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Cancerous tumours of the thyroid
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.
Differentiated thyroid cancer
Papillary carcinoma and follicular carcinoma are grouped together and called differentiated thyroid cancer. Together, these cancers make up more than 90% of all thyroid cancers. They are well-differentiated tumours, which means the cancer cells look similar to normal thyroid tissue.
Papillary carcinoma, or papillary thyroid cancer, is the most common type of thyroid cancer. More than 80% of all thyroid cancers are papillary carcinomas. They are most often diagnosed in young women.
Papillary carcinoma 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 carcinomas to start in more than one place in the thyroid at the same time (called multifocal cancer). They usually grow slowly and respond well to treatment.
There are several variants, or forms, of papillary carcinoma. When seen under a microscope, the cancer cells of each variant have unique features that make them look different from typical papillary carcinoma. The follicular variant happens most often. It has features of both papillary carcinoma and follicular carcinoma, but it is treated the same way as typical papillary carcinoma. Less common variants 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 carcinoma or the follicular variant.
Follicular carcinoma, or follicular thyroid cancer, is a less common type of thyroid cancer. Less than 10% of all thyroid cancers are follicular carcinomas.
Like papillary carcinoma, follicular carcinoma starts in follicular cells of the thyroid. Follicular carcinoma usually grows slowly and has a very good prognosis, or outcome, in most cases.
Hurthle cell carcinoma is a variant of follicular carcinoma. Some studies have shown that it can have a poorer prognosis than typical follicular carcinoma.
Poorly differentiated carcinoma
Poorly differentiated carcinoma, or poorly differentiated thyroid cancer, is considered a separate cancer between differentiated thyroid cancer and anaplastic carcinoma. It makes up about 2% of all thyroid cancers. It is an aggressive type of cancer that grows and spreads quickly. It is often diagnosed when it has already spread to other tissues in the neck or other parts of the body, such as the lungs or bone. There is a high risk that poorly differentiated carcinoma will come back, or recur, after treatments.
Poorly differentiated carcinoma starts in follicular cells in the thyroid. The cancer cells still have some features of normal follicular cells, but they look and act more abnormal than differentiated thyroid cancer cells. Sometimes poorly differentiated carcinoma can develop or progress from a differentiated thyroid cancer.
Poorly differentiated carcinoma is also called insular carcinoma because the cancer cells have a specific growth pattern. The tumour looks like a solid island or nest made of small, uniform cancer cells.
Anaplastic carcinoma, or anaplastic thyroid cancer, makes up less than 2% of all thyroid cancers. It is an aggressive type of cancer that 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 carcinoma usually develops in older people.
Anaplastic carcinoma starts in follicular cells in the thyroid. It may occur along with other types of thyroid cancer, such as papillary or follicular carcinoma. Because of this, doctors think it sometimes develops from a papillary or follicular carcinoma.
Anaplastic carcinoma is also called undifferentiated thyroid cancer. The cancer cells look abnormal and very different from normal thyroid tissue.
Medullary carcinoma, or medullary thyroid cancer, is an uncommon type of thyroid cancer. It makes up about 5% of all thyroid cancers. It most often develops in people in their 50s and 60s.
Medullary carcinoma starts in C cells (also called parafollicular cells) in the thyroid.
Most medullary carcinomas are sporadic, which means they happen by chance.
Sometimes medullary carcinoma is hereditary, or inherited. Hereditary medullary carcinoma is caused by a mutation of the RET gene, and it often occurs as part of a genetic condition called multiple endocrine neoplasia type 2 (MEN2). Two forms of MEN2 are associated with hereditary medullary carcinoma. MEN2A can cause medullary carcinoma along with tumours of the adrenal glands (called pheochromocytomas) and parathyroid glands. MEN2B can cause medullary carcinoma along with other tumours, including pheochromocytomas and neuromas.
When different people in the same family develop medullary carcinoma that is not associated with MEN2, it is called familial medullary thyroid carcinoma.
Rare thyroid cancers
The following thyroid cancers are rare:
A non-cancerous (benign) tumour that starts in neurons (nerve cells).