The Problem of Cancer (Genetics & Inherited Conditions)
Cancer is characterized by abnormal cell growth that leads to the invasion and destruction of healthy tissue by cells that meet certain criteria. Normal cells in the human body are continuously growing but are under normal cell control mechanisms. Cancer cells begin as normal cells that, as a result of genetic mutations, start to grow uncontrollably, escaping from the normal rules regulating cell growth and behavior. Cancer cells are characterized by six traits that differentiate them from normal healthy cells: cells that grow to abnormally large size; disregard for normal growth signals; lack of sensitivity to growth inhibition factors (antigrowth signals); avoidance of natural cell death (apoptosis); uninhibited replication; ability to form new vascular supply (angiogenesis); and ability for metastasis and invasion of neighboring tissues. Contact inhibition, in which cells contacting other cells prevent unrestrained growth, is lost in cancer cells. Normal cells also remain in one location, or at least in the same tissue, but malignant tumors, in their later stages, metastasize, allowing their cells to wander freely in the body, leading to the development of tumors in other organs. A final common feature is that cancer cells lose their normal cell shape.
The area where cancer begins to form a tumor is called the primary site. Most types of cancer begin in one place (the breast, lung, or bowel, for example) from which...
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How Cancer Cells Grow and Invade (Genetics & Inherited Conditions)
Cancer cells are able to break down the barriers that normally keep cells from invading other groups of cells. With the aid of a microscope, cancer cells can be observed breaking through the boundary between cells, called the basement membrane. Cancer cells can make substances that break down the intercellular matrix, the “glue” that holds cells together. The intercellular matrix is a complex mixture of substances, including collagen, a strong, fibrous protein that gives strength to tissues. Cancer cells produce collagenase, an enzyme that breaks down collagen. Cancer cells also produce hyaluronidase, which further breaks down the intercellular matrix. This causes cancer cells to lose their normal shape and allows them to push through normal boundaries and establish themselves in surrounding tissues. Cancer cells have jagged edges, are irregular in shape, have large nuclei, and have hard-to-detect borders, making them relatively easy to identify microscopically. Normal cells, on the other hand, have a regular, smooth edge and shape.
There are many steps involved in the process of metastasizing, not all of which are understood by researchers. First is the entry into a blood vessel or lymph channel. Lymph channels, or lymphatics, comprise a network of vessels that carry lymph from the tissues to the bloodstream. Lymph is a colorless liquid that drains from spaces between cells. It consists mainly of water,...
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The Genetics of Cancer (Genetics & Inherited Conditions)
Cancer has been known since antiquity, but it was not until the twentieth century that the underlying causes of cancer began to be explored. In 1910, Peyton Rous discovered a type of cancer in chickens called a sarcoma (a cancer of connective tissue) that could be passed on to other chickens. He demonstrated this by removing tumors from affected chickens, grinding the tumors up, filtering the grindate, and then injecting the filtrate into healthy chickens. Injected chickens invariably developed sarcoma tumors, suggesting that something smaller than the tumor cells was being passed on and was stimulating cancer development in otherwise normal cells. It is now known that the filtrate contained a cancer-causing virus, now called the Rous sarcoma virus. Similar types of viruses were discovered to be responsible for cancers in a variety of animals, but none was discovered in humans initially.
As the genetic material of some of the tumor viruses was later analyzed, all of them were discovered to contain genes called oncogenes, because they promoted oncogenesis (tumor development). Even more surprising was the discovery that humans have genes in their genome that are homologous (having a high degree of similarity) to viral oncogenes. The human genes did not seem to cause cancer under normal circumstances and were called proto-oncogenes. In cancer cells, some of these proto-oncogenes were discovered to have mutations or, in...
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How Cancer Develops (Genetics & Inherited Conditions)
The development of cancer is typically more complicated than implied above. Information gathered from the Human Genome Project helped improve our understanding of the role of genetics and genetic mutations not only in the development of cancer, but also in its treatment. The development of cancer generally requires mutations in more than a single proto-oncogene or tumor-suppressor gene. Any factors that increase mutation rates or decrease the ability of a cell to repair mutations will increase the likelihood that cancer will develop. Inheritance of already mutated genes can also greatly increase a person’s chance of developing cancer, which accounts for the above-normal occurrence of certain types of cancer in some families.
One of the best-studied cases of oncogenesis involves colorectal cancer, which takes years to develop from a small cluster of abnormal cells into life-threatening cancer. It involves the loss or mutation of three tumor-suppressor genes and one proto-oncogene. Often colorectal cancer runs in families, because the loss of the first gene, the APC tumor-suppressor gene, is often inherited, resulting in an increased chance of developing colorectal cancer. Loss of this gene causes increased cell growth and some other genetic changes. In the next step, the ras oncogene is mutated, causing even more cell growth. Two more tumor-suppressor genes are lost, DCC and p53, at...
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Cancer Treatment (Genetics & Inherited Conditions)
Cancers vary in their severity and rate of growth, which means that proper treatment depends on correctly diagnosing the type of cancer. For example, some forms of prostate cancer grow extremely slowly, and metastasis is rare until very late stages in the disease, sometimes many years after initial diagnosis. Treatment may comprise simply monitoring the tumor, avoiding carcinogenic exposure as much as possible, and possibly changing one’s lifestyle. On the other hand, some types of skin cancer progress so rapidly that aggressive treatment may be required, unless it is caught very early. Although survival rates for many types of cancer have risen, treatment for most cancers is still only partially successful, and the later a tumor is detected, the greater chance that it will be untreatable.
New therapies are constantly being developed, but most cancers are still treated using surgery (removal of tumors), chemotherapy, and radiation therapy, either singly or, more often, in combination. More important than the specific treatment used is detecting tumors in their earliest stages, before they have extensively invaded surrounding tissues or metastasized. Survival rates are high for most cancers when treated very early.
The very nature of cancer makes treatment difficult. Because the cells involved are difficult for the immune system to recognize as dangerous, the body is typically inefficient at destroying them. Many of...
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Targeted Therapy (Genetics & Inherited Conditions)
The Human Genome Project opened a new avenue of cancer therapy called targeted therapy. The availability of gene and protein databases led to the identification of hundreds of human proteins and kinases that may harbor mutations and play a role in cancer development. Targeted therapies, which may be classified as either monoclonal antibodies or small molecule kinase inhibitors, act directly on these kinases. These products promise greater efficacy than blanket chemotherapy while keeping associated adverse effects to a minimum. Because kinase mutations tend to be found only in cancerous cells, normal healthy cells are largely unaffected by the targeted therapy.
Targeted therapies that are available are directed against a variety of proteins and are effective for a number of types of cancer. Agents targeting the epidermal growth factor receptor (EGFR-type I) pathway, for example, disrupt the signals that mediate cell growth. Several EGFR agents have been approved by the Food and Drug Administration (FDA) for cancer treatment, such as cetuximab (Erbitux) and panitumumab (Vectibix) for colorectal cancer; cetuximab for head and neck cancer; cetuximab and erlotinib (Tarceva) for pancreatic cancer; trastuzumab (Herceptin) and lapatinib (Tykerb) for breast cancer; erlotinib for hepatocellular carcinoma; and cetuximab, erlotinib, and gefitinib for lung cancer.
Other targeted therapies block the activity of ABL, which is a...
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Innovations and Future Treatments (Genetics & Inherited Conditions)
Although the immune system cannot normally identify cancer cells accurately, there has been some success in immunological approaches. Research is progressing on development of vaccines against cancer, but so far this approach is still in its early experimental stages.
Photodynamic therapy also shows promise. It is based on the observation that certain chemicals, when ingested by single-celled organisms, release damaging oxygen radicals when exposed to light, thus killing the organisms. It has been observed that cancer cells retain these chemicals longer than normal cells. Treatment involves administering the chemical by injection, then waiting for a specified period for it to be retained by cancer cells and flushed out of normal cells. Then the tissue in which the cancer cells are located is exposed to laser light. This method works on any tissues that can be exposed to laser light, which includes any part of the body accessible to endoscopy.
Information from the Human Genome Project is being used not only to develop new, more specific therapies but also to control adverse events and to identify which patients will benefit most from a particular therapy. Researchers are exploring whether genes that predict cancer risk may also predict outcomes and susceptibility to symptoms such as fatigue and depression. Furthermore, biomarkers are being examined as predictors of cancer risk and treatment...
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Further Reading (Genetics & Inherited Conditions)
Bowcock, Anne M., ed. Breast Cancer: Molecular Genetics, Pathogenesis, and Therapeutics. Totowa, N.J.: Humana Press, 1999. Detailed information geared toward researchers and health professionals. The chapter “Hereditary Breast Cancer Genes” discusses BRCA1 and BRCA2 mutations among Ashkenazi women. Also addresses surgery, chemotherapy, drug resistance, and the MDR gene.
Bradlow, H. Leon, Jack Fishman, and Michael P. Osborne, eds. Cancer Prevention: Novel Nutrient and Pharmaceutical Developments. New York: New York Academy of Sciences, 1999. Examines several classes of nutrients and pharmaceutical agents believed to be important for tumor inhibition. Reviews novel preclinical models that facilitate analysis of chemopreventive agent efficacy and mechanisms of gene-nutrient interaction and provides information on clinical trials studying chemopreventive regimens.
Coleman, William B., and Gregory J. Tsongalis, eds. The Molecular Basis of Human Cancer. Totowa, N.J.: Humana Press, 2002. Surveys the known molecular mechanisms governing neoplastic transformation in the breast, prostate, lung, liver, colon, skin, leukemias, and lymphomas. Illuminates both recent developments and established concepts in epidemiology, molecular techniques, oncogenesis, and mutation mechanisms.
Cowell, J. K., ed. Molecular Genetics of Cancer. 2d ed. San Diego: Academic...
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Web Sites of Interest (Genetics & Inherited Conditions)
American Cancer Society. http://www.cancer.org. Comprehensive and searchable site covering all aspects of cancer.
National Cancer Institute. http://www.cancer.gov. Site links to comprehensive information on genetics and cancer, including a cancer-basics tutorial.
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Causes and Symptoms (Magill’s Medical Guide, Sixth Edition)
Cancer is a disease of abnormal cellular growth. Growth is a feature of all living things, but it must be precisely regulated for development to occur properly. All growing cells pass through a strictly regulated series of events called the cell cycle, where most cellular structures are duplicated. At the end of the cycle, one cell is separated into two “daughter cells,” each receiving one copy of the duplicated structures. The most important structures to be duplicated are the genes, which govern all cellular activities.
Human life begins as a fertilized egg which divides again and again; the adult human body is composed of a trillion cells, each with a specific job to perform. At adulthood, most cells stop duplicating. Some cells, however, must continue dividing to replace worn-out cells in places like the blood, skin, and intestine. Such growth is accurately controlled so excess cells are not produced. Sometimes, however, a mutation arises in one or more genes, resulting in needless cell duplication and ultimately loss of growth control: a malignant transformation. This is the start of cancer.
At first, these cells resemble their neighbors. For example, newly altered blood cells look like normal blood cells, and in most respects are. However, cancer cells differ in a number of ways from “normal” cells. First, cancer cells grow uncontrollably. They may or may not grow faster than normal cells, but cancer...
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Treatment and Therapy (Magill’s Medical Guide, Sixth Edition)
The most common cancer treatments fall into three categories: surgery, chemotherapy, and radiotherapy. The oldest treatment, going back several hundred years, is the surgical removal of tumors. If performed at an early stage, before metastasis, this method can be highly successful. Even so, surgery is much easier and less dangerous for some cancers (like that of the skin) than others (like that of the brain, which can be difficult to reach and remove). Surgery is not an option for blood and lymph cancers that are widely distributed.
The second most common type of cancer treatment for tumors is radiotherapy. The radiation of choice is X rays, which can penetrate the body to reach a tumor in very high dosages using modern equipment. X rays can be focused on a specific small area or be administered over the whole body in the case of metastasized cancer. Therapeutic radiation damages genes to such an extent that they become physically fragmented and nonfunctional, ending the life of the target cell.
Radiotherapy has major drawbacks. The most serious problem is that normal cells in the path of the radiation will also be killed. Bone marrow, the source of blood cells, is destroyed with whole-body cancer treatments. This problem can be overcome after radiotherapy by transplanting new bone marrow into the patient, so that a treated patient can begin to manufacture new blood cells. Ironically, radiation designed to kill...
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Perspective and Prospects (Magill’s Medical Guide, Sixth Edition)
In the mid-1990’s, evidence of a potential new basis for cancer emerged. The hypothesis suggested that overproduction of the enzyme telomerase, which synthesizes the telomeres at the ends of chromosomes, may cause uncontrolled growth in cells.
In normal human cells, the telomeres, long DNA repeats of TTAGGG, are slowly shortened and erode away as the cells age. The lengthening enzyme telomerase is not active in normal cells, so the ends of the chromosomes shorten more and more over a lifetime; in fact, these chromosomal changes have been postulated as one possible cause of cellular aging. When the telomeres become short enough, cell senescence is induced as the cells stop dividing. Tumor cells have active telomerase and do not lose their chromosomal ends. One source of immortality in cancer cells may be their long telomeres.
It is not clear whether lengthening of telomeres is an oncogenic event causing cells to become cancerous or whether these events are simply crucial in tumor formation. The importance of the telomerase activity in inducing cancer is quite controversial. Mice who have had telomerase genes turned off permanently show no ill effects and age normally. Although researchers have shown that oncogenes become operative if the telomerase is active in cell culture, it is unclear what role telomerase plays in actual biological systems. Researchers have suggested that perhaps the loss of that enzyme...
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For Further Information: (Magill’s Medical Guide, Sixth Edition)
American Cancer Society (ACS). http://www.cancer .org. This Web site is divided into sections for patients, family, and friends; survivors; health information seekers; ACS supporters; and professionals. Information on all cancers is wide ranging.
Bognar, David, et al. Cancer: Increasing Your Odds for Survival—A Resource Guide for Integrating Mainstream, Alternative, and Complementary Therapies. Alameda, Calif.: Hunter House, 1998. This is a resource guide for cancer patients and their families, covering the emotional shock and disorientation at diagnosis and how to deal with it.
Cairns, John. Matters of Life and Death: Perspectives on Public Health, Molecular Biology, Cancer, and the Prospects for the Human Race. Princeton, N.J.: Princeton University Press, 1998. Cairns is a prominent molecular biologist who has turned his attention to cancer. He writes eloquently but nontechnically about the biology and medical implications of cancer.
Cancer Care. http://www.cancercare.org. This site provides information, including links and resources, to help people who have cancer, and their families and friends, better cope with the disease.
Dollinger, Malin, et al. Everyone’s Guide to Cancer Therapy. 5th ed. Kansas City, Mo.: Andrews McMeel, 2008. A comprehensive review of treatments available, with topics that include cryotherapy, radio frequency treatment, genetic risk...
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Cancer (Encyclopedia of Genetic Disorders)
Cancer is not just one disease, but a large group of diseases characterized by uncontrolled and abnormal growth of the cells in the human body and the ability of these cells to spread to distant sites (metastasis). If the spread is not controlled, cancer can result in death.
Cancer, by definition, is a disease of the genes. Genes are formed from deoxyribonucleic acid (DNA) and located on chromosomes. They carry the hereditary instructions for the cell to make the proteins required for many body functions. Proteins are special chemical compounds that mostly contain carbon, hydrogen, oxygen, and nitrogen. They are required by our bodies to carry out all the processes that allow us to breathe, think, move, etc.
Throughout people's lives, the cells in their bodies are growing, dividing, and replacing themselves. Many genes produce proteins that are involved in controlling the processes of cell growth and division. A change (mutation) occurring in the DNA molecules can disrupt the genes and produce faulty proteins and cells. Abnormal cells can start dividing uncontrollably, eventually forming a new growth known as a "tumor" or "neoplasm" (medical term for cancer meaning "new growth"). In a healthy...
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Cancer (Encyclopedia of Medicine)
Cancer is not just one disease, but a large group of almost one hundred diseases. Its two main characteristics are uncontrolled growth of the cells in the human body and the ability of these cells to migrate from the original site and spread to distant sites. If the spread is not controlled, cancer can result in death.
One out of every four deaths in the United States is from cancer. It is second only to heart disease as a cause of death in the states. About 1.2 million Americans are diagnosed with cancer annually; more than 500,000 die of cancer annually.
Cancer can attack anyone. Since the occurrence of cancer increases as individuals age, most of the cases are seen in adults, middle-aged or older. Sixty percent of all cancers are diagnosed in people who are older than 65 years of age. The most common cancers are skin cancer, lung cancer, colon cancer, breast cancer (in women), and prostate cancer (in men). In addition, cancer of the kidneys, ovaries, uterus, pancreas, bladder, rectum, and blood and lymph node cancer (leukemias and lymphomas) are also included among the 12 major cancers that affect most Americans.
Cancer, by definition, is a disease of the genes. A gene is a small part of DNA, which is the master molecule of the cell....
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Cancer (Encyclopedia of Science)
Cancer is a disease of uncontrolled cell growth caused by exposure to carcinogens (cancer-causing substances), genetic defects, or viruses. Cancer cells can multiply and form a large mass of tissue called a tumor. Some tumors are limited to one location and can be surgically removed. These tumors may cause little harm and are therefore termed benign. Cancer cells of other tumors may spread, or metastasize (muh-TASS-tuh-size), to surrounding tissue or other organs of the body. Such aggressive tumors are termed malignant. Cancer is a word used usually to describe malignant, not benign, tumors. The study of cancer is called oncology.
How cancer cells are formed
The transformation of a normal cell into a cancer cell can occur when the genetic material (deoxyribonucleic acid or DNA) of a cell is changed, or mutated. A tumor is the result of multiple gene mutations within a single cell. Years or decades before a tumor forms, a cell can become weakened by...
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Cancer (Encyclopedia of Children's Health)
Cancer is a group of diseases characterized by uncontrolled growth of tissue cells in the body and the invasion by these cells into nearby tissue and migration to distant sites.
Cancer results from alterations (mutations) in genes that make up DNA, the master molecule of the cell. Genes make proteins, which are the ultimate workhorses of the cells, responsible for the many processes that permit humans to breathe, think, and move, among other functions. Some of these proteins control the orderly growth, division, and reproduction of normal tissue cells. Gene mutations can produce faulty proteins, which in turn produce abnormal cells that no longer divide and reproduce in an orderly manner. These abnormal cells divide uncontrollably and eventually form a new growth known as a tumor or neoplasm. A healthy immune system can usually recognize neoplastic cells and destroy them before they divide. However, mutant cells may escape immune detection and become tumors or cancers.
Studies of the origins of cancer have shown that a combination of genetic influences and environmental causes over time triggers gene mutations, which may explain why most cancers are seen in adults of middle age or older (60%) and cancer is...
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Cancer (Encyclopedia of Alternative Medicine)
Cancer is not just one disease, but a large group of almost 100 diseases. It is a genetic disease, with two main characteristics of uncontrolled growth of the cells in the human body and the ability of those cells to migrate from the original site and spread to distant sites.
One out of every four deaths in the United States is from cancer. It is second only to heart disease as a cause of death in the United States. About 1.2 million Americans are expected to be diagnosed with cancer each year, of which more than 500,000 are diagnosed as terminally ill. Cancer can attack anyone. Since the occurrence of cancer increases as people age, most cases are seen in adults, middle-aged or older. The most common cancers are skin cancer, lung cancer, colon cancer, breast cancer (in women), and prostate cancer (in men). In addition, cancer of the kidneys, ovaries, uterus, pancreas, bladder, rectum, blood (leukemia), and lymph nodes (lymphoma) are also included among the 12 major cancers that affect most Americans.
Cancer, by definition, is a disease of the genes. A gene is a small part of DNA, which is the master molecule of the cell. Genes make proteins, which are the ultimate workhorses of the cells. It is these proteins, along with other substances, that allow our...
(The entire section is 6108 words.)
Cancer (Encyclopedia of Nursing & Allied Health)
Cancer is characterized by uncontrolled growth of cells in the body and the ability of these malignant cells to spread (metastasize) to distant sites within the body. If the spread is not controlled, cancer can result in death. Cancer is not just one disease but a group of almost one hundred different types of malignant diseases.
Cancer, by definition, is a disease of the genes. A gene is a small part of DNA, which is the instruction manual of the cell. Genes hold the instructions to make proteins, which carry out many of the body's functions. It is these proteins that allow the human body to carry out all the processes that permit people to breathe, think, move, etc.
Throughout people's lives the cells in their bodies grow, divide, and replace themselves. Many genes produce proteins that are involved in controlling the processes of cell growth and division. An alteration (mutation) to the DNA molecule can disrupt some of the genes on the DNA molecule and produce faulty proteins. This causes a cell to loose restraint on growth. The abnormal cell begins to divide uncontrollably and eventually forms a new growth known as a tumor or neoplasm (medical terms for cancer meaning new growth). In a healthy body the immune system can recognize neoplastic (abnormal) cells and destroy them before they get a chance to divide. Even so, some mutant cells may escape immune detection and survive to develop into cancerous growths.
Tumors are divided into two general categories: benign or malignant. A benign tumor is slow growing and does not spread or invade surrounding tissue. Once it is removed it doesn't usually recur. A malignant tumor, on the other hand, invades surrounding tissue and can spread to other parts of the body. Malignant tumors can be removed; however, if the cancer cells have spread to the surrounding tissues, the tumor is likely to recur.
A majority of cancers are caused by changes in the cell's DNA that are due to the environment. Environ- mental factors that are responsible for causing the initial mutation in the DNA are called carcinogens. Internal factors can cause cancer as well. Certain hormones have been shown to have an effect the growth or control of a particular cell line. Hormones are substances made by one organ and passed through the bloodstream to perform a function in another organ.
While there is scientific evidence that both environ- mental and genetic factors play a role in most cancers, approximately 50% of all cancers are classified as hereditary (genetic). This means a faulty gene that leads to a cancer is passed from parent to child. This poses a greater risk for that particular type of cancer in certain descendants of the family. However, having a cancer- causing gene does not necessarily mean that person will automatically get cancer. Rather, it means that person is predisposed to a type of cancer, or more likely to get this cancer when compared to the general population. Cancers known to have a hereditary tendency in some cases include breast cancer, colon cancer, ovarian cancer, skin cancer, and prostate cancer.
Aside from genes, certain inherited physiological traits can contribute to cancers. For example, inheriting fair skin makes a person more likely to develop skin cancer, but only if they also have prolonged exposure to intensive sunlight.
There are many different types of cancers. Some of the most common types include:
- Carcinomas. These cancers arise in the epithelium (layers of cells in the skin covering the body's surface and lining the internal organs and various glands). About 80% of human cancers fall into this category. Carcinomas can be subdivided into two subtypes: adenocarcinomas, which are cancers that develop in an organ or a gland; and squamous cell carcinomas, cancers that originate in the skin.
- Melanomas. This form also originates in the skin, usually in the pigment cells (melanocytes), and can quick- ly metastasize to internal organs.
- Sarcomas. Cancers of the supporting tissues of the body, such as bone, muscle, cartilage, and fat.
- Leukemias. Cancers of the blood or blood-forming organs.
- Lymphomas. Cancer of the lymphatic system, the network of vessels and nodes that acts as a filtration system, distributing nutrients to blood and tissue and preventing bacteria and other foreign substances from entering the bloodstream.
- Gliomas. Cancers of nerve tissue.
The most common cancers are skin cancer, lung cancer, colon and rectal (colorectal) cancer, breast cancer (in women), and prostate cancer (in men). In addition, cancer of the kidneys, ovaries, uterus, pancreas, bladder, and blood and lymph-node cancer (leukemias and lymphomas) are also included among the 12 major cancers that affect most Americans.
Almost every tissue can give rise to cells that cause cancer and each of these cancers is very different in its symptoms and prognosis. However, there are basic and similar genetic processes that lead to tumor growth in the human body. Genes are responsible for producing proteins that regulate cell growth and division. When these genes do not function properly, the proteins are abnormal and cells can grow uncontrollably. This results in the formation of a tumor. As more genetic mutations (changes) occur in this tumor, it becomes more life-threatening and has a greater chance of spreading to other parts of the body.
Three classes of genes appear to play a role in the development of cancer:
- Proto-oncogenes encourage and promote the normal growth and division of cells. When they are defective, they become oncogenes. Oncogenes are overactive proto-oncogenes that cause excessive cell multiplication that can lead to tumors.
- Tumor suppressor genes act as brakes on cell growth. They prevent cells from multiplying uncontrollably. If these genes are defective there is no control over cell growth and tumors can result.
- DNA repair genes ensure that each strand of DNA is correctly copied during cell division. When these genes do not function properly, the replicated DNA is likely to have errors. This causes defects in other genes and can lead to tumor formation in some cases.
Approximately 50% of cancers have a hereditary component. In these cancers a child does not inherit caner
from the parents. Rather, a predisposition to cancer is inherited. For example, a faulty tumor suppressor gene may be inherited. This gene is not able to control cell growth but the corresponding gene inherited from the other parent is still functional. Cell growth is under control. However, as a child grows up, radiation, pollution, or any other environmental factor could change the functional gene, making it defective as well. Now, neither of these tumor suppressor genes are functioning, and it is likely that a tumor will develop. Defects in proto-oncogenes and DNA repair genes can be inherited, as well, leaving a person more vulnerable to cancer than the general population.
Some cancers seem to run in families. In these cancers there is no specific gene responsible for the clustering of cancer in a family. However, a particular type of cancer may be seen more often than in the general population. It is suggested that this is due to a combination of genetic and environmental factors.
Cancer kills one out of every four Americans. As of 2001 it was the second leading cause of death in the United States, surpassed only by heart disease. More than 1.2 million new cases of cancer are diagnosed every year in the United States. The National Cancer Institute estimates that approximately 8.4 million Americans alive in 2001 had a history of cancer. Some of these people were cured while others were still affected with the dis- ease and possibly undergoing treatment.
Anyone is at risk for developing cancer. Since the occurrence of cancer increases as a person ages, most cases are seen in adults middle-aged or older. Nearly 80% of cancers are diagnosed in people 55 years of age and older.
Lifetime risk is the term used to refer to the probability that an individual will develop cancer over the course of his or her lifetime. In the United States men have a one-in-two lifetime risk of developing cancer. For women, the risk is one in three. Overall, African Americans are more likely to develop cancer than Caucasians and are 33% more likely to die of cancer than Caucasians.
The major risk factors for cancer are tobacco, alcohol, diet, sexual and reproductive behavior, infectious agents, family history, occupation, environment, and pollution.
Eighty-to-ninety percent of lung cancer cases occur in smokers. Smoking has also been shown to be a contributory factor in cancers of mouth, pharynx, larynx, esophagus, pancreas, uterine cervix, kidney, and bladder. Smoking accounts for at least 30% of all cancer deaths. Scientists have shown that inhaling secondhand smoke (passive smoking) can increase one's risk of developing cancer.
Excessive consumption of alcohol is a risk factor in certain cancers such as liver and breast cancer. Alcohol, in combination with tobacco, significantly increases the chances that an individual will develop mouth, pharynx, larynx, and esophageal cancers. The combined effect of tobacco and alcohol is greater than the sum of their individual effects. This is called synergy.
Diet and physical activity
One-third of all cancer deaths are due to a poor adult diet. High-fat diets have been associated with cancers of the colon and rectum, prostate, endometrium, and possibly breast. Consumption of meat, especially red meat, has been associated with increased cancer at various sites such as the colon and prostate. Additionally, a high-calorie diet and low level of physical activity can lead to obesity, which increases the risk for cancer at various sites including the breast, colon and rectum, prostate, kidney, and endometrium.
Sexual and reproductive behavior
The human papilloma virus, a sexually transmitted disease, has been shown to cause cancer of the cervix. Having many sexual partners and becoming sexually active early has been shown to increase one's chances of contracting this disease and, therefore, developing cervical cancer. In addition, it has also been shown that women who do not have children or those who have children late in life have an increased risk for both ovarian and breast cancer.
Hormone replacement therapy
As women go through menopause, a physician may recommend hormone replacement therapy. This involves taking female hormones (called estrogen and progesterone) to control certain symptoms such as hot flashes and vaginal dryness that occur during this time of a woman's life. Taking estrogen alone can increase the risk for uterine cancer. However, progesterone is often prescribed at the same time to counteract the cancerous effects of estrogen. There is a questionable relationship between hormone replacement therapy and breast cancer as well. As of 2001, this relationship was not fullyunderstood.
Certain cancers tend to occur more commonly among members of a family. Much of the time this seems to happens by chance, or is due to a common family habit such as cigarette smoking or extended sun exposure. However, certain cancers can occur in excess in some families due to a genetic predisposition that is passed from generation to generation. For example, if the BRCA1 gene is defective in a family, members of that family may have an increased risk to develop breast, colon, ovarian, or prostate cancer. Other defective genes can make persons susceptible to other types of cancer. Therefore, inheriting particular genes can increase a person's chance of developing cancer.
There is ample evidence that occupational hazards account for 4% of all cancer deaths. For example, asbestos workers have an increased incidence of lung cancer. Similarly, bladder cancer is associated with dye, rubber, and gas workers; skin and lung cancer with people who are smelters, gold miners and arsenic workers; leukemia is seen more frequently in people who work with glue and varnish; liver cancer is more prevalent in PVC manufacturers; and lung, bone, and bone marrow cancer is associated with radiologists and uranium miners.
High-frequency radiation has been shown to cause human cancer. Ultraviolet radiation from the sun accounts for a majority of melanoma cases. Other sources of radiation are x rays, radioactive substances, and rays that enter the earth's atmosphere from outer space. Virtually any part of the body can be affected by these types of radiation, especially bone marrow and the thyroid gland.
Additionally, being exposed to substances such as certain chemicals, metals, or pesticides, can increase the risk of cancer. Asbestos is an example of a well-known carcinogen, increasing the risk for lung cancer. This risk is increased even further for a smoker who is exposed to asbestos over a period of time.
Causes and symptoms
Cancer is a progressive disease and goes through several stages. Each stage can produce a number of symptoms. Unfortunately, many types of cancer do not display any obvious symptoms or cause pain until the disease has progressed to an advanced stage. Early signs of cancer are often subtle and are easily mistaken for signs of other less-dangerous diseases.
Despite the fact that there are several hundred different types of cancers producing very different symptoms, the American Cancer Society (ACS) has established the following seven symptoms as possible warning signals of cancer:
- changes in the size, color, or shape of a wart or a mole
- a sore that does not heal
- persistent cough, hoarseness, or sore throat
- a lump or thickening in the breast or elsewhere
- unusual bleeding or discharge
- chronic indigestion or difficulty in swallowing
- any change in bowel or bladder habits
Other diseases can produce similar symptoms. However, it is important to have these symptoms checked as soon as possible, especially if they linger. The earlier a cancer is diagnosed and treated, the better the chance of it being cured. Many cancers, for example breast cancer, may not have any early symptoms. Therefore, it is important to undergo routine screening tests, such as breast self-exams and mammograms.
If a person has symptoms of cancer, a physician will begin with a complete medical history and a thorough physical examination. The doctor will examine different parts of the body in order to identify any variations from the normal size, feel, and texture of the organ or tissue. In addition to the physical exam, the doctor may order various tests.
Laboratory tests on blood and urine are often used to obtain information about a person's health. If cancer is suspected, a special test can be done that measures the amount of certain substances, called tumor markers, in the blood, urine, or particular tissues. These proteins are released from some types of cancer cells. Thus, the levels of these substances may be abnormal when certain cancers are present. However, laboratory tests alone cannot be used to make a definitive diagnosis of cancer. Blood tests are generally more useful in monitoring the effectiveness of the treatment or in following the course of the disease and detecting any signs of recurrence.
A doctor may look for tumors by examining images of areas inside the body. The most common way to obtain these images is by using x rays. Other techniques used to examine the insides of the body include computed tomography (CT scan), magnetic resonance imaging (MRI), and ultrasonography.
The most definitive diagnostic test is a biopsy. In this technique a piece of tissue is surgically removed for examination under a microscope. A biopsy provides information about the cellular nature of an abnormality: the stage it has reached, the aggressiveness of the cancer, and the extent of its spread. Further analysis of the tissue obtained by biopsy defines the cause of the abnormality. Since a biopsy provides the most accurate analysis, it is considered the gold standard of diagnostic tests for cancer.
Regular screening examinations conducted by healthcare professionals can result in the early detection of various types of cancer. Early detection means treatment is more likely to succeed. For example, the ACS recommends an annual mammogram (x ray of the breast) for women over the age of 40 years, to screen for breast cancer. It also recommends a sigmoidoscopy, in which a thin, lighted tube with a tiny camera is used to view the inside of the colon, every five years for people over the age of 50. This technique can assess the presence of colorectal cancer. Self-examinations for cancers of the breast, testes, mouth, and skin can also help in detecting tumors before the symptoms become serious.
Evolutions in molecular biology and the genetics of cancer have led to the development of several tests designed to assess one's risk of getting certain types of cancer. Genetic testing involves looking closely at certain genes that have been linked to particular cancers. As of 2001 there were many limitations to genetic testing. Tests could be uninformative and to a very small proportion of individuals tested. Additionally, concerns exist about insurance coverage and employment discrimination for someone who has an increased risk for cancer. As of 2001 these tests were reserved only for very specific individuals. A hereditary cancer clinic can help assess who may benefit from this type of testing.
The aim of cancer treatment is to remove all or as much of the tumor as possible and to prevent the recur- rence or spread of the primary tumor. While devising a treatment plan for cancer, the likelihood of curing the cancer must be weighed against the side effects of the treatment. If the cancer is aggressive and a cure is not possible, then treatment should be aimed at relieving the symptoms and controlling the cancer for as long as possible.
Cancer treatment can take many different forms, and it is always tailored to an individual. The decision on which type of treatment is the most appropriate depends upon the type and location of the cancer and the extent to which it has already spread. A physician will also consider an affected person's age, sex, general health status, and personal treatment preferences. Treatment can be local, meaning that it affects cancer cells in the tumor and the surrounding area only. Surgery and radiation are local treatments. Treatment can also be systemic, meaning that the treatment travels through the bloodstream and affects cancer and other cells throughout the entire body. Chemotherapy, immunotherapy, and hormone therapy are examples of systemic treatments.
Surgery can be used for many purposes:
- Treatment. Treatment of cancer by surgery involves removal of the tumor to cure the disease. This is typically done when the cancer is localized to a discrete area. Along with the cancer, some of the surrounding tissue is also removed to ensure that no cancer cells remain in the area. Since cancer usually spreads via the lymphatic system, lymph nodes near the tumor site may be removed for examination.
- Prevention. Preventive or prophylactic surgery involves removal of an abnormal-looking area that is likely to become malignant over time. For example, 40% of people with a colon disease called ulcerative colitis ultimately die of colon cancer. Rather than live with the fear of developing colon cancer, these people may choose to have their colons removed in order to reduce their risk of colorectal cancer.
- Diagnosis. The most definitive tool for diagnosing cancer is a biopsy. Sometimes a biopsy can be performed by inserting a needle through the skin and aspirating a small amount of fluid or tissue. At other times the only way to obtain a tissue sample is through surgery.
- Cytoreductive surgery. This is a surgical procedure in which the surgeon removes as much of the cancer as possible. The remaining cancer cells are then treated with radiation therapy, chemotherapy, or both.
- Palliative surgery. This type of surgery is intended to relieve cancer symptoms or slow the progression of disease. It is not designed to cure the cancer. For example, if the tumor is very large or has spread to many places in the body, removing the entire tumor may not be an option. However, by decreasing the size of the tumor, pain may be alleviated. This is known as debulking surgery.
Radiation uses high-energy rays to kill cancer cells. This technique may be used instead of surgery. It also may be utilized before surgery to shrink a tumor or after surgery to destroy any remaining cancer cells.
Radiation can be either external or internal. In the external form, the radiation comes from a machine that aims the rays at the tumor. In internal radiation (also known as brachytherapy), radioactive material is sealed in needles, seeds, or wires and placed directly in or near the tumor. Radiation may lead to various side effects, such as fatigue, hair loss, and a susceptibility to infections. However, these side effects can usually be controlled.
Chemotherapy is the use of drugs to kill cancer cells. The entire body is exposed to the drugs (systemic therapy) in an effort to destroy the hard-to-detect cancer cells that have spread and are circulating in the body. The cancer cells are affected more dramatically than normal cells because they are rapidly dividing. Chemotherapeutic drugs can be injected into a vein, the muscle, or the skin, or they may be taken by mouth.
When chemotherapy is used before surgery, it is known as primary, or neoadjuvant chemotherapy. Its purpose is usually to reduce the size of the tumor. The more common use of chemotherapy is in adjuvant therapy. In this technique, chemotherapy is given after surgery to destroy any remaining cancer cells and to help prevent cancer from recurring. Chemotherapy can also be used in conjunction with radiation.
Side effects of chemotherapy vary but can include susceptibility to infections, fatigue, poor appetite, weight loss, nausea, diarrhea, and hair loss. Decreased fertility can be a long-term side effect in some instances.
Bone marrow failure is a complication of chemotherapy. When high-dose chemotherapy is utilized, bone
|Recommendations for cancer screening|
|SOURCE: U.S. Preventative Services Task Force, Dept. of Health and Human Services.|
|Chest x ray||Not recommended on a routine basis|
|Sputum cytology||Not recommended on a routine basis|
|Fecal occult blood||Yearly after age 50|
|testing (FOBT) or|
|Papanicolaou (Pap)||Every 3 years from onset of sexual activity to|
|Mammography alone||Every 1 years at ages 509; starting at ages|
|or mammography||409 may be recommended if high-risk|
|and breast physical|
marrow failure is anticipated. Bone marrow transplantation (BMT) or peripheral stem cell transplantation (PSCT) are techniques used to treat this complication. Both techniques provide healthy stem cells for an affected person. Stem cells are immature cells that mature into blood cells. Transplanted stem cells replace the patient's stem cells that have been damaged or destroyed by chemotherapy or radiation. This procedure allows an individual to undergo very aggressive treatment for cancer. Those who receive BMT or PSCT have an increased risk of infection, bleeding, and other side effects due to the chemotherapy and radiation. Graft-versus-host dis- ease may also occur. This complication develops when the donated marrow reacts against the recipient's tissues. It can occur any time after the transplant. Drugs may be given to reduce the risk of graft-versus-host disease and to treat the problem if it occurs.
Immunotherapy, also called biological therapy, is the use of treatments that promote or support the body's immune system response to cancer. The side effects of immunotherapy are variable but include flu- like symptoms, weakness, loss of appetite, and skin rash. These symptoms will subside after the treatment is completed.
Hormone therapy is used to fight certain cancers that depend on hormones for their growth. Drugs can be used to block the production of hormones or change the way they work. Additionally, organs that produce hormones may be removed. As a result of this therapy, the growth of the tumor slows and survival may be extended for several months or years.
|COMMON PATHOGENS AND THE CANCERS ASSOCIATED WITH THEM|
|Causative Agent||Type Of Cancers|
|Papillomaviruses||Cancer of the cervix|
|Hepatitis B virus||Liver cancer|
|Hepatitis C virus||Liver cancer|
|Epstein-Barr virus||Burkitt's lymphoma|
|Cancers of the upper pharynx||Hodgkin's lymphoma, Non-Hodgkin's lymphoma, Gastric cancers|
|Human immunodeficiency virus (HIV)||Kaposi's sarcoma Lymphoma|
|Helicobacter pylori||Stomach cancer Lymphomas|
Alternative and complementary therapies
There are certain cancer therapies that have not been scientifically tested and approved. If these unproven treatments are used instead of the standard therapy, this is known as alternative therapy. If they are used along with standard therapy, this is known as complementary therapy. Alternative therapy is considered dangerous because some of these unproven treatments might have life- threatening side effects. Additionally, persons who use alternative therapy may lose the opportunity to benefit from standard, proven therapy. However, some complementary therapies may help to relieve symptoms of cancer, decrease the magnitude of side effects from treatment, or improve a patient's sense of well being. The American Cancer Society recommends that anyone considering alternative or complementary therapy consult a health care team before doing so.
Palliative care, which focuses on the terminally ill, is an extremely important aspect of care. Its goal is to prevent and relieve pain and suffering through symptom management, and addresses not only the physical, but the practical, emotional, and spiritual needs of patients, their families, and caregivers. By viewing dying as a natural process, palliative care helps facilitate what can be termed a "good" death, free from suffering and stress. Many patients and their families are unaware that suffering at the end of life is no longer necessary. Health care professionals can relieve a patient and their loved ones of much anxiety by informing them that appropriate symptom management is readily available.
Hospice care is the environment in which palliative care is given to terminally ill patients. Hospice care can be provided either at home, or in a home-like facility called a hospice. Hospice care focuses on providing the best possible palliative care for the patient until the patient dies.
Most cancers are curable if detected and treated in their early stages. The prognosis for a person with cancer is affected by many factors, particularly the type of cancer and stage of the cancer, the extent to which it has metastasized, and its aggressiveness. In addition, a person's age, general health status, and effectiveness of the treatment being pursued are important factors.
To help predict the outcome of cancer and the likeli- hood of recovery from the disease, five-year survival rates are used. In the United States, as of 2001, the five- year survival rate for all cancers combined was 59%. This means that 59% of people with cancer are expected to be alive five years after they are diagnosed. These people may be free of cancer, or they may be undergoing treatment. It is important to note that, while this statistic can give some information about the average survival of people with cancer in a given population, it cannot be used to predict the course of cancer for an individual. No two people are exactly alike. The five-year survival rate does not account for differences in detection methods, types of treatments, additional illnesses, and personal behavior of the individual.
Health care team roles
Family physicians, internists, gynecologists, or pediatricians generally make an initial diagnosis of cancer. Other physicians, notable radiologists, and oncologists provide chemotherapeutic and radiologic treatment. Nurses provide emotional and educational support, home care, home hospice care, and case management. Counselors and psychologists may provide emotional support to patients and their families. Epidemiologists collect and maintain data related to cancer.
According to experts from leading universities in the United States, a person can reduce the chances of getting cancer by following these guidelines:
- eating plenty of fruits and vegetables
- exercising vigorously for at least 20 minutes every day
- avoiding excessive weight gain
- avoiding tobacco (including second hand smoke)
- decreasing or avoiding consumption of animal fats and red meats
- avoiding excessive amounts of alcohol
- avoiding the midday sun (between 11 A.M. and 3 P.M.) when the sun's rays are the strongest
- avoiding risky sexual practices
- avoiding known carcinogens in the environment or work place
Certain drugs being used for treatment can also be suitable for prevention. For example, tamoxifen (Nolvadex) has been very effective against recurrence of breast cancer and is now thought to be helpful in the prevention of breast cancer. Similarly, retinoids derived from vitamin A are being tested for their ability to slow the progression of, or prevent, head and neck cancers.
Benign growth that does not spread to other parts of the body. Recovery is favorable with treatment.
Biopsyhe surgical removal and microscopic examination of living tissue for diagnostic purposes.
Bone marrowpongy material that fills the inner cavities of the bones. The progenitors of all the blood cells are produced in this bone marrow.
Carcinogenny substance capable of causing cancer by mutating a cell's DNA.
Chemotherapyreatment with anticancer drugs.
Epitheliumayer of cells covering the body's surface and lining the internal organs and various glands.
Hormone therapyreatment of cancer by changing the hormonal environment, such as testosterone and estrogen.
Immunotherapyreatment of cancer by stimulating the body's immune system.
Malignant general term for cells that can break loose from an original tumor, invade, and then destroy other tissues and organs.
Metastasishe spread of cancer from one part of the body to another.
Radiation therapyreatment using high-energy radiation from x-ray machines, cobalt, radium, or other sources.
Soren open wound or a bruise or lesion on the skin.
Tumorn abnormal growth resulting from a cell that lost its normal growth control restraints and started multiplying uncontrollably.
X raysigh energy radiation used in high doses, either to diagnose or treat disease.
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American College of Occupational and Environmental Medicine. 55 West Seegers Road, Arlington Heights, IL60005. (708) 228-6850. <<a href="http://www.acoem.org">http://www.acoem.org>.
American College of Radiology. 1891 Preston White Drive, Reston, VA 20191. (703) 648-8900.<<a href="http://www.acr.org">http://www.acr.org>.
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L. Fleming Fallon, Jr., M.D., Dr.P.H.
Cancer (Encyclopedia of Public Health)
Cancer is the end product of a multistep process (carcinogenesis) that occurs over many years. The term "cancer" actually refers to numerous distinct diseases characterized by abnormal cell growth and differentiation. Cancers are categorized by the organ and/or cell of origin. For example, squamous cell carcinoma of the lung arises from pulmonary epithelial tissue, whereas adenocarcinoma of the breast arises from mammary duct epithelium. The natural history of a cancer is highly dependent on the organ and cell type from which it is derived. In addition, prognosis is influenced by the stage and histologic grade of the cancer. Staging is generally designated by the TNM (tumor, nodes, metastasis) staging system, which takes into account the size of the primary tumor(T), the extent to which local lymph nodes (glands) are involved (N), and whether or not distant metastases are present (M). The histologic grade, determined by microscopic examination of the biopsy specimen, provides an objective assessment of the degree of cellular differentiation.
INCIDENCE, PREVALENCE, AND MORTALITY
The worldwide burden of cancer is a major health problem, with more than 8 million new cases and 5 million deaths per year. The burden from cancer may be described in terms of incidence (number of new cases per 100,000 each year), prevalence (number of people at a given point in time with a cancer diagnosis), and mortality (number of cancer deaths). With few exceptions, cancer incidence, prevalence, and mortality rates are higher in industrialized countries (e.g., United States, European nations) than in developing countries (e.g., African nations, China). Incidence rates for specific cancers can be dramatically affected by the use of screening procedures to identify asymptomatic disease. This is illustrated by the dramatic increase in the incidence of prostate cancer that accompanied the introduction of prostate-specific antigen (PSA) screening in the late 1980s. Similarly, prevalence rates may be a poor index for comparing cancers, because they are dependent upon incidence, natural history, and treatment efficacy. For example, due to the relatively short life expectancy of individuals with pulmonary neoplasms, the prevalence of lung cancer is much lower than that of prostate cancer, despite the higher mortality rates associated with lung cancer. In addition to the impact of screening and natural history, prevalence rates increase as treatment improves, because therapeutic advances enable individuals to live longer following a cancer diagnosis.
Worldwide, lung cancer is the leading cause of cancer mortality, followed by stomach cancer. Smoking remains the leading preventable cause of cancer, and mortality and incidence rates of lung cancer rise and fall with smoking rates. The current trend shows a leveling off of smoking-related cancers in developed countries, possibly because of health-promotion and disease-prevention efforts. Geographical variations occur in cancer incidence and mortality, with Africa and Asia generally having lower rates than North America and Europe. However, it has been noted that differences in data collection and diagnostic practices make worldwide cancer comparisons somewhat difficult. Overall, worldwide incidence rates of breast, colon and rectum, and prostate cancers are highest in developed countries, while cancers of the cervix, mouth and pharynx, esophagus, and liver are higher in developing countries. Migration studies generally report that migrants from developing countries to developed countries adopt cancer incidence rates equivalent to those of their new country. For example, studies of Japanese and Chinese immigrants living in the United States show that their risks for prostate and breast cancers increase dramatically the longer they reside in the United States. Similar trends for increased risk are seen among African immigrants in European countries. Geographical variations in cancer incidence and mortality also exist in the United States. Each cancer site shows some variation, such as higher prostate cancer mortality rates in the South Central and southern Atlantic states, and higher breast cancer mortality rates in the northeastern states. The Atlas of United States Mortality, published by the Centers for Disease Control and Prevention, provides detailed geographic information on cancer mortality rates in the United States, and is available online at http://www.cdc.gov/nchs/data/atlasmet.pdf.
In the United States, cancer is the second leading cause of death; although there has been a slight decline in the number of people dying from cancer since 1990, with more than one-half of those who develop cancer being cured or surviving for over five years. Because cancer is many diseases, some cancers are more common and/or more curable than others. For example, although nonmelanoma skin cancer, mainly caused by overexposure to ultraviolet (UV) radiation from the sun, is responsible for the largest number of new cancer cases each year, mortality rates associated with it are low.
Incidence, prevalence, and mortality in the United States vary by cancer site, between whites and blacks, and between men and women. By order of incidence, the three most common cancers in men are prostate, lung, and colorectal; in women the three most common cancers are breast, lung, and colorectal. These cancer sites represent more than one-half of both new cases of cancer and deaths from cancer each year. Lung cancer is the leading cause of death from cancer for men and women, accounting for almost one-third of cancer deaths.
Although cancer risk increases with age, malignant diseases are an important cause of morbidity and mortality in the pediatric population. The most frequent cancers in children are leukemias, tumors of the nervous systems, lymphomas, soft-tissue sarcomas, and kidney tumors. Other than lung cancer, which increases dramatically after age forty, three out of every four deaths from cancer occur in individuals older than sixty years of age.
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 polymorphismsifferent 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 chemopreventionhe 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.
SCREENING AND EARLY DETECTION
Mammography has been shown to reduce breast cancer mortality among women over the age of fifty, and Pap smear screening has dramatically reduced mortality from cervical cancer. In addition, there is growing evidence that fecal occult blood testing and endoscopic screening significantly reduce mortality from colorectal cancer. Identification of mutations is becoming an important tool for identifying individuals at high risk of various cancers. For instance, DNA repair-gene mutations (e.g., MSH2, PMS1) have been associated with a higher risk of colon cancer, as have mutations in the tumor suppressor genes BRCA1 and BRCA2 in breast cancer. Although it is believed that inherited risk for cancer accounts for a small proportion of total cancer cases each year, identifying this risk may help researchers determine how cancer develops and progresses, and may provide a tool for targeting prevention or treatment strategies.
Prognosis is dependent on the type of cancer diagnosed, the stage of the disease at the time of diagnosis, and the effectiveness of currently available therapy. Surgery, radiation, chemotherapy, hormonal therapy, and immunologic therapy form the basis of modern cancer treatment. Surgery is generally the treatment of choice for localized tumors, although radiation often is an appropriate alternative. Lasers are being used for small noninvasive tumors of the skin, cervix, and throat. Radiation therapy is often recommended as primary therapy (e.g., for Hodgkin's disease and early stage tumors of the head and neck), and is an important adjunct to lumpectomy for the treatment of breast cancer. Radiation therapy also plays an important role in the symptomatic management of patients with advanced cancer (e.g., bone or brain metastases). In contrast to surgery and radiation, chemotherapy is a systemic, rather than local, therapy, because the drugs are distributed throughout the body. Chemotherapy generally is required to treat advanced cancers that are not amenable to surgical removal or radiation therapy. Chemotherapy is often used after surgery (adjuvant therapy) to reduce the risk of relapse. The most common indication for adjuvant chemotherapy is following surgery for localized breast or colorectal cancer.
Hormone therapy represents a very important category of cancer treatment for breast cancer (tamoxifen and raloxifene) and prostate (androgen blockers) cancer. In addition, immunotherapy (also called biologic therapy) is being used to boost the immune system to fight cancer cells. Monoclonal antibodies are one type of immunotherapy that can be used to fight specific cancer cells or to carry chemotherapeutic agents to a tumor. Interferon is another immunotherapy that has shown promise in slowing the growth of tumors. Each of these treatments has advantages and disadvantages, and should be discussed with a physician.
CANCER IN DEVELOPING COUNTRIES
Cancer trends are of great concern to the public health community. As developing countries become more industrialized, incidence and mortality rates for cancers of the breast, colon, rectum, and prostate begin to rise. Also, smoking is increasing worldwidelong with lung cancer incidence and mortality rates. Liver cancer shows the same trends as lung cancer, but for a different reason. Infection with the hepatitis B or C viruses is a major risk factor for liver cancer. In some countries, where a vaccine for hepatitis B is widely used to vaccinate infants, liver cancer incidence in later life has declined; however, incidence rates in developing countries, where vaccination is not widely available, appear to be increasing. Another virus, the human papillomavirus (HPV), is an important risk factor for cervical cancer. Cervical cancer and HPV are more common in equatorial countries (e.g., in Latin America, sub-Saharan Africa, and Southeast Asia) and less common in countries in northern latitudes. Screening and treatment for early stages of cervical cancer have made significant inroads for reducing the incidence and mortality of this disease.
HOWARD L. PARNES
(SEE ALSO: Breast Cancer; Carcinogen; Cervical Cancer; Colorectal Cancer; Environmental Determinants of Health; Environmental Tobacco Smoke; Genetics and Health; Geography of Disease; Geriatrics; Incidence and Prevalence; Lung Cancer; Melanoma; Mortality Rates; Nutrition; Occupational Safety and Health; Oral Cancer; Ovarian Cancer; Prevention; Preventive Health Behavior; Prostate Cancer; Screening; Skin Cancer)
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