Mitosis is supposed to happen only when the body needs new cells, and to stop when the job is done. A cut heals, the new skin cells fill the gap, and division slows back down. Cancer is the name for a large group of diseases in which some cells keep dividing when they should stop. About 2 in 5 Canadians are expected to be diagnosed with cancer at some point in their lives, which makes understanding it, preventing it and catching it early one of the biggest jobs in public health.
How normal cells control division
- Checkpoints: at several points in the cell cycle the cell checks that its DNA is undamaged and copied correctly before it carries on. If something is wrong, it pauses to repair the damage.
- Contact inhibition: most normal cells stop dividing when they are crowded against their neighbours, so they form a neat single layer.
- Programmed cell death: a cell with damage it cannot repair usually destroys itself, removing the problem.
- Specialization: normal cells mature into a specific type and do their job; many mature cells divide rarely or not at all.
Cancer begins with mutations, changes in the DNA of the genes that control these checks. One mutation is usually not enough. Over years, a cell lineage can collect several, until its descendants ignore stop signals, skip checkpoints and avoid programmed death. That is one reason most cancers are more common in older people: there has been more time for mutations to build up.
- A lump of abnormal cells that stays in one place
- Often surrounded by a capsule; does not invade nearby tissue
- Does not spread to other parts of the body
- Usually removable by surgery; can still be dangerous if it presses on a vital organ such as the brain
- Cells invade and destroy nearby healthy tissue
- Cells can break off and travel in the blood or lymph
- Can set up new tumours elsewhere: this spread is called metastasis
- Harder to treat once it has spread, so early detection matters
Seeing the difference in the rate of division
Because cancer cells divide more often, a sample of tumour tissue usually contains a larger share of cells in mitosis than the normal tissue around it. Scientists summarize this with the mitotic index, the percentage of cells caught in mitosis. Medical laboratory technologists prepare and stain tissue samples, and pathologists examine them for signs such as a high mitotic index, oddly shaped nuclei and cells that have lost their normal specialized look.
What raises the risk
| Cause (carcinogen or risk factor) | Cancers it is linked to |
|---|---|
| Tobacco smoke | Lung, mouth, throat, esophagus, bladder and many others; the largest preventable cause of cancer in Canada |
| Ultraviolet (UV) radiation from the Sun and tanning beds | Skin cancers, including melanoma |
| Human papillomavirus (HPV) infection | Cervical cancer and some cancers of the throat, mouth and genitals |
| Radon, a radioactive gas that can build up in basements | Lung cancer; the leading cause of lung cancer in people who have never smoked |
| Asbestos fibres (old insulation and building materials) | Mesothelioma (lining of the lungs) and lung cancer |
| Alcohol | Mouth, throat, liver, breast and colorectal cancers |
Screening, prevention and treatment
Screening means testing healthy people with no symptoms, to find cancer or pre-cancerous changes early, when treatment works best. As of 2025, Ontario's organized programs include mammograms for breast cancer (every two years for ages 40 to 74, with people in their 40s able to book without a referral), an HPV test for cervical cancer (every five years starting at 25), a take-home stool test called FIT for colorectal cancer (every two years from 50 to 74), and lung scans for people aged 55 to 74 with a long smoking history. Prevention means lowering risk: not smoking, using sunscreen, testing homes for radon, limiting alcohol and getting the HPV vaccine.
| Treatment | How it works | Why it has side effects |
|---|---|---|
| Surgery | Removes the tumour and sometimes nearby lymph nodes | Recovery from the operation; works best before the cancer has spread |
| Radiation therapy | High-energy radiation aimed at the tumour damages the DNA of cells so they cannot divide | Healthy tissue in the beam can also be damaged |
| Chemotherapy | Drugs carried through the blood that kill or stop rapidly dividing cells | Other fast-dividing cells (hair follicles, the gut lining, bone marrow) are hit too: hair loss, nausea, low blood counts |
| Immunotherapy and targeted drugs | Help the immune system recognize cancer cells, or block a specific molecule that the cancer cells depend on | Usually more specific, but can trigger immune reactions against healthy tissue |