Cancer is not a unique disease, but rather a large class of diseases characterized by out-of-control cell growth. The masses of abnormal growing cells can interfere with the function of vital organs (e.g. essential parts of the digestive, nervous, and circulatory systems), and they can release hormones that alter the global body function.
The World Health Organisation estimates that, worldwide, there were 14 million new cancer cases and 8.2 million cancer-related deaths in 2012 (their most recent data). In the next 15 years, the number of cancer cases in Canada is expected to increase by 40%. This is largely attributed to our growing and aging population.
How does cancer spread?
Normal cells strongly interact with their surrounding environment; a kind of scaffolding glue that holds the cells in place, named extracellular matrix. Cancer cells sometimes acquire strategies to detach from the extracellular matrix. For example, they may produce a substance to destroy it, or develop the ability to “swim” across the matrix. The result is the same, these cancer cells are thus unstuck from the original tumor site and can reach the blood circulation, or more often the lymphatic system, which is an extended network of canals used by the immune system in our body. The cancer cells circulating in the blood or the lymphatic system can unfortunately reattach at a new site and thus form a new tumor; in other words, a metastasis or secondary tumor.
The understanding of this process is important because cancer mortality is mainly due to metastatic tumors. Only 10% of cancer deaths are caused by the primary tumors. Some scientists, from the Massachusetts Institute of Technology for example, say that finding a way to stop cancer cells from sticking to new sites could interfere with metastatic disease, and halt the growth of secondary tumors. Consequently, there is a lot of research that aims to identify the molecules used by cancer cells to detach from the extracellular matrix and to migrate to other sites in the body. Ultimately, the researchers hope to develop targeted therapies that will inactivate these molecules.
Important risk factors at play
In addition to understanding the biology of how cancer forms, many researchers have tried to understand why cancer incidence is rising. In Canada, the main reason is increasing life span; older people are significantly more at risk of cancer. But there are many other factors that can contribute to cancer, including heredity, lifestyle and environmental hazards. These factors can include our family history of cancer, our diet, smoking, sunbathing, alcohol consumption, exposures to different chemicals in the workplace, and even to certain viruses like the human papillomavirus (HPV) that can cause cervical, anal and oral cancers. One troubling factor is obesity, which is on the rise in Canada. Experts predict that cancer rates will increase nearly 6 times faster in women than in men over the next 20 years with obesity partly to blame. As several obesity-related cancer types only affect women, the growing number of people of both sexes who are severely overweight is likely to have a greater effect on incidence of cancer among women. Overall, cases of ovarian cancer, oral cancer, and melanoma are predicted to rise the most. This is because of obesity, but also because of the increased prevalence of HPV and more exposure to ultraviolet radiation. Cancer rates are predicted to rise by at least 0.5% for men and 3% for women, meaning an estimated 4.5 million women and 4.8 million men will be diagnosed with cancer by 2035.
Healthy lifestyle and prevention
Education regarding sunbathing and using sunbeds, smoking, alcohol consumption, healthy diet, exercise, and safe sex practice play an important role. Early screening programs also have an impact on cancer diagnosis, many of which might have gone previously undetected, thereby allowing early intervention to cure the disease.
In short, research has shown us the benefits of a healthy lifestyle in cancer prevention. As individuals, we can stack the odds in our favour by not smoking, maintaining a healthy weight, being more active and drinking less alcohol. Indeed, 4 out of 10 cancers could have been prevented by adopting these changes. History taught us that adopting bad habits can have terrible consequences; lung cancer was a rare disease before cigarettes were commercialized but has since become the leading cause of cancer death among both men and women. Conversely, better food hygiene and quality has decreased the incidence of Helicobacter pylori infection, a bacterium associated with stomach cancer; leading to its lower frequency. Similarly, the incidence of cervical cancer is expected to fall due to the the vaccine against HPV, the main cause of this cancer.
Canadian cancer society’s Advisory Committee on Cancer statistics. Canadian Cancer Statistics 2015. Toronto, ON: Canadian Cancer Society, 2015
Cancer Research UK. Causes of Cancer, Cancer Controversies and cancer Clusters. Cancer Statistics for UK. National Report 2014
Maria Kyrgiou, Ilkka Kalliala, Georgios Markozannes, Marc J Gunter, Evangelos Paraskevaidis, Hani Gabra, Pierre Martin-Hirsch, Konstantinos K Tsilidis. Adiposity and cancer at major anatomical sites: umbrella review of the literature. BMJ 2017;356:j477 | doi: 10.1136/bmj.j477
White KK, Park SY, Kolonel LN, Henderson BE, Wilkens LR. Body size and breast cancer risk: the Multiethnic Cohort. Int J Cancer 2012; 131:E705-16. doi:10.1002/ijc.27373.
Wang F, Xu Y. Body mass index and risk of renal cell cancer: a dose-response meta-analysis of published cohort studies. Int J Cancer 2014;135:1673-86. doi:10.1002/ijc.28813.
Carter BD, Abnet CC, Feskanich D, et al. Smoking and mortality–beyond established causes. N Engl J Med 2015;372:631-40
Julia Hamer HBSc, Ellen Warner MD MSc: CMAJ 2017 February 21;189:E268-74. doi: 10.1503/cmaj.160464
IOM (Institute of Medicine) Fulfilling the Potential of Cancer Prevention and Early Detection. Curry SJ, Byers T, Hewitt M, editors. Washington, DC: The National Academies Press; 2003.
World Health Assembly. Cancer prevention and control. WHA Resolution 58.22. Geneva, Switzerland: WHO; 2005.
World Cancer Research Fund. Food, Nutrition, and the Prevention of Cancer: A Global Perspective. Washington, DC: American Institute for Cancer Research; 1997.
PMID Cancer Causes Control. 2017 Feb;28(2):145-154. doi: 10.1007/s10552-016-0846-y. Epub 2017 Jan 27. GomezSL, Canchola AJ, Nelson DO, Keegan TH, Clarke CA, Cheng I, Sariff Marco, Derouen M, Catalano R, Satariano WA, Davidson Allen K, Glaser SL. Recent declines in cancer incidence: related to the great recession?
Connor J. Alcohol consumption as a cause of cancer. Addiction 2017; 112: 222–228.
World Cancer Research Fund/American Institute for Cancer Research (AICR) Food, Nutrition, Physical Activity, and the Prevention of Cancer: A Global Perspective. Washington, DC: AICR; 2007