The human body is made up of trillions of cells, grouped together to form tissues and organs such as muscles, bones, lungs, and liver, and all the other parts of the body. Genes inside each cell control the ways it grows, works, reproduces, dies, and is replaced by new cells.
Normally, the cells obey orders from the genes. They reproduce and die off to make room for new, healthy cells in a process called senescence that happens millions of times a day, often without our notice, and the body remains healthy. But this natural process goes awry in cancer. When cells divide to form new cells, sometimes mistakes, known as mutations, are made in the reproduction of the genetic material. The first mistake often makes it more likely that there will be a second mistake; that mistake makes the situation even more unstable, increasing the likelihood of a third mistake; and so on.
The body has a defense against this downward slide of accumulating errors: cells have “spellcheckers” that look for these mutations and, if errors are detected, can stop cell division. If the damage cannot be repaired, the cell puts out a signal for the immune system to kill it, or the cell simply self-destructs in a process known as apoptosis.
However, if a cell accumulates enough mutations without being eliminated through apoptosis, then that cell and all of its descendants can become malignant and resistant to normal processes of cell death. These cells continue to grow in an unregulated fashion. When cancerous cells spread beyond the organ or part of the body in which they initially arose to invade and damage distant organs, this is referred to as metastasis. For example, cancer in the colon can break through the wall of the colon and metastasize to the liver or the lungs, disrupting normal body functions and devastating healthy tissue.
Ironically, cancer cells use normal cellular machinery involved in wound healing, embryonic development, and normal growth to carry out these very abnormal and destructive behaviors. When a normal gene is inappropriately activated by a malignant cell to support cancer cell growth, this gene is called an oncogene. This “hijacking” of normal cell functions by cancer makes targeting and killing cancer cells while sparing healthy ones a difficult challenge. Targeting cancer cells is a major focus of current cancer research.
Cancer develops when the balance between accumulating genetic mutations, and the safeguards built into cells to defend against and repair the resulting damage, is upset. The genes that a cell activates in this surveillance-and-repair process are called tumor suppressor genes. The workings of this balance are reflected in the epidemiology of cancer in human populations. Things that tip the balance in favor of the wayward cell completing its transformation into malignancy increase one’s risk of developing cancer.
Smoking, exposure to radiation and exposure to certain chemicals increases the likelihood of the cell making a mistake on the next division. Exposure to these known carcinogens has been associated with increased risk of leukemia, lymphoma (lymph node cancer), lung cancer, melanoma, and bladder cancer.
Defects in the immune system – either hereditary, acquired through diseases such as HIV infection, or caused by medical treatments given to suppress immunity – can lead to lymphoma and other cancers.
Hormones increase cell division and growth in some tissues. But the more times a cell divides, the greater the chance for a mistake which can lead to cancer. Menopausal hormone therapy – in which women are given hormones to relieve hot flashes and other symptoms of menopause – has been associated with a higher risk of breast cancer.
Hereditary deficiencies in the genetic machinery involved in mutation repair can lead to cancer. For example, mutations in the BRCA genes have been linked to breast and ovarian cancer.
Imagine cancer as a broken light switch, with the light permanently stuck in the “on” position when it should be turned off. This can happen to anyone, anytime, but it is more likely to happen if you repeatedly turn the light switch on and off (as in the hormone supplement example), use a hammer to turn the light on and off (via exposure to a carcinogen), or lose your screwdriver so you can’t fix the switch when it breaks (as when the body’s immune system or mutation-repair machinery are not working properly).
Simply put, cancer is the uncontrolled growth of cells. If this runaway growth of cells continues, it forms lumps or tumours. These can be benign, causing no harm, or malignant, disrupting normal body functions and devastating healthy tissue.
A cancer is named after the part of the body where it began, even if it spreads to other organs. Colon cancer that involves the lungs is still called colon cancer. Lung cancer can spread to the lining of the brain, but is still called lung cancer.
Cancer tends to be associated with aging. The number of new cancer cases continues to rise steadily as the Canadian population grows and ages. In fact, 69 percent of all new cancer cases and 62 percent of cancer deaths will occur among Canadians 50–79 years of age. However, many other factors can increase a person’s risk of developing cancer.
Having a risk factor, or even multiple factors, doesn’t mean a person will necessarily develop cancer, and not having them doesn’t mean a person won’t develop cancer. However, reducing or managing risk factors can help reduce the chances that a person will develop cancer at some point in their lifetime. See “Cancer Prevention Tips” for some things you can do to reduce your risk.