Many genetic and environmental factors increase the risk of developing cancer. However, not all people who are exposed to carcinogens or who have other risk factors develop cancer. (See also Overview of Cancer Overview of Cancer A cancer is an abnormal growth of cells (usually derived from a single abnormal cell). The cells have lost normal control mechanisms and thus are able to multiply continuously, invade nearby... read more .)
Some families have a significantly higher risk of developing certain cancers. Sometimes the increased risk is due to a single gene and sometimes it is due to several genes interacting together. Environmental factors—common to the family—may alter this genetic interaction and cause cancer.
Genes and chromosomes
An extra or abnormal chromosome Chromosome abnormalities Chromosomes are structures within cells that contain a person's genes. A gene is a segment of deoxyribonucleic acid ( DNA) and contains the code for a specific protein that functions in one... read more may increase the risk of cancer. For example, people with the most common type of Down syndrome, who have three instead of the usual two copies of chromosome 21, have a 12 to 20 times higher risk of developing acute leukemia Acute Myeloid Leukemia (AML) Acute myeloid leukemia is a life-threatening disease in which the cells that normally develop into the types of white blood cells called neutrophils, basophils, eosinophils, and monocytes become... read more , but paradoxically, a lower risk of developing carcinomas.
Abnormalities (mutations) affecting critical genes are believed to contribute to the development of cancer. These genes produce proteins that regulate growth and alter cell division and other basic cell properties.
Gene mutations causing cancer may result from the damaging effects of chemicals, sunlight, medications, viruses, or other environmental agents. In some families, these abnormal cancer-causing genes are inherited.
The two major categories of genes involved with cancer are
Tumor suppressor genes
Oncogenes are mutated or amplified forms of genes that in their normal state regulate cell growth. These oncogenes include HER2, which causes breast cancer Breast Cancer Breast cancer occurs when cells in the breast become abnormal and divide uncontrollably. Breast cancer usually starts in the glands that produce milk (lobules) or the tubes (ducts) that carry... read more and EGFR, which causes some lung cancers Lung Cancer Lung cancer is the leading cause of cancer death in both men and women. About 85% of cases are related to cigarette smoking. One common symptom is a persistent cough or a change in the character... read more . Some oncogenes inappropriately signal cells to multiply in an uncontrolled manner, leading to a cancer. The mutation of normal genes to oncogenes is not entirely understood, but many factors may contribute, including
Toxins at work, in the air, or in chemicals (for example, in tobacco smoke)
Infectious agents (for example, certain viruses)
Tumor suppressor genes normally suppress the development of cancers by coding for proteins that repair damaged DNA or suppress the growth of cancerous cells. Cancer is more likely when DNA damage impairs tumor suppressor gene function, allowing affected cells to multiply continuously. Suppressor gene mutations, inherited from a parent, may underlie a certain percentage of cases of breast cancer, usually occurring at a young age and in multiple family members.
Some cancers, such as Wilms tumor Wilms Tumor Wilms tumor is a specific kind of kidney cancer that occurs mainly in young children. The cause of Wilms tumor is not known, but some children may have a genetic abnormality that increases their... read more , retinoblastoma Retinoblastoma Retinoblastoma is a cancer of the retina, the light-sensing area at the back of the eye. Retinoblastomas result from a genetic mutation. The child may have a white pupil or cross-eyes or occasionally... read more , and neuroblastoma Neuroblastoma Neuroblastoma is a common childhood cancer that grows in parts of the nervous system or adrenal glands. What causes neuroblastoma is often not known. Symptoms depend on where neuroblastomas... read more , occur almost exclusively in children. These cancers result from suppressor gene mutations that are either inherited or that occur during fetal development. However, most other cancers are more common in adults, particularly in older people. In the United States, more than 60% of cancers occur in people older than 65. The increased cancer rate is probably due to a combination of increased and prolonged exposure to carcinogens and weakening of the body’s immune system.
Numerous environmental factors increase the risk of developing cancer.
Tobacco smoke contains carcinogens that substantially increase the risk of developing cancers of the lungs, mouth, throat, esophagus, kidneys, and bladder.
Pollutants in the air or water, such as asbestos, industrial waste, or cigarette smoke, can increase the cancer risk. Many chemicals are known to cause cancer, and many others are suspected of doing so. For example, asbestos exposure may cause lung cancer and mesothelioma Mesothelioma Mesothelioma is cancer of the thin, transparent, two-layered membrane that covers and lines the inside of the chest wall and abdomen. The most common symptoms are persistent chest pain and shortness... read more (cancer of the pleura). Exposure to pesticides is associated with a higher risk of some types of cancer (for example, leukemia Overview of Leukemia Leukemias are cancers of white blood cells or of cells that develop into white blood cells. White blood cells develop from stem cells in the bone marrow. Sometimes the development goes awry... read more and non-Hodgkin lymphoma Non-Hodgkin Lymphomas Non-Hodgkin lymphomas are a diverse group of cancers of types of white blood cell called lymphocytes. Often, lymph nodes in the neck, under the arms, or in the groin enlarge rapidly and painlessly... read more ). The time between exposure to the chemicals and development of the cancer may be many years.
Exposure to radiation Sources of Radiation Exposure Radiation injury is damage to tissues caused by exposure to ionizing radiation. Large doses of ionizing radiation can cause acute illness by reducing the production of blood cells and damaging... read more is a risk factor for the development of cancer. Extended exposure to ultraviolet radiation, primarily from sunlight, causes skin cancers. Ionizing radiation is carcinogenic. X-rays (including computed tomography [CT]) use ionizing radiation, and people who have many tests have a small increased risk of cancer (see also risks of radiation in medical imaging Risks of Radiation in Medical Imaging Imaging tests that use radiation, usually x-rays, are a valuable tool in diagnosis, but exposure to radiation has some risks (see also Radiation Injury). Different diagnostic tests require different... read more ).
Exposure to the radioactive gas radon, which is released from soil, increases the risk of lung cancer. Normally, radon disperses rapidly into the atmosphere and causes no harm. However, when a building is placed on soil with a high radon content, radon can accumulate within the building, sometimes causing levels in the air that are sufficiently high enough to cause harm. Radon is breathed into the lungs, where it may eventually cause lung cancer. In exposed people who also smoke, the risk of lung cancer is further increased.
Many other substances have been investigated as possible causes of cancer, but more study is needed to identify those chemicals that increase the risk of cancer.
The risk of cancer varies according to where people live, although the reasons for the geographic differences are often complex and poorly understood. This geographic variation in cancer risk is probably multifactorial: a combination of genetics, diet, and environment.
For example, the risk of colon cancer Colorectal Cancer Family history and some dietary factors (low fiber, high fat) increase a person’s risk of colorectal cancer. Typical symptoms include bleeding during a bowel movement, fatigue, and weakness... read more and breast cancer Breast Cancer Breast cancer occurs when cells in the breast become abnormal and divide uncontrollably. Breast cancer usually starts in the glands that produce milk (lobules) or the tubes (ducts) that carry... read more is low in Japan, yet when people immigrate from Japan to the United States, the risk of colon and breast cancer increases and eventually equals that of the rest of the US population. In contrast, people living in Japan have extremely high rates of stomach cancer Stomach Cancer A Helicobacter pylori infection is a risk factor for stomach cancer. Vague abdominal discomfort, weight loss, and weakness are some typical symptoms. Diagnosis includes endoscopy and biopsy... read more . When people from Japan immigrate to the United States, their risk of stomach cancer declines to that of the United States, possibly due to a change in diet, although the decline may not be evident until the next generation.
Substances consumed in the diet can increase the risk of cancer. For instance, a diet high in unsaturated fat, and obesity by itself, have been linked to an increased risk of colon, breast, and possibly prostate cancer Prostate Cancer Prostate cancer begins in a small area of the prostate gland, an organ found only in males. The risk of prostate cancer increases as men age. Symptoms, such as difficulty urinating, a need to... read more . People who drink large amounts of alcohol are at much higher risk of developing liver cancer Hepatocellular Carcinoma Hepatocellular carcinoma is a cancer that begins in the liver cells and is the most common of the primary liver cancers. Having hepatitis B or hepatitis C or fatty liver disease, or drinking... read more , head and neck cancer Overview of Mouth, Nose, and Throat Cancers Cancers of the mouth, nose, and throat develop in almost 65,000 people in the United States each year. These cancers are more common among men because males who smoke continue to outnumber females... read more , and esophageal cancer Esophageal Cancer Esophageal cancers develop in the cells that line the wall of the esophagus (the tube that connects the throat to the stomach). Tobacco and alcohol use, human papillomavirus infections, and... read more . A diet high in smoked and pickled foods or in barbecued meats increases the risk of developing stomach cancer Stomach Cancer A Helicobacter pylori infection is a risk factor for stomach cancer. Vague abdominal discomfort, weight loss, and weakness are some typical symptoms. Diagnosis includes endoscopy and biopsy... read more . People who have overweight or obesity have a higher risk of cancer of the breast, lining of the uterus (endometrium), colon, kidneys, and esophagus.
Medications and medical treatments
Certain medications and medical treatments may increase the risk of developing cancer. For example, estrogens in oral contraceptives may slightly increase the risk of breast cancer in people who are currently taking them or who took them within the past few years. The hormones estrogen and progestin that may be given to women during menopause (hormone therapy Hormone Therapy for Menopause Menopause is the permanent end of menstrual periods and thus of fertility. For up to several years before and just after menopause, estrogen levels fluctuate widely, periods become irregular... read more ) also slightly increase the risk of breast cancer.
Diethylstilbestrol (DES) increases the risk of breast cancer in women who took the medication and in daughters of these women who were exposed before birth. DES also increases the risk of cervical and vaginal cancer in daughters of women who took the medication. Tamoxifen, a medication used to treat breast cancer, increases the risk of endometrial cancer Cancer of the Uterus The most common type of cancer of the uterus develops in the lining of the uterus (endometrium) and is called endometrial cancer. Endometrial cancer usually affects women after menopause. It... read more (cancer of the inner lining of the uterus).
Long-term use of testosterone, danazol, or other male hormones (androgens) may slightly increase the risk of liver cancer.
Treatment of cancer with certain chemotherapy medications (alkylating agents) and with radiation therapy may increase the risk of people developing a second cancer years later.
Several viruses are known to cause cancer in humans, and several others are suspected of causing cancer. The human papillomavirus Human Papillomavirus (HPV) Infection Human papillomavirus (HPV) causes warts. Some types of HPV cause skin warts, and other types cause genital warts (growths in or around the vagina, penis, or rectum). Infection with some HPV... read more (HPV), causes genital warts and is also is a major cause of cervical cancer Cervical Cancer Cervical cancer develops in the cervix (the lower part of the uterus). Most cervical cancers are caused by human papillomavirus (HPV) infection. Cervical cancer usually results from infection... read more and vulvar cancer Vulvar Cancer Vulvar cancer usually develops in the labia, the tissue that surrounds the opening of the vagina. The cancer may appear to be a lump, an itchy area, or a sore that does not heal. A sample of... read more in women, and penile cancer Penile Cancer Cancers of the penis are usually types of skin cancers. Skin cancer can occur anywhere on the penis, but it most commonly occurs at the glans penis (the cone-shaped end of the penis), especially... read more and anal cancer Anal Cancer Risk factors for anal cancer include certain sexually transmitted infections. Bleeding with bowel movements, pain, and sometimes itching around the anus are typical symptoms. A manual examination... read more in men. HPV also causes some cancers of the mouth and throat. Hepatitis B virus Hepatitis B, Chronic Chronic hepatitis B is inflammation of the liver that is caused by the hepatitis B virus and that has lasted more than 6 months. Most people with chronic hepatitis B have no symptoms, but some... read more or hepatitis C virus Hepatitis C, Chronic Chronic hepatitis C is inflammation of the liver that is caused by the hepatitis C virus and that has lasted more than 6 months. Hepatitis C often causes no symptoms until after it has badly... read more can cause liver cancer. Some human retroviruses, such as HIV Human Immunodeficiency Virus (HIV) Infection Human immunodeficiency virus (HIV) infection is a viral infection that progressively destroys certain white blood cells and can cause acquired immunodeficiency syndrome (AIDS). HIV is transmitted... read more , cause lymphomas and other cancers of the blood system. Some viruses cause types of cancer in certain countries but not in others. For instance, the Epstein-Barr virus causes Burkitt lymphoma Burkitt Lymphoma Burkitt lymphoma is a very fast-growing non-Hodgkin lymphoma, a cancer of white blood cells that originates from B cells (B lymphocytes). Lymphomas (see also Overview of Lymphoma) are cancers... read more (a type of cancer) in Africa and cancers of the nose and pharynx in Asia.
Some bacteria also may cause cancer. Helicobacter pylori Helicobacter pylori Infection Helicobacter pylori (H. pylori) infection is a bacterial infection that causes stomach inflammation ( gastritis), peptic ulcer disease, and certain types of stomach cancer. The... read more , which causes stomach ulcers, can increase the risk of stomach cancer Stomach Cancer A Helicobacter pylori infection is a risk factor for stomach cancer. Vague abdominal discomfort, weight loss, and weakness are some typical symptoms. Diagnosis includes endoscopy and biopsy... read more and lymphomas Overview of Lymphoma Lymphomas are cancers of lymphocytes, which reside in the lymphatic system and in blood-forming organs. Lymphomas are cancers of a specific type of white blood cells known as lymphocytes. These... read more .
Some parasites Overview of Parasitic Infections A parasite is an organism that lives on or inside another organism (the host) and benefits (for example, by getting nutrients) from the host at the host's expense. Although this definition actually... read more can cause cancer. Infection with Schistosoma haematobium Schistosomiasis Schistosomiasis is infection caused by certain flatworms (flukes), called schistosomes. People acquire schistosomiasis by swimming or bathing in fresh water that is contaminated with the flukes... read more can cause chronic inflammation and scarring of the bladder, which may lead to cancer. Another type of parasite, Clonorchis sinensis, has been linked to pancreatic cancer Pancreatic Cancer Smoking, chronic pancreatitis, male sex, being Black, and possibly long-standing diabetes are risk factors for pancreatic cancer. Abdominal pain, weight loss, jaundice, and vomiting are some... read more and bile duct cancer Tumors of the Bile Ducts and Gallbladder Tumors, both noncancerous and cancerous, within the bile ducts or gallbladder are rare. Ultrasonography or MRI/MRCP can usually detect a tumor in the bile ducts or gallbladder. These cancers... read more .
Inflammatory disorders often increase the risk of cancer. Such disorders include ulcerative colitis Ulcerative Colitis Ulcerative colitis is a chronic inflammatory bowel disease in which the large intestine (colon) becomes inflamed and ulcerated (pitted or eroded), leading to flare-ups (bouts or attacks) of... read more and Crohn disease Crohn Disease Crohn disease is an inflammatory bowel disease where chronic inflammation typically involves the lower part of the small intestine, the large intestine, or both and may affect any part of the... read more (which can result in colon cancer and bile duct cancers).