Cancer Health Article

CAUSES OF CANCER

Environmental and lifestyle factors such as tobacco use, diet, alcohol consumption, and exposure to sunlight play a primary role in the development of the majority of cancers. In addition, exposure to occupational factors and to specific pathogens (e.g., viruses, bacteria), hormones, and radiation also contributes to cancer at particular sites. However, the question still remains as to why one person exposed to a given environmental or lifestyle risk factor develops cancer and another person does not. The importance of hereditary factors (gene-environment interactions) cannot be overemphasized in this regard. True "hereditary cancers," those attributable to specific genes that are passed from one generation to another, account for only a small proportion of cancer cases, however.

Exposure to carcinogens in tobacco smoke accounts for almost one-third of cancer cases, especially cancers of the lung, respiratory tract, esophagus, bladder, pancreas, and, most likely, cancers of the stomach, liver, and kidneys. Carcinogens found in the environment and the workplace (e.g., asbestos, benzene, vinyl chloride compounds, dyes, arsenic, petroleum products) and cancers associated with exposure to these chemicals (e.g., lung and bladder) are higher in urban areas than in rural areas. Diet also influences the risk of cancer, although researchers are unsure of the mechanisms involved. In general, evidence supports an increased risk of various cancers (e.g., colon, rectum) with a high intake of red meats, and a decreased risk of various cancers (e.g., lung, colon, stomach) with a high intake of vegetables and fruits. Other food constituents, such as vitamins and minerals, are also being investigated for their ability to prevent cancer.

Other possible causes of cancer include pathogens, such as hepatitis B and C viruses in liver cancer, and the Helicobacter pylori bacterium in stomach cancer. Hormonal factors contributing to cancer have focused on estrogen, progesterone, and testosterone, and their role in reproductive organ cancers. These steroid hormones are being investigated because they influence the growth of cells, particularly those of the prostate, ovary and cervix, and breast. Radiation exposure, especially UV radiation from the sun, is a significant contributor to cancer of the skin, and using sunscreens has been shown to reduce skin cancer risk.

Interactions between genes and environmental exposures are of great importance in determining one's risk of developing cancer. For instance, genes and nutrients can interact to increase or decrease the risk of cancer depending on genetic variations known as polymorphisms—different forms of the same gene that may either increase or decrease the risk of cancer. For example, different polymorphisms in the gene that determines how vitamin D is metabolized can influence the risk of prostate cancer; one polymorphism is associated with increased risk of prostate cancer and another is associated with decreased risk. Polymorphisms in the genes that are responsible for repairing radiation damage to skin cells also play a role in increasing or decreasing cancer risk.

CANCER PREVENTION AND TREATMENT

Many cancer risk factors are avoidable. Preventing cancer by attention to diet and by quitting or never starting smoking are the most significant strategies to reduce cancer risk. Prevention of cancer is being investigated in clinical trials on dietary patterns (high intake of vegetables and fruits; low intake of saturated fats) and dietary constituents such as vitamins, minerals, and soy. Future progress may depend partly on strategies such as chemoprevention—the use of natural or synthetic substances to prevent cancer cells from forming, progressing, or recurring. For example, the antiestrogen hormone tamoxifen has been shown to reduce the risk of developing breast cancer by 50 percent among women at high risk for this disease. It also has been shown to reduce the risk of developing a new primary breast cancer in the opposite breast among women with a history of breast cancer. Chemopreventive agents also are being investigated for prevention of colon, rectum, prostate, and lung cancers.