In patients with cancer, cells grow in an abnormal way and multiply uncontrollably, invading healthy tissue.

The body has natural systems to prevent the growth of cancerous cells by either repairing the damage to the DNA or forcing the cell to die, if the damage is too great. Cancer occurs when these inbuilt defence mechanisms fail.

The term cancer describes a group of diseases that share this characteristic. Cancers are usually classified according to the location in the body where the abnormal cell growth began.

Cancerous cells can grow abnormally in blood or form masses called tumours. Some tumours are benign – meaning they can’t spread and therefore aren’t cancerous.

Malignant tumours, which are cancerous, are collections of abnormal cells that do spread and invade healthy tissue. Any cancer can interfere with the way the body functions, eventually becoming life-threatening.

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How does cancer start?

Cancer starts when healthy cells begin to grow abnormally. Typically, cells grow, divide and die in a normal way. In people with cancer, cells don’t follow this process – instead, they grow when they aren’t needed, remain when they should have died and divide without stopping.

These abnormal cells can remain localised or spread throughout the body and invade other healthy tissues and organs.

How does cancer spread?

The location in the body where cancer begins is called the primary site. Cancerous cells can multiply in the primary site, or spread by breaking away from it.

When they break away, cancerous cells travel through the bloodstream or the lymphatic system to another part of the body, where they start multiplying again. This process is known as metastasis.

Cancer that forms as a result of spreading from the primary site is referred to as metastatic or secondary cancer.

What causes cancer?

Cancer’s pattern of abnormal cell growth is genetic, and it’s caused by changes to the genes that control cell function or mutations to the DNA that make up the genes that control cell function.

Cancer-causing genes can be inherited from family members. They can also develop in the body as a result of lifestyle choices and environmental exposures that damage DNA.

Smoking is one of the most well-established causes of cancer, and as many as one in every nine cancers are caused by smoking. Alcohol consumption, as well as lifestyle and environmental factors such as diet, radiation and chemical exposure, can also lead to cancer.

Some cancers are attributed to inherited genes, while others develop without specific causes. Approximately 40% of cancers are thought to occur due to genetics, and 60% are attributed to environmental or other factors.

What different types of cancer exist?

Cancerous cells can develop in the skin or tissues covering internal organs (carcinoma), in connective tissues (sarcoma), in the blood-forming tissue (leukaemia), in the immune system (lymphoma and multiple myeloma) and in the brain or spinal cord (central nervous system).

Generally, cancer is classified depending on the organ the abnormal cell growth begins in. For example, cancer that begins in the breast is called breast cancer.

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Treating and preventing cancer

Cancer treatment can involve a combination of surgery, chemotherapy, radiation therapy and other forms of therapies such as immunotherapy. Recent advances in immunotherapy have shown promising results, and it’s likely that it will become a routine cancer treatment in the near future.

Traditionally, the treatment approach depends on the type of cancer being treated, and secondary cancers are often treated in the same way as primary cancers since a specific mutation may respond to a particular treatment regime irrespective of location.

Cancerous tissue can continue to mutate after treatment and if a relapse occurs, a different type of treatment may be required.

An emerging body of research is indicating that knowing a tumour’s genomic profile could be more important for successful treatment than knowing its location or size. As each tumour’s genomic profile is unique, this approach is often referred to as personalised or precision medicine.

Raising awareness of the well-established cancer risk factors, encouraging patients to learn their family history, and referring at-risk patients for regular screening checks risk are all important preventative measures.