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| Chemotheraphy |
abnormal cells in a part of the body. It is also defined as the uncontrolled growth of abnormal cells in the body. Cancerous cells are also called
malignant cells.
Chemotheraphy is treatment of disease by the use of chemical substances,
especially the treatment of cancer by cytotoxic and other drugs
The People's Republic of China is both the world's largest producer and
largest consumer of tobacco, which has led to an impending cancer
epidemic in the most populous country on Earth.
How lung cancer
develops - Lung cancer, like all other forms of cancer, results from
uncontrolled cell growth. Most cases of lung cancer start in the tissue that
lines the bronchi— that is, the airways leading from the trachea to the lungs,
Cancer is a disease in which cells multiply without control, destroy
healthy tissue, and endanger life. About 100 kinds of cancer affect human
beings. The disease is a leading cause of death in many countries. Cancer occurs
in most species of animals and in many kinds of plants as well as in human
beings.
Cancer
strikes people of all ages but especially middle-aged people and the elderly.
It occurs equally among people of both sexes. The disease can occur in any part
of the body and may spread to other parts. However, the organs most often
affected are the skin, the digestive organs, the lungs, the cervix (neck of
the womb), and the female breasts.
Scientists
do not know exactly why cancer develops. But they have found that certain agents play an
active role in causing the disease. These agents, called carcinogens, include
the tar in tobacco, a variety of other chemicals, and certain kinds of
radiation. In many cases, cancer can be prevented if a known agent is avoided
or eliminated. The elimination of cigarette smoking, for example, would prevent
most cases of lung cancer. Scientists also believe that some people may inherit
a tendency to develop cancer.
The
methods of diagnosing and treating cancer have improved greatly since the
1930's. Today, about half of all cancer patients survive at least five years
after treatment. People who remain free of cancer that long after treatment
have a good chance of remaining permanently free of the disease. But much
research remains to be done to find better ways of prevention and cure.
This
article discusses how cancer develops, the major kinds of cancer, its causes,
the main methods of diagnosis and treatment, and cancer research.
How cancer develops
Cancer
develops as a result of abnormal cell reproduction. The body of an adult is
made up of hundreds of billions of cells. Each minute, several billion of these
cells die and are replaced by several billion new cells. The new cells are
produced by division. In this process, a cell divides into two identical cells.
Each of these new cells
then doubles in size and becomes capable of dividing. In this way, the new
cells that are constantly being produced replace those that die. Normal cells
divide % when chemical signals reach the DNA in the nucleus. The cells reproduce
at exactly the rate required to replace dying cells, never at a faster rate.
Cancer
develops as a result of abnormal cell reproduction. The genes in cancer cells
no longer respond to growth-regulating signals. The cells continue to multiply,
and gradually form a mass called a tumour or neo-plasm. Some
tumours are noncancerous, or benign. A benign
tumour does not spread to surrounding healthy tissue or other parts of the
body.
Cancer
produces malignant tumours. A malignant tumour invades,
compresses, and eventually destroys surrounding healthy tissue. In addition,
cells can break j| away from a malignant tumour. These cells are carried by the
blood or lymph (fluid from body tissues) to other II parts of the body, where
they continue to multiply and m so form
secondary tumours. The spread of cancer from the original tumour to one or more
other body sites is called metastasis. Cancer's
ability to spread to other parts of the body makes the disease extremely
difficult % to treat unless it is detected early.
About 100 kinds
of cancer have been recognized in human beings. They are classified in two
ways: (1) by a cancers primary body site—that is,
the part of the body where the cancer first develops; and (2) by the type of
body tissue in which the cancer originates.
Classification by body site. The
primary body sites that cancer strikes most often are the skin; the female
breasts; and the organs of the digestive, respiratory, reproductive,
blood-forming, lymphatic, and urinary systems. The occurrence of cancer in
these sites varies from country to country. Cancer of the stomach, for example-
is much more common in Japan than in the United States. But a far higher
percentage of Americans than Japanese develop breast cancer and prostate cancer. The
following discussion deals with the kinds of cancer that occur most often in
many countries.
Skin cancer is the most common form of cancer in the world.
Most skin cancers do not spread. As a result, these cancers are easy to treat. Malignant
melanoma, a rare form of skin cancer, is much more serious. This type of
cancer begins in the skin's pigment cells and can spread rapidly to other parts
of the body if not treated early. Malignant melanoma can develop as a result of
overexposure to the sun's ultraviolet rays.
Lung cancer is strongly linked with smoking. For several
decades it has been known that smokers are much more likely to develop lung
cancer. The risk of developing this type of cancer has risen with the increase
in air pollution. It is the single greatest cause of death among men and women
in most industrial countries. Lung cancer is also on the increase in many of
the developing countries of Africa and Asia.
Stomach cancer was a very common disease 100 years ago, but it
now occurs much less frequently in Western countries. The causes of stomach
cancer have been linked to diet, especially nitrates used in food preservation
or water contaminated with nitrates. This type of cancer is rarely found among
people who eat fresh food, or those who eat a well balanced diet. Other factors
which increase the risk of stomach cancer include alcohol, smoking, and
stomach ulcers.
Breast cancer is the most common cancer affecting women in
affluent societies, although it is fast being overtaken by lung cancer. It is
rare in Latin America, the West Indies, eastern Europe, Asia, and the Pacific.
Factors such as the age at which a woman has her first pregnancy, the number
of children, and diet seem to be linked with the development of the disease.
Women who have several children and eat a relatively low-fat diet are least
likely to suffer from this form of cancer.
Colo-rectal cancer occurs mainly in
industrial countries. It has been linked to the lack of fibre in diets. Rural
people moving to cities change from a diet of fresh fruit and vegetables to one
of meat and processed foods. This low-fibre diet increases their risk of
developing cancer of the large bowel.
Cancer of the cervix occurs mostly in countries where women live under poor
conditions. Regular screening and improved diet and personal hygiene can reduce
the incidence of this disease.
Cancer of the oesophagus has been linked to alcohol
consumption, smoking, drinking very hot tea, and poor diet. This form of cancer
occurs frequently among members of the Chinese community in Singapore.
Primary liver cancer is a tropical disease
found in many parts of the Far East and Africa, where hepatitis B is also
common. Hepatitis is a disease that involves inflammation of the liver (see Hepatitis).
Liver
cancer is widespread in Hong Kong and the hot, humid, coastal regions of China.
Burkitt's lymphoma, a serious health problem
in parts of tropical Africa and New Guinea, is closely associated with
infection by the Epstein-Barr virus. This cancer of the
lymphatic system affects mostly children. See Epstein-Barr virus.
Cancers of the blood-forming and lymphatic systems. Cancer
of the bone marrow and other blood- forming organs is called leukaemia. It
involves the multiplication of immature white blood cells at the expense of
vital blood elements. Cancer of the lymphatic organs, and of other organs
composed of lymphoid tissue, is called lymphoma. It
involves the overproduction of certain cells in this tissue (see Lymphatic
system).
Both
leukaemia and lymphoma have several forms. One of the most common forms of
leukaemia is acute leukaemia. The most common form of lymphoma is Hodgkin's
disease, named after the English doctor Thomas Hodgkin, who first
described it. However, non- Hodgkin's lymphoma is the
most common type of cancer of the lymphatic organs.
In the past, leukaemia and lymphoma were among
the most difficult of all cancers to cure. But methods of treatment developed
since the early 1960's have greatly improved. Leukaemia is the most important
form of cancer found in children. Exposure to ionizing radiation in the womb or
during childhood is believed to be an important factor in the development of
leukaemia in children. See Hodgkin's disease;
Leukaemia.
Classification
by body tissue. Cancers are identified according to the type of body tissue in
which they originate. They can thus be divided into two main groups: carcinomas and sarcomas.
Carcinomas are cancers that start in epithelial tissue—the tissue
that forms the outer layer of the skin or that lines an internal body surface
or organ. Sarcomas are cancers that begin in connective tissue— the
tissue that forms the body's supporting structures, such as bones and
cartilage. Leukaemia and lymphoma are sometimes classed separately from
carcinomas and sarcomas. But they are also classed as forms of sarcoma because
the tissues they affect—blood and lymph—are forms of connective tissue.
The great
majority of cancer cases are carcinomas. They include most cancers of the skin
and breast and of the digestive, reproductive, respiratory, and urinary systems.
Sarcomas also occur in all these organs and systems but far less often than
carcinomas do.
Most
experts agree that people develop cancer mainly through repeated or prolonged
contact with one or more cancer-causing agents, called carcinogens. In
addition, scientists suspect that a person may inherit a tendency to develop
the disease.
Carcinogenesis
begins
with a stage called initiation. During this stage the DNA, which contains the
genetic 5 blueprint of the cell,
undergoes mutation (a change in the hereditary material). See Mutation. Mutation may
occur accidentally during cell division. It may also occur as a result of
external agents entering the body. External agents include ionizing radiation,
cigarette smoke, certain viruses, and some chemicals.
Although
initiation does not result in cancer, it leaves, the damaged cell susceptible
to promotion. During promotion, the initiated cell begins to
divide and pass on the genetic changes. Eventually a tumour forms which may go
on to become cancerous.
Many
cells within a person's body become initiated and progress to form foci (small
benign tumours). Most i of the time, however, these foci never progress
further. The reasons why some people go on to develop the disease are still
only partly understood. Many cancers are probably caused by a combination of
two or more agents rather than by a single one.
If a
particular population group shows an unusually high cancer rate, experts look
for substances in the local, environment that may cause the cancer. Scientists
may test a substance in laboratory animals. If a high percentage of the animals
develop cancer, the agent might also cause cancer in people. Scientists then
compare a group of people exposed to a substance believed to be a carcinogen with groups that have not been
exposed to the substance. A higher occurrence of cancer in the first group
confirms that the substance is a carcinogen.
Laboratory tests and population studies indicate
that three main groups of carcinogens can cause cancer in human beings. These
groups consist of (1) various chemicals, (2) certain forms of radiation, and
(3) viruses.
Chemicals. Scientists have identified hundreds of chemicals
that can cause cancer in animals.
These chemicals are also a cancer hazard to human beings if they become widespread in the food supply, the
general environment, or the living or working
environments of individual groups of people.
In most
cases, a chemical carcinogen enters the food supply as a food
additive (chemical used in food processing) or through use in
agriculture. Some widely used food additives have been discovered to be carcinogens,
as have certain insecticides and other chemicals widely used in
agriculture. Moulds that sometimes develop on such food
crops as maize and groundnuts are also suspected of containing carcinogens.
Scientists are seeking ways to control these moulds.
In
discharging waste products, some factories release chemical carcinogens into
the environment. These carcinogens may pollute the air or drinking water and
so endanger entire communities. Local, national, and international agencies
act to stop this practice before it becomes a serious threat.
Other
carcinogens that may occur in a person's living or working environment include
(1) the tars in tobacco, (2) certain industrial chemicals, (3) certain natural
food chemicals, and (4) certain chemicals used in drugs.
Cigarette
smoking is the principal cause of lung cancer. It is also linked with other
cancers, including cancers of the mouth and pharynx, larynx, trachea, oesophagus,
pancreas, kidney, bladder, and cervix. Research indicates that cigarette smoke
can even lead to cancer in nonsmokers who live or work closely with smokers.
Some
industrial chemicals create a cancer hazard for people who work with them. Such
chemicals include aniline dyes, arsenic, asbestos, chromium and iron compounds,
lead, nickel, vinyl chloride, and certain products of coal, lignite, oil
shale, and petroleum. Unless industrial plants carefully control the use of
such chemicals, excessive amounts may escape or be released into the
environment. The chemicals then create a cancer hazard for people in
surrounding areas.
Some
chemicals naturally present in food may become a cancer threat if consumed in
large quantities. Diets high in fats have been associated with cancers of the
breast, colon, and prostate gland. High consumption of salt-cured,
salt-pickled, and smoked foods has been linked to cancers of the digestive
system. Diets rich in fruit, vegetables, and whole-grain breads and cereals
seem to reduce the risk of certain cancers.
Radiation. Certain kinds of radiation produce cancer in
people exposed to these radiations for long periods. For example, most cases of
skin cancer are caused by ultraviolet rays from the sun. Even in small doses,
ionizing radiation can cause genetic mutation which increases the risk of
cancer developing later in life. Diagnostic X rays are a potential cancer
hazard. However, the dose of radiation a person receives with each X ray is
carefully controlled, and the usefulness of X-ray imaging in medicine and
dentistry is believed to outweigh any hazards connected with their use.
Viruses. Experiments have shown that certain kinds of viruses cause cancer
in animals. Certain cancerous human tissues contain viruses similar to those
that cause cancer in animals. A virus has been linked to a rare form of leukaemia
in human beings, and some
forms of
liver cancer in humans may be caused by viruses.
Inherited
tendencies. Some cancers, including those of the breast and colon, occur among
relatives at a higher than average rate. Leukaemia and lymphoma often occur in
children with inherited immune deficiency disorders. Scientists therefore
conclude that some people inherit a tendency to develop a certain type of
cancer. But only a few kinds of cancer have been proved to be hereditary. One
such kind is retinoblastoma, a rare cancer of the eye
that occurs mainly in children under 3 years of age.
The
alteration of genes that control cell development or division may cause some types of
cancers. Researchers have identified certain genes, called protooncogenes, that are
vital to early tissue development. These genes may become changed or rearranged
by chemicals or viruses. In this altered state, the genes are called oncogenes. The
oncogenes then produce proteins that can transform a healthy cell into a
cancerous one. Scientists have identified about 50 oncogenes that may cause
cancer in certain organs, including the bladder, breasts, liver, lungs, colon,
and pancreas.
Other
kinds of genes, called suppressor genes, control
cell division in normal cells. Suppressor genes also delay the division of
damaged cells until the DNA (deoxyribonucleic acid)
molecules of the damaged cells are repaired. If repair is not possible,
suppressor genes cause the damaged cells to self-destruct. If these genes are lost
during cell division or changed by chemicals or drugs, control over cell
division may be lost. This can lead to the development of cancer.
Cancer
detection and diagnosis
Only a doctor can diagnose cancer. But in many
cases, a doctor is consulted only after the disease has advanced. A person
should therefore be alert to any physical change that may be a symptom of
cancer. Early detection greatly increases the chances of a cure.
Possible carcinogens
(cancer-causing agents)
Carcinogen or agent
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Site of cancer
|
Tobacco
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Lungs, mouth, oesophagus, bladder
|
Alcohol
|
Liver
|
Salted fish
|
Nose and throat
|
Scalding tea
|
Oesophagus
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Sunlight
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Skin
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Ionizing radiation
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Bone marrow
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Tar, soot
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Skin, scrotum, lung, bladder, digestive organs
|
Hepatitis B virus + afla- toxins
|
Liver
|
Epstein-Barr virus
|
Lymphatic system
|
Papilloma virus
|
Cervix
|
Human T-cell leukaemia virus
|
Bone marrow
|
Asbestos
|
Lung, lining of chest and abdominal cavity
|
Benzene
|
Bone marrow
|
Vinyl chloride
|
Liver
|
Source:
P. Pritchard
led.). Oncology for Nurses and
Health Care Professionals vol. 1, 2nd
edn.
London 1988, 33-5.
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