Cancer (Understanding the Basics): Causes, Risk factors and Prevention

Cancer is a generic term for a large group of diseases that can affect any part of the body. Other terms used are malignant tumours and neoplasms. One defining feature of cancer is the rapid creation of abnormal cells that grow beyond their usual boundaries, and which can then invade adjoining parts of the body and spread to other organs, the latter process is referred to as metastasizing. Metastases are a major cause of death from cancer.

Another clearer way to define and understand it is this: Cancer is a term used for diseases in which abnormal cells divide without control and can invade other tissues. Cancer cells can spread to other parts of the body through the blood and lymph systems. Cancer is not just one disease, but many diseases. There are more than 100 kinds of cancer.

The problem

Cancer is a leading cause of death worldwide, accounting for 8.8 million deaths in 2015. The most common causes of cancer death are cancers of:

Lung (1.69 million deaths)
Liver (788 000 deaths)
Colorectal (774 000 deaths)
Stomach (754 000 deaths)
Breast (571 000 deaths)

What causes cancer?

Cancer arises from the transformation of normal cells into tumour cells in a multistage process that generally progresses from a pre-cancerous lesion to a malignant tumour. These changes are the result of the interaction between a person's genetic factors and 3 categories of external agents, including:

physical carcinogens, such as ultraviolet and ionizing radiation;
chemical carcinogens, such as asbestos, components of tobacco smoke, aflatoxin (a food contaminant), and arsenic (a drinking water contaminant); and
biological carcinogens, such as infections from certain viruses, bacteria, or parasites.

WHO, through its cancer research agency, International Agency for Research on Cancer (IARC), maintains a classification of cancer-causing agents.

Ageing is another fundamental factor for the development of cancer. The incidence of cancer rises dramatically with age, most likely due to a build-up of risks for specific cancers that increase with age. The overall risk accumulation is combined with the tendency for cellular repair mechanisms to be less effective as a person grows older.

Risk factors for cancers

Tobacco use, alcohol use, unhealthy diet, and physical inactivity are major cancer risk factors worldwide and are also the 4 shared risk factors for other noncommunicable diseases.

Some chronic infections are risk factors for cancer and have major relevance in low- and middle-income countries. Approximately 15% of cancers diagnosed in 2012 were attributed to carcinogenic infections, including Helicobacter pylori, Human papillomavirus (HPV), Hepatitis B virus, Hepatitis C virus, and Epstein-Barr virus.

Hepatitis B and C virus and some types of HPV increase the risk for liver and cervical cancer, respectively. Infection with HIV substantially increases the risk of cancers such as cervical cancer.

Reducing the cancer burden

Between 30–50% of cancers can currently be prevented. This can be accomplished by avoiding risk factors and implementing existing evidence-based prevention strategies. The cancer burden can also be reduced through early detection of cancer and management of patients who develop cancer. Many cancers have a high chance of cure if diagnosed early and treated adequately.

Modify and avoid risk factors

Modifying or avoiding key risk factors can significantly reduce the burden of cancer. These risk factors include:

tobacco use including cigarettes and smokeless tobacco
being overweight or obese
unhealthy diet with low fruit and vegetable intake
lack of physical activity
alcohol use
sexually transmitted HPV-infection
infection by hepatitis or other carcinogenic infections
ionizing and ultraviolet radiation
urban air pollution
indoor smoke from household use of solid fuels.

Tobacco use is the single most important risk factor for cancer and is responsible for approximately 22% of cancer-related deaths globally.

Pursue prevention strategies

The number of new cancer cases can be reduced and many cancer deaths can be prevented. Research shows that screening for cervical and colorectal cancers as recommended helps prevent these diseases by finding precancerous lesions so they can be treated before they become cancerous. Screening for cervical, colorectal, and breast cancers also helps find these diseases at an early stage, when treatment works best. Some government agencies, like the United State's CDC, offer free or low-cost mammograms and Pap tests nationwide, and free or low-cost colorectal cancer screening in six states.

Vaccines (shots) also help lower cancer risk. The human papillomavirus (HPV) vaccine helps prevent most cervical cancers and several other kinds of cancer, and the hepatitis B vaccine can help lower liver cancer risk. Vaccination against these HPV and hepatitis B viruses could prevent 1 million cancer cases each year.

To prevent cancer, people may:

increase avoidance of the risk factors listed above;
vaccinate against HPV and hepatitis B virus;
control occupational hazards;
reduce exposure to ultraviolet radiation;
reduce exposure to ionizing radiation (occupational or medical diagnostic imaging).

A person’s cancer risk can be reduced with healthy choices like avoiding tobacco, limiting alcohol use, protecting your skin from the sun and avoiding indoor tanning, eating a diet rich in fruits and vegetables, keeping a healthy weight, and being physically active.

Detection, care and treatment of cancer »

Reference(s)
1). World Health Organization: Cancer - WHO Fact Sheets. Accessed 06.08.2017. Available here: http://www.who.int/mediacentre/factsheets/fs297/en/
2). Centers for Disease Control: Cancer Prevention and Control. Accessed 06.08.2017. Available here: https://www.cdc.gov/cancer/index.htm