The majority of cancers, some 90–95% of cases, are due to genetic mutations from environmental and lifestyle factors. The remaining 5–10% are due to inherited genetics. Environmental refers to any cause that is not inherited, such as lifestyle, economic, and behavioral factors and not merely pollution. Common environmental factors that contribute to cancer death include tobacco use (25–30%), diet and obesity (30–35%), infections (15–20%), radiation (both ionizing and non-ionizing, up to 10%), lack of physical activity, and pollution. Psychological stress does not appear to be a risk factor for the onset of cancer, though it may worsen outcomes in those who already have cancer.
Exposure to particular substances have been linked to specific types of cancer. These substances are called carcinogens.
Tobacco smoke, for example, causes 90% of lung cancer. It also causes cancer in the larynx, head, neck, stomach, bladder, kidney, esophagus and pancreas. Tobacco smoke contains over fifty known carcinogens, including nitrosamines and polycyclic aromatic hydrocarbons.
Diet, physical inactivity, and obesity are related to up to 30–35% of cancer deaths. In the United States, excess body weight is associated with the development of many types of cancer and is a factor in 14–20% of cancer deaths. A UK study including data on over 5 million people showed higher body mass index to be related to at least 10 types of cancer and responsible for around 12,000 cases each year in that country. Physical inactivity is believed to contribute to cancer risk, not only through its effect on body weight but also through negative effects on the immune system and endocrine system. More than half of the effect from the diet is due to overnutrition (eating too much), rather than from eating too few vegetables or other healthful foods.
Some specific foods are linked to specific cancers. A high-salt diet is linked to gastric cancer. Aflatoxin B1, a frequent food contaminant, causes liver cancer. Betel nut chewing can cause oral cancer. National differences in dietary practices may partly explain differences in cancer incidence. For example, gastric cancer is more common in Japan due to its high-salt diet while colon cancer is more common in the United States.
Worldwide approximately 18% of cancer deaths are related to infectious diseases. This proportion ranges from a high of 25% in Africa to less than 10% in the developed world. Viruses are the usual infectious agents that cause cancer but cancer bacteria and parasites may also play a role.
Breast cancer
Breast cancer is a disease in which cells in the breast grow out of control. There are different kinds of breast cancer. The kind of breast cancer depends on which cells in the breast turn into cancer.
Breast cancer can begin in different parts of the breast. A breast is made up of three main parts: lobules, ducts, and connective tissue. The lobules are the glands that produce milk. The ducts are tubes that carry milk to the nipple. The connective tissue (which consists of fibrous and fatty tissue) surrounds and holds everything together. Most breast cancers begin in the ducts or lobules.
Breast cancer can spread outside the breast through blood vessels and lymph vessels. When breast cancer spreads to other parts of the body, it is said to have metastasized.
The primary risk factors for breast cancer are being female and older age. Other potential risk factors include genetics, lack of childbearing or lack of breastfeeding, higher levels of certain hormones, certain dietary patterns, and obesity. One study indicates that exposure to light pollution is a risk factor for the development of breast cancer.
If all adults maintained the healthiest possible lifestyles, including not drinking alcoholic beverages, maintaining a healthy body composition, never smoking, eating healthful food, and other actions, then almost a quarter of breast cancer cases worldwide could be prevented. The remaining three-quarters of breast cancer cases cannot be prevented through lifestyle changes.
Risk factors for breast cancer
- Breast cancer is strongly related to age, with only 5% of all breast cancers occurring in women under 40 years old.
- There is strong evidence that alcohol consumption increases risk of breast cancer.
- Obesity has been linked to an increased risk of developing breast cancer by many scientific studies.
- High total processed meat and red meat consumption are associated with increased breast cancer risk.
- Lower age of first childbirth, compared to the average age of 24, having more children (about 7% lowered risk per child), and breastfeeding (4.3% per breastfeeding year, with an average relative risk around 0.7) have all been correlated to lowered breast cancer risk in premenopausal women, but not postmenopausal women, in large studies.
- Breathing secondhand smoke increases breast cancer risk by 70% in younger, primarily pre-menopausal women.
- Male individuals have a much lower risk of developing breast cancer than females. In developed countries, about 99% of breast cancer cases are diagnosed in female patients.
Epidemiology of breast cancer
Worldwide, breast cancer is the most common invasive cancer in women. (The most common form of cancer is non-invasive non-melanoma skin cancer; non-invasive cancers are generally easily cured, cause very few deaths, and are routinely excluded from cancer statistics.)
Cervical Cancer
Cancer is a disease in which cells in the body grow out of control. Cancer is always named for the part of the body where it starts, even if it spreads to other body parts later. When cancer starts in the cervix, it is called cervical cancer. The cervix connects the vagina (birth canal) to the upper part of the uterus. The uterus (or womb) is where a baby grows when a woman is pregnant.
Anyone with a cervix is at risk for cervical cancer. It occurs most often in people over age 30. Long-lasting infection with certain types of human papillomavirus (HPV) is the main cause of cervical cancer. HPV is a common virus that is passed from one person to another during sex. At least half of sexually active people will have HPV at some point in their lives, but few women will get cervical cancer.
Screening tests and the HPV vaccine can help prevent cervical cancer. When cervical cancer is found early, it is highly treatable and associated with long survival and good quality of life.
Colorectal (Colon) Cancer
Colorectal cancer is a disease in which cells in the colon or rectum grow out of control. Sometimes it is called colon cancer, for short. The colon is the large intestine or large bowel. The rectum is the passageway that connects the colon to the anus.
Sometimes abnormal growths, called polyps, form in the colon or rectum. Over time, some polyps may turn into cancer. Screening tests can find polyps so they can be removed before turning into cancer. Screening also helps find colorectal cancer at an early stage, when treatment works best.
Lung cancer
Lung cancer is the leading cause of cancer death. Not counting some kinds of skin cancer, lung cancer is the second most diagnosed cancer in both men and women in the United States. After increasing for decades, lung cancer rates are decreasing nationally, as fewer people smoke cigarettes and as lung cancer treatments improve. People with lung cancer are living longer after their diagnosis because more cases are found early, when treatment works best.
Cigarette smoking is the number one cause of lung cancer. Lung cancer also can be caused by using other types of tobacco (such as pipes or cigars), breathing secondhand smoke, being exposed to substances such as asbestos or radon at home or work, and having a family history of lung cancer.
Prostate cancer
Prostate cancer is cancer of the prostate. Prostate cancer is the second most common cancerous tumor worldwide and is the fifth leading cause of cancer-related mortality among men. The prostate is a gland in the male reproductive system that surrounds the urethra just below the bladder. It is located in the hypogastric region of the abdomen.
Globally, it is the second-most common cancer. It is the fifth-leading cause of cancer-related death in men. In 2018, it was diagnosed in 1.2 million and caused 359,000 deaths.
Skin Cancer
Skin cancer is the most common cancer in the United States. Some people are at higher risk of skin cancer than others, but anyone can get it. The most preventable cause of skin cancer is overexposure to ultraviolet (UV) light, either from the sun or from artificial sources like tanning beds.