Cancer includes a group of diseases that are characterized by an abnormal growth of any kind of cell that divides in an uncontrollable way and has the ability to infiltrate and destroy normal body tissue 1. Indeed, a single cancerous cell surrounded by healthy tissue will replicate at a higher rate than other cells 2. This will render the nutrient supply and elimination of metabolic waste products critical 2. Once a tumor mass has formed, the healthy tissue will not be able to compete with the cancer cells for the inadequate supply of nutrients from the blood stream 2. Healthy cells will be displaced by tumor cells until the tumor reaches a diffusion-limited maximal size 2. Therefore, the tumor cells will continue dividing because they do so without regard to nutrient supply. 2
There are more than a hundred distinct types of cancer that are variable in terms of characteristics, occurrence and response to treatment (Figure 4) 1. The distinction between benign and malignant tumors is key in oncology 1. A benign tumor remains confined to its original location, while a malignant tumor is capable of invading surrounding normal tissue and spreading throughout the body via the circulatory or lymphatic systems (metastasis) 1. Only malignant tumors are properly referred to as cancers, and it is their ability to invade and metastasize that makes cancer so dangerous 1.
Tumors are classified according to the type of cell that formed them. Cancers are composed of three main groups: carcinomas, sarcomas, and leukemias or lymphomas. Carcinomas are malignancies of epithelial cells and account for approximately 90% of human cancers. Sarcomas are considered to be rare in humans. They are tumors that form in connective tissues such as muscle, bone, cartilage and fibrous tissue. Leukemias or lymphomas account for approximately 8% of human malignancies and arise from blood-forming cells and cells of the immune system, respectively. Tumors are classified according to tissue of origin (e.g. lung or breast carcinomas) and the type of cell involved 1.
Although there are many types of cancer, only a few occur frequently 1. It was reported that more than one million cases of cancer are diagnosed annually in the United States, and more than 500,000 Americans die of cancer each year 1. In the United States and Switzerland, the ten most common cancers include breast, prostate, lung, colon/rectum, lymphomas, bladder, uterus, skin, kidney cancer and leukemia 1 (this rating can differ depending on the country and habits). It is important to note that breast, prostate, lung, and colon/rectum cancers are carcinomas forming solid tumors 3, and account for more than 50% of all cancer cases 1 3. Lung cancer is by far the most lethal one (approx. 30% of all cancer deaths) 1.
A considerable number of risk factors and causes are associated with the development of cancer 1. This includes genetic factors as well as radiation, chemicals and viruses 1. Radiation and chemical carcinogens act by damaging DNA and inducing mutations 1. These carcinogens referred to as initiating agents, include solar ultraviolet radiation, carcinogenic chemicals in tobacco smoke (mainly benzo(a)pyrene, dimethyl nitrosamine, and nickel compounds), and aflatoxin 1. Tumor promotors are mainly carcinogens that contribute to cancer development by stimulating cell proliferation (e.g. phorbol esters, hormones like estrogens) 1. In addition to chemicals and radiation, microbes, especially viruses (e.g. Human Papillomavirus) but also some bacteria (Helicobacter pylori) are considered as cancer causative agents 1 4. The four most common types of cancer are listed below:
1. Lung cancer (see also respiratory diseases)
Lung cancer is the main cause of cancer death in men and the second in women3 5. It encompasses the malignant tumors of the trachea, bronchi and lung. The tumor constituting cells are very aggressive and spread very quickly 6. Active or passive smoking is the main risk factor (responsible for 90% of lung cancer death in developed countries) 6. Other risk factors associated with this disease are: hormonal factors, viruses 5, exposure to radon, occupational exposure to polycyclic aromatic hydrocarbons, asbestos, crystalline silica, metals (beryllium, arsenic, chromium, nickel), atmospheric pollution with fine particles (<PM10), diesel emissions, domestic pollution caused by the use of solid fuels and exposure to gamma rays and plutonium 5 6.
2. Breast cancer
Breast cancer is reported to be the most common cause of cancer death among women worldwide 7. It develops in the glandular cells that constitute the breast 6.
In the international classification of diseases, there are more than twenty different subtypes of breast cancer 3 6. Incidence rates are higher in developed countries and increase during the reproductive years 7. The risk factors are associated with the menstrual cycle, childbearing, breastfeeding, menopause, endogenous hormones, oral contraceptives, hormonal therapy for menopause, other exogenous hormones (diethylstilboestrol and fertility drugs), diet, alcohol, smoking, ionizing radiation, electromagnetic fields, environmental oestrogens, family history and genetic factors 7 6.
3. Prostate cancer
Prostate cancer is the most common cancer in men 8. It is the second leading cause of death from malignancies 8. As in breast cancer, it develops in the glandular cells 6. The risk factors include age, family history, ethnicity, hormones, nutritional and environmental factors 6 8. Diets high in red meat and dairy products appear to increase risk of prostate cancer. On the contrary, diets containing selenium, lycopene and vitamin E have protective effects 8. It was reported that under the age of 55, prostate cancer is rare 6.
4. Colorectal cancer
Colorectal cancer (CRC) is the third leading cancer in both sexes after lung and breast or prostate cancers 6,9,10. Colorectal cancer usually develops from cells of the intestinal mucosa 6. It includes malignant tumors of the colon, of the recto-sigmoid junction and rectum. The latter two constitute the majority of colorectal cancer 6. The risk factors associated with CRC include consumption of red and processed meat, high body mass index, a bodyfat percentage higher than average, lack of physical activity and above average body size 6. Alcohol and tobacco consumption as well as exposure to x- and γ-rays are also considered as potential risk factors 6. It is reported that between 5% and 10% of colorectal cancers are attributed to hereditary origin, such as familial adenomatous polyposis and non-polyposis hereditary colorectal cancer 6. In 20% of cases, colorectal cancer develops in subjects with family history of CRC 6. Finally, inflammatory bowel diseases such as ulcerative colitis and Crohn’s disease, increase the risk of colorectal cancer 6.
REFERENCES
1. Cooper, G. The Cell: A Molecular Approach. 2nd edition. in Sunderland (MA): Sinauer Associates.The Development and Causes of Cancer. Available from: https://www.ncbi.nlm.nih.gov/books/NBK9963/ (2000).
2. Brannon-peppas, L. & Blanchette, J. O. Nanoparticle and targeted systems for cancer therapy. Adv. Drug Deliv. Rev. 56, 1649–1659 (2004).
3. OFSP. Le cancer en Suisse. Office fédérale de la santé publique, Division stratégies de la santé (2019). Available at: https://www.bag.admin.ch/bag/fr/home/zahlen-und-statistiken/zahlen-fakten-nichtuebertragbare-krankheiten/krebserkrankungen-schweiz.html.
4. Hussain, S. P., Hofseth, L. J. & Harris, C. C. Radical causes of cancer. Nature 3, 276–285 (2003).
5. Subramanian, J. & Govindan, R. Lung Cancer in Never Smokers : A Review. J. Clin. Oncol. 25, 561–570 (2007).
6. Arndt, V. et al. Le cancer en Suisse, rapport 2015. (Office fédéral de la statistique (OFS) Neuchâtel 2016, 2016).
7. Key, T. J., Verkasalo, P. K. & Banks, E. Reviews Epidemiology of breast cancer. Lancet 44, 133–140 (2001).
8. Giovannucci, E. A Review of Epidemiologic Studies of Tomatoes , Lycopene , and Prostate Cancer. Soc. Exp. Biol. Med. 852–859 (2002).
9. Walsh, J. M. E. & Terdiman, J. P. Colorectal Cancer Screening. JAMA 289, 1288–1296 (2003).
10. Vernon, S. W. Participation in Colorectal Cancer Screening : a Review. J. Natl. Cancer Inst. 89, 1406–1422 (1997).