Causes of cancer

What are the risk factors of cancer?

There were attempts to link various cancer-related risk factors to one another. It is estimated that 43 per cent of all new cancer cases could be avoided by modifying the risk factors for cancer that individuals can affect through their own actions. Smoking is the largest single cause, accounting for up to 20 per cent of all new cases of cancer. [1, 2] It should be noted that the influence of the different population-level variables would vary depending on regional conditions and population health and socio-economic behaviour.

Categorizing RIsk factors of Cancer

Cancer risk factors can be roughly divided into the following categories:

• Internal factors, such as age, sex, metabolism of substances foreign to the body, inherited genetic defects and non-inherited gene disorders, as well as the type of skin
• Lifestyle-related factors,
• Occupational exposures, e.g. many chemicals, radioactive materials and asbestos,
• Environmental exposure, e.g. radon and UV radiation, as well as small particles.

Key points

• The majority of cancers are related to ageing. The longer a person lives, the more likely it is that these cells develop cancer that causes lesions.
• It is projected that by adjusting the risk factors that people can affect, more than 40 per cent of all new cancer cases may be prevented.
• Tobacco is the single largest risk factor, accounting for up to 20 per cent of all new cancer cases.
• Cancer development is a series of events over a period of many years, during which originally healthy genetic cell material is altered, and the cells transform into tissue separate from the regulatory systems of the body through a number of intermediate stages of the body. The malignant tissue inevitably provokes symptoms as it develops. In most cancer cases, it is difficult to determine the exact cause of the disease. Cancers are a variety of diseases, and their causes, development, symptoms and treatment can vary greatly from one another.
• The creation of most cancers includes human factors related to behaviour and living conditions. While individual susceptibility can be a factor associated with some cancers, for a cancer to develop, external factors are necessary almost always.

Age

Most cancers are associated with aging. The longer a person lives, the more likely it is that these cells develop cancer that causes lesions. The ability of cells to avoid and recover from these defects weakens with age.

Gender

There are significant differences between men and women in the incidence of also other cancers than gender-related ones (such as ovarian cancer and prostate cancer). For example, in men, the age-adjusted morbidity of laryngeal cancer is almost ten times that of women. The disparity is now less than three-fold in lung and lip cancers, although it was 15-fold until the 1960s. These developments can be explained primarily by the fact that men and women's smoking patterns have changed over the last decades over different ways.

Living habits

Tobacco:

The use of tobacco products is the single most important factor in increasing cancer risk. The hazards of tobacco are due to a large number of carcinogenic compounds: those which are already in the tobacco product and those that form during smoking.

It is well understood what effect smoking has on the production of lung cancer. The greater the risk of getting lung cancer, the younger people start smoking, the more they smoke regularly, and the longer they continue to smoke. [4, 5] If a person has smoked 20 cigarettes a day for 50 years, his or her risk of developing lung cancer is nearly 50 times greater than that of non-smokers. The risk of lung cancer, after quitting smoking, exceeds those of non-smokers of the same age quite rapidly but it does not fall to the same level. People who have smoked the longest benefit from giving up. Smoking is a major cause of larynx cancer, and also has an impact on oral, pharyngeal, kidney, pancreatic, oesophageal, cervical, and bladder cancer. Smoking could increase breast cancer risk slightly, too. [6]

In recent years, strong evidence has accumulated in studies about the carcinogenic potential of using snus. The risk of developing oral and pharyngeal cancer, pancreatic and gastric cancer and oesophageal cancer is significantly higher for those who use snus than for those who do not use tobacco products. [7-9] Over the last few years, electronic smoking has increased significantly. It is studying its impact on health.

With the combination of smoking and other factors, the risk of cancer also increases considerably. Smoking and outdoor work together, for example, increase the risk of lip cancer by as much as 15-fold, while outdoor work or smoking alone only doubles the risk of lip cancer. [10] Smoking increases the effect of asbestos and other chemicals used in the workplace, which are not especially harmful by themselves.

Alcohol:

There is a direct causal link between alcoholic beverage use and many cancer types. There is compelling evidence that heavy alcohol consumption raises the risk of cancers of the throat, pharynx, larynx, oesophagus, liver, bowel and breast. [11]

For example, four regular doses of alcohol (50 grams of ethanol) roughly double the risk of oral and pharyngeal cancer. Other factors that contribute to the alcohol impact. Drinking alcohol and smoking together, for example, very significantly increase the risk of nasal, pharynx and larynx cancers.

There is no safe level of alcohol intake about breast cancer, although the risk of cancer is increased in direct proportion to the amount of alcohol consumed on a daily basis. The biggest risk factor is the volume of ethanol consumed. Strong alcohol consumption raises cancer risk and causes many apparent health issues. The type of alcoholic drinks consumed does not affect cancer risk significantly.

Nutrition:

Diet is a dynamic combination of components, having joint and opposite effects that are hard to accurately predict. Food is known to have the greatest effect on the occurrence of oesophagus, colon, rectum, kidney, prostate, lung, and breast cancer.

Nutritional fat increases the risk of breast, colon and pancreatic cancer according to a number of animal tests. However, information about people is not yet sufficiently convincing to allow for a definite judgment of the effects of fat on cancer risk. Cancer tissue requires energy and minerals, so nutrition can have an effect not only on cancer production but also on its growth.

It is probable that dietary fibres can protect against colorectal cancer. [12] In addition, diets high in fruits and vegetables lower the risk of many cancers.

There was a comprehensive study of the effect of red and processed meat on the incidence of some types of cancer. High processed meat intake is associated with increased risk of colorectal and gastric cancer. Red meat is also a potential risk factor for both colorectal and pancreatic and prostate cancer. [13]

The key interest in the vitamin and cancer connection has been in carotenoids, vitamins A, E, C , and D, and folate. Vitamin products have also been tested for cancer prevention, but none of these have yet been shown to prevent cancer alone or in any combination. [14] Therefore, the key factor for cancer prevention appears to be a healthy overall lifestyle, not specific dietary factors.

Some processes in food preparation cause chemical changes which lead to the formation of cancer-causing substances in the food. Smoking and grilling fatty foods over an open fire or other high temperature will produce small quantities of polycyclic aromatic hydrocarbons ( PAHs) on the food's surface, which will increase cancer risk. High salt and salt-conserving foods raise the risk of gastric cancer.

Exercise and weight control.

There are several studies that have identified the correlation between physical activity and cancer risk. Accumulated scientific evidence suggesting that physical activity especially protects against breast, colon, endometrial, and prostate cancer.

Moderate physical activity affects the metabolism of certain hormones and improves the general mechanisms of body defence. Exercise reduces the amount of fat tissue and balances the situation of different growth factors in the body. People who exercise more regularly often have more safe ingredients for cancer in their diets.

A total amount of about one hour of daily physical activity in the form of everyday or beneficial exercise, such as walking or biking to work or to the shop, climbing the stairs or working the yard, is sufficient according to recent studies. Specifically, people who do sedentary work can frequently exercise to stop overweight. Swift workout several days a week will increase the protective impact of physical activity on cancer. [15]

Reproduction and hormones.

There is a strong connection between female genital cancer and breast cancer and sexual and reproductive behaviour. If a woman has multiple sexual partners, she may have a greater risk of developing cervical cancer. The number of their male partner's sexual partners also affects the risk of developing cervical cancer between women. This may be explained by the fact that sexual contact with infectious viruses is significant in cervical cancer pathogenesis.

To give birth at an early age and to have several children protects against breast cancer. If the woman has a very large number of children, the protective effect is emphasised. [16] Infertility also constitutes a risk factor for ovarian and endometrial cancers. Early sexual maturity and late menopause increase menstrual cycles and the incidence of the above cancers.

Breast, endometrial and ovarian cancers are more frequently diagnosed during menopause in women who use long-term hormone therapy. [17-19] Progesterone, combined with replacement oestrogen therapy, increases the risk of breast cancer but, in turn, protects against endometrial cancer. [17, 18, 20] Accordingly, the effects of oestrogen replacement therapy in the treatment of menopause should be assessed individually in relation to the risk of cancer, inter alia.

For men, elevated levels of male hormone, i.e. androgen, are likely to be associated with an increased risk of prostate cancer.

Cosmetics.

In everyday life, we are exposed, involuntarily or accidentally, to a wide range of chemicals. In several studies, inter alia, links with the use of deodorants and hair dyes to cancer risk were investigated. Some studies have found that the use of hair dyes increases the incidence of, for example, bladder and breast cancer slightly, but there is insufficient evidence of their definite role as a potential risk factor for cancer.

Infections.

Some viral infections increase the risk of developing cancer. Bacteria-infections may also raise the risk of certain cancers. The most widely studied family of cancer-causing viruses are papillomaviruses (HPV, human papilloma virus), some of which contribute to chronic inflammation and eventually cervical cancer. [25] In addition, papillomaviruses can cause other cancers, such as pharyngeal cancer. [26] Long-term follow-up studies are underway to determine to what degree HPV vaccines often prevent cancer. [27]

The Helicobacter pylori bacterium raises the risk of gastric cancer. [28] Chronic inflammation of the liver (infections with hepatitis B and C viruses) is associated with increased risk of liver cancer. A vaccination program for hepatitis B virus ( HBV), initiated in Taiwan in 1984, led first to a decrease in HBV prevalence, and subsequently to a substantial reduction in the incidence of liver cancer in vaccinated age groups. [29]

Work-related exposures

The impact of work-related exposures, such as chemicals, on cancer risk, has been studied in hundreds of separate studies.

Sedentary work decreases physical activity, which has cancer-protective properties. In many cases, the lack of physical activity and excessive sitting are detrimental to health. [30] At the moment, shift work is also known to increase the risk of cancer. [32]. For certain occupations, ultraviolet radiation from the sun plays a part in the prevention of cancer.

For example, lip cancer is most frequently found in fishermen and farmers, while the risk of cutaneous melanoma is the greatest in indoor workers whose irregular skin is easily burned during holidays.

Environmental exposures

Pollutions of the living environment. Air pollution, mostly emissions from manufacturing and traffic, greatly raises the risk of lung cancer. Air pollution is a risk factor which is marginal compared with tobacco. However, studies are currently underway, among other items, on how wood-heated houses and small particle emissions are related to cancer.

Drinking water processing by surface water chlorination produces mutation-causing compounds which can slightly increase the risk of genitourinary cancer, for example. The arsenic tends to have a similar impact in drilled well water.

Over recent years, measurement results describing the state of the environment have been increasingly obtained, and they can be used to better assess the causes of cancer risk. An example of this is an environmental health analysis of people who once lived in houses that were built and then demolished on top of a former Myllypuro landfill in Helsinki. According to the first study, some more cancer and asthma were identified in the residents of these houses compared to comparison residents[35], but the latest studies show that those living in the area of Myllypuro were not diagnosed with cancer more frequently than those living in any other region of Helsinki in the 2000s.

Ionising radiation

Ionising radiation exists everywhere, as radiation can be contained e.g. in the natural radioactive compounds of the earth's surface, which are carried by nutrition through the human body. Radon inhalation causes a dose of radiation primarily only in the lungs. The only clearly defined impact on radon is the increased risk of lung cancer. In smokers, the risk of radon-caused lung cancer is higher than in non-smokers. It is estimated that radon in indoor air causes roughly in conjunction with smoking.

X-ray radiation is used in radiology, and radiation treatments use various forms. Therefore, in health care, it should always be carefully assessed if the benefit obtained with the use of radiation is greater than the risk it creates. Cancer patients seeking radiation treatment undergo extremely high dosages of radiation, and other tissues may also be exposed to radiation in addition to the tumour. Patients treated with radiation therapy have an increased risk of developing leukaemia and certain other cancers[38-43], but treatment benefits are much greater than the drawbacks.

It has been estimated that ionising radiation causes 1-3 per cent of all cancers. Radiation increases the risk of leukaemia and of thyroid, breast, lung and bladder cancer particularly strongly.

Non-ionising radiation

Non-Ionising radiation involves ultraviolet radiation, as well as the magnetic and electric fields. Ultraviolet radiation is obtained primarily from sunlight and sunbeds, and it causes skin cancers. Repeated burning of the skin is the main cause of cutaneous melanoma due to excessive UV radiation, particularly in infancy and youth.

Power cords and electrical devices generate very low frequency magnetic fields around them (50-60 Hz). Their effect on cancer risk and, above all, the risk of childhood leukaemia was tested several times, but the findings were inconsistent. The risk of cancer in children or adults in the vicinity of power lines is not increased according to studies. [44, 45]

The strength of magnetic fields with radio frequencies is million hertz. They are generated by radar, radio transmitters, cell phones and base stations among others. Radio-frequency radiation does not induce mutation, and animal studies have not raised cancers. Mobile phone usage was not shown to increase the risk of cancer in humans in epidemiological studies either (Interphone Study Group 2010), but the issue with the studies was the relatively short follow-up duration of ten years and the unreliability of questionnaire-based knowledge use. New large-scale, ongoing follow-up studies are hoped to provide more information in different countries.

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