How cancer begins
Cancer begins with an alteration to the structure of deoxyribonucleic acid (DNA) that is found in all human cells. This is known as a genetic mutation. The DNA provides the cells with a basic set of instructions, such as when to grow and when to reproduce.
The mutation in the DNA changes these instructions, so the cells keep growing. This causes them to reproduce uncontrollably, producing a lump of tissue known as a tumour.
How cancer spreads
Left untreated, cancer can grow and spread to the other parts of the body, usually through the lymphatic system.
The lymphatic system is made up of a network of vessels (channels) and glands called lymph nodes, which are distributed throughout the body, much like your blood circulation system. These glands produce many of the specialised cells needed by your immune system (the body’s natural defence system against infection and illness).
Once the cancer reaches the lymphatic system, it is capable of spreading to any other part of your body, including your:
The most common types of thyroid cancer, the differentiated thyroid cancers (DTCs - papillary carcinomas and follicular carcinomas), spread much slower than other types of cancer. In most cases where a DTC is diagnosed, it is usually limited to the thyroid gland, or has only spread to nearby lymph nodes.
The rarer types of thyroid cancer are more aggressive and spread faster. By the time it has been diagnosed, medullary thyroid carcinoma may have spread to lymph nodes. In the most advanced cases, it will have also spread to the bones or lungs, or both. Anaplastic cancer usually spreads to the windpipe and, in some cases, the lungs.
Risk factors for thyroid cancer
Radiation exposure
Exposure to radiation in childhood has been identified as a risk factor for thyroid cancer.
Two types of exposure that are associated with thyroid cancer have been identified.
The first type of exposure is exposure to nuclear fall-out. Nuclear fall-out is a type of radioactive waste that is released during a nuclear explosion. It can also occur when a nuclear reactor malfunctions, releasing high levels of nuclear waste into the atmosphere.
The second type of exposure is exposure to radiation that is used for medical treatments. It is thought that many of the cases of thyroid cancer that have been reported in recent years were actually caused by exposure to radiation that occurred between 1910 and 1960. At this time, the risks of radiation treatment were not properly understood. Most cases involved children who received radiation treatments to their neck.
Today there are much stricter regulations regarding the medical use of radiation to prevent the risk of people developing thyroid cancer. It is hoped that this will lead to a drop in the number of reported cases in future years.
Sex
Women are three times more likely to develop thyroid cancer than men.
It was thought that this may be due to the hormones that are released during a woman’s period or during pregnancy. However, despite extensive research in this area, no evidence has been found to support this theory. The reason why women have an increased chance of developing thyroid cancer is still not understood.
Health conditions
There are a number of health conditions that increase your risk of developing thyroid cancer. These conditions are:
- Hashimoto's thyroiditis - a common condition where the immune system attacks the thyroid gland
- Cowden’s syndrome - a rare genetic condition that causes abnormal growths to develop on your skin and inside your mouth and nose
- thyroid adenoma - a non-cancerous growth of the thyroid gland
- familial adenomatous polyposis - a genetic condition that causes abnormal growths in your intestine (part of the digestive system)
Occupational exposure
Research from Sweden suggests that people whose job involves exposure to solvents (a type of chemical used in manufacturing, engineering and the textile industry) have a slightly higher risk of developing thyroid cancer than the population at large.
Medullary thyroid carcinomas and genetic mutations
Inherited genetic mutations cause 1 in 4 cases of medullary thyroid carcinoma. A genetic mutation occurs when the DNA instructions that are carried in certain genes are altered. This means that some of the body’s processes do not work in the normal way.
There are three types of inherited medullary thyroid carcinomas:
- the familial medullary thyroid cancer (FMTC)
- the multiple endocrine neoplasia (MEN) syndrome type 2A
- the multiple endocrine neoplasia (MEN) syndrome type 2B
Cases of thyroid cancer related to the MEN2A or MEN2B mutations usually develop during childhood or as a teenager. Cases of thyroid cancer related to the FMTC mutations usually develop during adulthood.
All of the mutated genes (FMTC, MEN2A and MEN2b) are passed down through families and this is known as autosomal dominant inheritance. This means that if either of your parents has one of the mutations that causes medullary thyroid cancer, there is a 1 in 2 chance that you will also be born with the same mutation.
Because of this fact, if you have a parent with a history of medullary thyroid carcinoma, you should consider having your blood tested to determine whether you have any of the mutated genes. If the tests prove positive, it is usually recommended that you have your thyroid gland surgically removed as a precaution.
