Cancer is the name given to a group of related diseases characterized by the uncontrolled and excessive growth of abnormal cells. More than 200 different types of cancer have been identified.
When these extra cells form a mass or a solid lump of tissue, it is called a tumor. Most cancers form tumors, but not all do. Tumors may be either benign or cancerous.
Benign tumors do not spread to other parts of the body and are rarely life-threatening. Many breast lumps, for example, are benign. Benign tumors are not cancer.
Malignant tumors crowd out healthy cells, interfere with bodily functions, and draw nutrients from body tissues. Malignant tumors can also spread to other parts of the body via the blood or lymphatic system, forming satellite tumors, called secondary cancers or metastases. Malignant tumors are cancer.
Cancer can occur anywhere in the body and skin cancer is the most commonly reported cancer. Breast cancer is the next most common cancer in women, and in men, it is prostate cancer. Lung cancer and colorectal cancer are also common cancers that occur in both women and men. Cancer can be classified into one of five types:
- Carcinomas – these begin in the skin or tissues that line the internal organs
- Central nervous system cancers – these develop in the brain and spinal cord
- Leukemias – these begin in the blood and bone marrow
- Lymphomas – these start in the immune system
- Sarcomas – these develop in connective tissue such as the bone, cartilage, fat, or muscle.
Cancers are named according to the origin of the cancer (where it first starts) even if it has spread. For example, if prostate cancer has spread to the liver it is still called metastatic prostate cancer.
How does it start?
Cancer starts when a mutation in a normal cell causes irreversible damage to the cell's DNA. If the normal control mechanisms of the body are unable to contain this cell, then it begins to divide at an abnormally fast rate, leading to more mutations and a mass of cells, all of which can proliferate to form more cells.
Are some people more likely to get cancer than others?
Experts have identified several key factors that make some people more likely than others to develop cancer. The most common ones are:
- High intake of dietary carcinogens: Certain foods, including those that are processed, salted, pickled, smoked, charred from a grill or barbecue, or treated with nitrites have been associated with a higher risk of cancer. Saturated fats from red meat are also associated with several different types of cancer, including cancer of the colon, rectum, and prostate gland. You can reduce your risk by not eating processed or burnt meats and eating more fiber, fruit, and vegetables.
- Genetic predisposition: Certain types of cancer, such as colon and breast cancer, often run in families, and people can inherit this predisposition towards cancer, although other non-genetic (environmental) factors must be present for cancer to develop
- Estrogen exposure in women: Women who are exposed to higher levels of estrogen, either via estrogen-containing medication or naturally through going through menstruation early or menopause late, are at increased risk for some cancers, such as those of the breast, ovaries, or uterus. This risk is reduced in women who have had a baby before the age of 35, who exercise regularly, or who consume a low-fat diet
- Exposure to carcinogens: People exposed to certain substances have a higher risk of developing certain types of cancer. There are more than two hundred known carcinogens including alcohol, asbestos, coal tar, diesel exhaust, formaldehyde, tobacco, and UV radiation
- Exposure to some infectious organisms: Certain viruses (such as hepatitis B, hepatitis C, and human papillomavirus), bacteria (eg, Helicobacter pylori) and parasites (eg, Schistosoma hematobium) have been associated with a higher risk of cancer
- Radiation: Over-exposure to X rays, nuclear radiation, ultraviolet radiation (from sunlight), and even cosmic radiation (aircrew have higher rates of cancer than non-aircrew) can cause DNA injury that may lead to cancer.
Several other factors also increase the risk of cancer such as chronic inflammation, immunosuppression, and obesity.
There are several treatment options available for cancer. Treatment plans are developed depending on the type of cancer; its location; the extent of cancer and the stage at which it is diagnosed; and the health and well-being of the patient. Treatment may consist of one type of therapy or be a combination of several. The most common types of treatment are listed below.
- Surgery: Involves removal of the tumor and sometimes, surrounding tissue and lymph nodes. Surgery can be performed using conventional instruments, cryosurgery (using liquid nitrogen or argon gas), light (photodynamic therapy), high temperatures (hyperthermia), or laser
- Radiation therapy: Uses high doses of radiation to kill cancer cells and shrink tumors. The two main types of radiation therapy are external beam and internal radiation
- Chemotherapy: Uses specific drugs to kill cancer cells by halting their growth or preventing their multiplication at some point in their life cycle. Drugs may be administered intravenously (into a vein), orally (by mouth), by injection into a muscle, topically (applied to the skin) or in other ways, depending on the drug and the type of cancer. Chemotherapy is often given in cycles of alternating treatment and rest periods
- Immunotherapy: A type of biological therapy where drugs are given or procedures are undertaken that help the immune system attack cancer directly or stimulate the immune system in a more general way. Examples of treatments include checkpoint inhibitors, cytokines, treatment vaccines, and adoptive cell transfer
- Targeted therapy: These treatments target changes in cancer cells that help them to , divide and spread. Monoclonal antibodies are a type of targeted therapy which attach to specific targets on the outer surface of cancer cells
- Hormone therapy: Drugs are used to block the body’s ability to produce hormones or interfere with how hormones behave in the body
- Stem cell transplants: Procedures that restore blood-forming stem cells in people who have had theirs destroyed, either by chemotherapy or high-dose radiation therapy
- Precision medicine: Treatment is tailored to each individual, based on a genetic understanding of their cancer. Although not yet routine, doctors hope it will one day be the standard treatment of the future
- Alternative and complementary therapy: Includes nonpharmacological treatments such as acupuncture, herbal supplements, and homeopathy. It is important to note; however, that many of these treatments do not have research to support their effectiveness.
Many other drugs may be used during the treatment of cancer, such as analgesics to relieve pain or antiemetics to prevent or treat nausea or vomiting.
Some cancer drugs may affect healthy cells in the body, in addition to cancerous cells, and cause side effects such as an increased risk of infection, bruising or bleeding, and extreme tiredness. Some cancer drugs cause hair loss.
There are more than 250 different cancer drugs. These can be organized into different categories depending on the way they work, for example:
- Alkylating agents: Interfere with DNA linking and include the alkyl sulfonates, ethylenimines, nitrogen mustards, nitrosoureas, and triazines
- Anthracyclines: Damage DNA in cancer cells, causing them to die (eg, daunorubicin, doxorubicin, and epirubicin)
- Antimetabolites: Affect DNA synthesis (eg, capecitabine, fluorouracil, methotrexate)
- Aromatase inhibitors: Block the enzyme aromatase, that converts androgens into estrogen (eg, anastrozole, exemestane, letrozole)
- CDK 4/6 inhibitors: Target enzymes, called CDK4 and CDK6 that are important for cell division (eg, abemaciclib, ribociclib, palbociclib)
- HDAC (histone deacetylase) inhibitors: Have a number of different actions including inhibiting DNA repair, stopping the proliferation of cancer cells, and stimulating cell death (eg, belinostat, panobinostat, romidepsin, vorinostat)
- mTOR inhibitors: blocks an enzyme that regulates growth factors that stimulate cell growth and the formation of new blood vessels (eg, everolimus, temsirolimus)
- PARP inhibitors: Used for the treatment of cancers with mutations in their DNA repair genes (eg, olaparib, niraparib, rucaparib)
- Retinoids: Inhibit tumor growth, blood vessel formation and tumor spread (eg, alitretinoin, bexarotene)
- Topoisomerase inhibitors: Block an enzyme, topoisomerase, that breaks and rejoins DNA strands and is vital for cell division and growth (eg, irinotecan, topotecan)
- Kinase inhibitors: Block kinase enzymes that help control important cell functions, such as cell signaling, metabolism, division, and survival (eg, imatinib, nilotinib, sorafenib)
- Vinca alkaloids: Extracted from the pink periwinkle plant they inhibit tubulin which is the main constituent of the microtubules of living cells, causing cell death (eg, vinblastine, vinorelbine, vincristine).