancer is aggressive, which means that it grows and spreads quickly. It is often diagnosed when the cancer has already spread to other tissues in the neck or other parts of the body. Anaplastic thyroid cancer usually develops in older people.

Anaplastic thyroid cancer starts in follicular cells in the thyroid. It may happen along with other types of thyroid cancer, such as papillary or follicular thyroid cancer. Because of this, doctors think it sometimes develops from a papillary or follicular thyroid cancer.

Medullary thyroid cancer is also called medullary carcinoma. It makes up about 5% of all thyroid cancers. It most often develops in people in their 50s and 60s.

Medullary thyroid cancer starts in C cells (also called parafollicular cells) in the thyroid.

Most medullary thyroid cancers are sporadic, which means they happen by chance. But sometimes medullary thyroid cancer is hereditary, or inherited. Hereditary medullary thyroid cancer is caused by a mutation of the RET gene, and it often happens as part of a genetic condition called multiple endocrine neoplasia type 2 (MEN2). Two forms of MEN2 are associated with hereditary medullary thyroid cancer. MEN2A can cause medullary thyroid cancer along with tumours of the adrenal glands (called pheochromocytomas) and parathyroid glands. MEN2B can cause medullary thyroid cancer along with other tumours, including pheochromocytomas and neuromas.

When different people in the same family develop medullary thyroid cancer that is not associated with MEN2, it is called familial medullary thyroid cancer.

The following thyroid cancers are rare:

• non-Hodgkin lymphoma, or thyroid lymphoma
• soft tissue sarcoma
• squamous cell carcinoma
• mucoepidermoid carcinoma