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Cancer

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A Metastasizing Breast Cancer Cell photographed by a scanning electron microscope.

Cancer is a group of more than 100 different diseases characterized by uncontrolled growth and spread of abnormal cells. If the spread of cancer is not controlled, it can result in death. [1] Such abnormal cell activity can be caused by both external (chemical, radiation, and viruses) and internal (hormones, immune conditions, and inherited mutations) factors. Causal factors may act together, or in sequence, to initiate or promote carcinogenesis.[2]

Cancer can arise in many sites and behave differently depending on its organ of origin. Breast cancer, for example, has different characteristics than lung cancer. It is important to understand that cancer originating in one body organ takes its characteristics with it even if it spreads to another part of the body. For example, metastatic breast cancer in the lungs continues to behave like breast cancer when viewed under a microscope, and it continues to look like a cancer that originated in the breast.[3]

The main categories of cancer include:

  • Melanoma - cancer that begins in the melanin-producing skin cells.
  • Carcinoma - cancer that begins in the skin or in tissues that line or cover internal organs.
  • Sarcoma - cancer that begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue.
  • Leukemia - cancer that starts in blood-forming tissue such as the bone marrow and causes large numbers of abnormal blood cells to be produced and enter the blood.
  • Lymphoma and myeloma - cancers that begin in the cells of the immune system.
  • Central nervous system cancers - cancers that begin in the tissues of the brain and spinal cord.[4]

Contents

Characteristics

Cancer cell diagram.jpg

Abnormality

Cells are the structural units of all living things. Each of us has trillions of cells, as does a growing tree. Cells make it possible for us to carry out all kinds of functions of life: the beating of the heart, breathing, digesting food, thinking, walking, and so on. However, all of these functions can only be carried out by normal healthy cells. Some cells stop functioning or behaving as they should, serving no useful purpose in the body at all, and become cancerous cells.[5]

Uncontrollability

The most fundamental characteristic of cells is their ability to reproduce themselves. They do this simply by dividing. One cell becomes two, the two become four, and so on. The division of normal and healthy cells occurs in a regulated and systematic fashion. In most parts of the body, the cells continually divide and form new cells to supply the material for growth or to replace worn-out or injured cells. For example, when you cut your finger, certain cells divide rapidly until the tissue is healed and the skin is repaired. They will then go back to their normal rate of division. In contrast, cancer cells divide in a haphazard manner. The result is that they typically pile up into a non-structured mass or tumor.[6]

Not all tumors are cancerous; tumors can be benign or malignant.

  • Benign tumors aren't cancerous. They can often be removed, and, in most cases, they do not come back. Cells in benign tumors do not spread to other parts of the body.
  • Malignant tumors are cancerous. Cells in these tumors can invade nearby tissues and spread to other parts of the body. The spread of cancer from one part of the body to another is called metastasis.[7]

Invasiveness

Sometimes tumors do not stay harmlessly in one place. They destroy the part of the body in which they originate and then spread to other parts where they start new growth and cause more destruction. This characteristic distinguishes cancer from benign growths, which remain in the part of the body in which they start. Although benign tumors may grow quite large and press on neighboring structures, they do not spread to other parts of the body. Frequently, they are completely enclosed in a protective capsule of tissue and they typically do not pose danger to human life like malignant tumors (cancer) do.[8]

Cellular Effectors

In general, cancer cells have defects in normal cellular functions that allow them to divide, invade the surrounding tissue, and spread by way of vascular and/or lymphatic systems. These defects are the result of gene mutations sometimes caused by infectious viruses.

Normal cells divide only when they receive the proper signals from growth factors that circulate in the bloodstream or from a cell they directly contact. For example, if a person loses blood, a growth factor called erythropoietin which is produced in the kidneys circulates in the bloodstream and tells the bone marrow to manufacture more blood cells. When a cell receives the message to divide, it goes through the cell cycle, which includes several phases for the division to be completed. Checkpoints along each step of the process make sure that everything goes the way it should. Many processes are involved in cell reproduction and all these processes have to take place correctly for a cell to divide properly. If anything goes wrong during this complicated process, a cell may become cancerous.

A cancer cell is a cell that grows out of control. Unlike normal cells, cancer cells ignore signals to stop dividing, to specialize, or to die and be shed. Growing in an uncontrollable manner and unable to recognize its own natural boundary, the cancer cells may spread to areas of the body where they do not belong.[9]

Mutations

Main Article: Mutation

In a cancer cell, several genes change (mutate) and the cell becomes defective. There are two general types of gene mutations. One type, dominant mutation, is caused by an abnormality in one gene in a pair. An example is a mutated gene that produces a defective protein that causes the growth-factor receptor on a cell's surface to be constantly "on" when, in fact, no growth factor is present. The result is that the cell receives a constant message to divide. This dominant "gain of function gene" is often called an oncogene (onco = cancer).

The second general type of mutation, recessive mutation, is characterized by both genes in the pair being damaged. For example, a normal gene called p53 produces a protein that turns "off" the cell cycle and thus helps to control cell growth. The primary function of the p53 gene is to repair or destroy defective cells, thereby controlling potential cancerous cells. This type of gene is called an anti-oncogene or tumor suppressor gene. If only one p53 gene in the pair is mutated, the other gene will still be able to control the cell cycle. However, if both genes are mutated, the "off" switch is lost, and the cell division is no longer under control.[10]

Abnormal Behavior

Abnormal cell division, can occur either when active oncogenes are expressed or when tumor suppressor genes are lost. In fact, for a cell to become malignant, numerous mutations are necessary. In some cases, both types of mutations - dominant and recessive - may occur. A gene mutation may allow an already abnormal cell to invade the normal tissue where the cancer started, or to travel in the bloodstream (metastasize) to remote parts of the body, where it continues to divide.

A normal cell can become damaged in different ways. A cell can become abnormal when part of a gene is lost (deleted), when part of a chromosome is rearranged and ends up in the wrong place (translocation), or when an extremely small defect occurs in the DNA, which results in an abnormal DNA "blueprint" and production of a defective [[protein[[ occurs. Abnormal cell division can also be caused by viruses. In this case, genes may be normal, but the protein may not function normally because the cell contains a cancer-producing virus.

How a specific cancer cell behaves depends on which processes are not functioning properly. Some cancer cells simply divide and produce more cancer cells, and the tumor mass stays where it began. Other cancer cells are able to invade normal tissue, enter the bloodstream, and metastasize to a remote site in the body.[11]

Cancer Statistics

Overall, cancer incidence rates have been decreasing since 2001 in men and women, and for all races/ethnicities. For women, a long term increase in overall incidence rates leveled beginning in 1999 after increasing since 1979. Breast, lung, and colorectal cancers remain the three leading incident cancers among women.

It is estimated that 1,444,920 men and women (766,860 men and 678,060 women) will be diagnosed with and 559,650 men and women will die of cancer of all sites in 2007.[12]

Estimated new cases and deaths from cancer in the United States in 2008:

  • New cases: 1,437,180 (does not include nonmelanoma skin cancers)
  • Deaths: 565,650[13]

A new report from the nation's leading cancer organizations shows cancer death rates decreased on average 2.1 percent per year from 2002 through 2004, nearly twice the annual decrease of 1.1 percent per year from 1993 through 2002.[14] From 2000-2004, the median age at diagnosis for cancer of all sites was 67 years of age. Approximately 1.1% were diagnosed under age 20; 2.7% between 20 and 34; 5.9% between 35 and 44; 13.6% between 45 and 54; 20.9% between 55 and 64; 25.8% between 65 and 74; 22.6% between 75 and 84; and 7.4% 85+ years of age.[15]

Risk Factors

The search for cause(s) of cancer has been going on for centuries. Early researchers said that cancer was a natural result of aging. As cells degenerated, it was believed that some simply became malignant. Others said cancer was hereditary, and investigations into genetics began. Then some began to consider chemical links while still others questioned whether viruses or bacteria were at fault. Finally, the "irritation" theory became popular, and researchers began trying to identify irritants - such as tobacco and coal tar - that would cause cancer in laboratory animals. Ultimately, though, cancer experts were forced to confront the fact that although all these factors might be involved, none of them invariably cause cancer. Not every animal or person exposed to an irritant or a particular chemical in the laboratory developed cancer, nor did all elderly people or everyone with a family history of cancer get it. As a result, scientists had to abandon the theory that cancer had a single cause.

However, despite the fact that there is yet no absolute agreement among the cancer research community in terms of what actually causes cancer, scientists are certain that many factors can be linked to cancer. These factors, including many other possible causes of cancer suggested by cancer researchers, are believed to be "cancer risk factors." These risk factors include eating habits, lifestyle, living or working environments, genetics, and many others.

Following are some major cancer risk factors identified by researchers with the support of scientific statistics. Cancer risk factors are not limited to those listed above. There are still other risk factors such as ethanol use, use of certain medications, hormones, and reproductive and sexual behavior. With further scientific research, more cancer risk factors will be identified in the future.[16]

Smoking

Cigarette smoking alone is directly related to at least one-third of all cancer deaths annually in the United States. Cigarette smoking is the most significant cause of lung cancer and the leading cause of lung cancer death in both men and women. Smoking is also responsible for most cancers of the larynx, oral cavity, and esophagus. In addition, it is highly associated with the development of, and deaths from bladder, kidney, pancreatic, and cervical cancers. Tobacco smoke contains thousands of chemical agents, including 60 substances that are known to cause cancer (carcinogens).

The health risks with cigarette smoking are not limited to smokers. Exposure to environmental tobacco smoke significantly increases a nonsmoker's risk of developing lung cancer. Environmental tobacco smoke is the smoke that nonsmokers are exposed to when they share air space with someone who is smoking.[17]

Diet

The life style factor that has received the most attention in recent years is diet. Evidence suggests that about one-third of the cancer deaths each year that occur in the United States are related to dietary factors. These include types of food, preparation methods, portion size, variety, and overall caloric balance.

A high-fat diet has been associated with an increased risk for cancer of the prostate, endometrium, and colon and rectum. It is believed that a high-fat diet is a cancer promoter, with numerous theories to explain the effects of excess fat. For instance, excess fat seems to be involved in the production of free radicals, which play a role in many types of cancer. A high-fat diet also increases the flow of bile acids into the intestine, which can promote colon cancer.

Study results suggest that certain food additives, as well as preparation methods, can either cause or promote cancer. Even some so-called natural methods of preserving foods are not considered safe. For example, pickled, cured, and smoked products appear to promote stomach cancer, possibly due to nitrites used in curing as well as to other compounds produced during smoking and pickling. The decrease in gastric cancer incidence is largely due to modern refrigeration and a reduction in pickled, cured, and smoked food products.[18]

Genetics

By definition, cancer is really a disease of genes. Genes are very small molecules in our cells, which determine almost everything in our body. Genes that control the genetics and heredity of each cell are strung like beads on a necklace along the cell's DNA in the cell nucleus. In a benign or malignant tumor, several of the genes regulating these processes are abnormal (mutated). Abnormal genes may be inherited or damaged by carcinogens, viruses, errors in cell division, and as yet unknown factors.

A number of the most common cancers, including breast, colon, ovarian, and uterine cancer, recur generation after generation in some families. In addition, certain genetic factors may predispose those affected to specific cancers. A few rare cancers, such as the eye cancer, retinoblastoma, and a type of colon cancer, have been linked to specific genes that can be tracked within a family.

Although it is helpful to know the role that our genetic heritage may play as a possible cause of cancer, scientists believe that environmental influences and our behaviors may outweigh the risks inherent in our family tree (pedigree).[19]

Occupation and Environment

Scientists have long been aware of the linkage between one's health conditions and their occupation and environment.

People who have direct contact to carcinogenic agents in the workplace are at the highest risk for developing cancer. For example, a recent study suggests that people with brain cancer are more likely to have worked in certain occupations than similarly aged people without brain cancer. Many cancer-causing chemicals have been identified and many of them are banned from manufacture in the United States.

More recently, investigators have identified a link between the environment and skin cancer. The environmental factor is something we depend on for our life: sunlight. Scientists have found that ultraviolet light causes mutations of genes, producing a carcinogenic effect. Now, we not only know that tumors may appear years after the damaging effects of sunlight, but also the risks from exposure to ultraviolet light are greater for light-skinned people. Statistics show that in the U.S. alone, about a million new cases of skin cancer (basal and squamous cell carcinomas) occur annually, rivaling the incidence of all other types of cancer combined.

The common body surfaces that are exposed to carcinogens are the skin, nasal passages, and lung. The primary internal body surface that has contact with carcinogens is the urinary bladder.[20]

Infectious Agents

Because viruses can invade and alter cells' genetic material, viral infections are implicated in some cancers. The Epstein-Barr virus, for example, is associated with Burkitt lymphoma, a tumor found mainly among children in Africa. The hepatitis B virus is responsible for much of the liver cancer around the world. The highest rates of hepatitis B infection in the world is in China, Taiwan, Japan, and Thailand with equally high rates of liver cancer in these countries. The human papilloma virus that causes genital warts has been shown to play an important causative role in cervical cancer. The human T-cell leukemia virus, a close relative of the virus that causes acquired immunodeficiency syndrome (AIDS), is associated with a cancer known as Kaposi sarcoma and some types of Non-Hodgkin lymphomas.[21]

References

External links

See Also

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