Cancer (medicine)

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Tumors Spread

Tumors are malignant only if they can invade other parts of the body. Malignant tumors extend into neighboring tissue or travel to distant sites, forming secondary growths known as metastases. To metastasize, tumor cells break through a nearby blood vessel to enter the circulatory system or through a lymphatic vessel wall to enter the lymphatic system. Most metastases occur in organs that are the next site downstream in the circulatory system or the lymphatic system and contain a network of capillaries, or small blood vessels. For example, cancer of the large intestine often travels through the bloodstream to the liver, the organ immediately downstream from the intestines. In the lymphatic system, tumor cells can spread to surrounding lymph nodes, or lymph glands. Normally, lymph nodes filter out and destroy infectious materials circulating in the lymphatic system.

The unique receptors on the surface of a cell may also play a role in where tumors metastasize. Specialized molecules on a cell’s surface identify where in the body the cell belongs. Similar cells adhere to one another when their surface receptors are compatible. Most often cells from different tissues and organs have incompatible surface receptors. However, some tissue types share similar surface receptors, enabling cancerous cells to move between them and proliferate. Prostate cells and bone cells, for example, have similar surface receptors. This gives prostate cancer cells a natural affinity for bone tissue, where they can settle to form a new tumor.

Many cancers shed cells into the bloodstream early in their growth. Most of these cells die in the bloodstream, but some lodge against the surface of the blood vessel walls, eventually breaking through them and into adjacent tissue. In some cases, these cells survive and grow into a tumor. Others may divide only a few times, forming a small nest of cells that remain dormant as a micrometastasis. They may remain dormant for many years, only to grow again for reasons not yet known.

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Causes of Cancer

Scientists do not fully understand the causes of cancer, but studies show that some people are more likely to develop the disease than others. Scientists called epidemiologists study particular populations to identify why cancer rates vary (see Epidemiology). One method they use is to compare cancer patients with healthy people in terms of behavior such as diet, exercise, and smoking and traits such as gender, age, and race. Population studies provide useful information about risk factors that increase the likelihood of developing cancer.

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Carcinogens

One of the greatest risk factors for cancer is prolonged or repeated exposure to carcinogens—chemical, biological, or physical agents that cause the cellular damage that leads to cancer. The details of how carcinogens cause cancer remain unclear. One theory is that exposure to carcinogens, when combined with the effects of aging, causes an increase in chemicals in the body called free radicals. An excessive number of free radicals causes damage by taking negatively charged particles called electrons from key cellular components of the body, such as DNA. This may make genes more vulnerable to the mutating effects of carcinogens.

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Tobacco Smoke

Smoking causes up to 30 percent of cancer deaths in the United States and Canada, making tobacco smoke the most lethal carcinogen in North America. Smoking is associated with cancer in the lungs, esophagus, respiratory tract, bladder, pancreas, and probably cancers of the stomach, liver, and kidneys. The risk of cancer increases depending on the number of cigarettes smoked per day, the cigarette’s tar content, and how many years a person smokes. Starting to smoke while young significantly increases the risk of developing cancer.

Each year in the United States, several thousand nonsmoking adults die of lung cancer caused by exposure to the smoke of others’ cigarettes, called secondhand smoke or environmental tobacco smoke. Nonsmoking spouses of smokers are 30 percent more likely to develop lung cancer than those married to nonsmokers. Breathing secondhand smoke also increases the risk of cancer in the children of smokers and in nonsmokers who work in smoky places. For this reason smoking has been banned in many places such as restaurants and bars.

Cigars, pipes, and smokeless tobacco have also been implicated in increased risk for cancer. Cigars contain most of the same cancer-producing chemicals as cigarettes, and people who smoke cigars have a 30 percent higher risk of developing cancer than nonsmokers. Oral cancers occur more frequently in people who use smokeless tobacco, or snuff. Snuff users, for example, are 50 times more likely to develop cancers of the cheek or gum than nonusers.

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Diet

Diet can also contribute to cancer. Saturated fats from red meats, such as hamburger or steak, and high-fat dairy products are linked with several cancers. High salt intake increases the risk of stomach cancer. Adult obesity increases the risk for cancer of the uterus in women and also appears to increase the risk for cancers in the breast, colon, kidney, and gallbladder. Alcohol consumption increases the risk of cancer of the esophagus and stomach, especially when combined with smoking.