Cancer
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Cancerany of a group of related diseases characterized by uncontrolled multiplication and disorganized growth of the affected cells; it may arise in any of the body's tissues. Cancer cells infiltrate and destroy adjacent tissues, eventually gain access to the circulatory system, are transported to distant parts of the body, and ultimately destroy the host. Concomitant with their capacity for unrestrained growth, cancer cells and the tissues they constitute lose their normal appearance, as viewed through a microscope, and assume aberrant functions.
Not all abnormal growths are malignant; those that are not are referred to as benign tumours. In contrast to malignant growths, benign tumours consist of an orderly growth of cells that often are identical to or very closely resemble their normal counterparts. They are not aggressive and do not invade surrounding tissues, spread to distant sites, or kill the host. Such tumours are usually surrounded by a capsule of fibrous connective tissue. Cancer was known in antiquity.
Malignant tumours have been found in Egyptian and Pre-Columbian mummies, about 5,000 and 2,400 years old, respectively. They are documented in ancient medical writings, such as the Edwin Smith and Ebers papyruses, both written about 3,500 years ago. Cancer is also seen in other species, such as birds, reptiles, and fishes. The precise equivalence between cancers in higher living forms and apparently similar growths in invertebrates and plants remains to be established.
Statistics of cancer incidence and mortality show striking geographic differences, varying significantly from country to country; and within countries differences of incidence and mortality occur between the sexes, various ethnic groups, and various occupations.
Epidemiologic analysis of such statistics continues to provide valuable clues and insights into the myriad factors that appear to be involved in the causation of cancer. This article examines the main causes of cancer, the spread of cancer, the various types that are known today, and the methods of treatment that are being used.
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[edit] Causes
It is now known that cancer can be caused by a variety of factors acting either singly or in concert. These include a wide variety of chemical substances, various types of ionizing radiation, and various classes of viruses. This knowledge has accrued from a composite of epidemiologic studies of cancer in humans and from experimental studies in the laboratory. Although much is known about how cancer is caused, the precise mechanism or group of mechanisms involved continues to elude researchers.
Chemicals Chemicals numbering in the hundreds are known to induce cancer in laboratory animals, and some of these have also been shown to be carcinogenic for humans. While it is well established that long exposure to certain chemicals gives rise to cancer in humans, it is most difficult to determine accurately what proportion of human cancer is due to such exposure. The difficulty arises from the fact that the length of time between exposure and the appearance of cancer is usually prolonged, lasting some 20-30 years; that exposure is more often than not to a variety of chemicals, so that identifying the carcinogen may be difficult, if not impossible; and that carcinogenesis, i.e., the induction and development of cancer, is a process that involves many factors and many phases.
Experiments on laboratory animals have established that the majority of carcinogenic chemicals are not capable of inducing cancer in their original form. Chemical carcinogens are toxic compounds that are foreign to the host. Once they gain entrance into the body by ingestion, inhalation, or absorption, they are modified by metabolic processes in the host's tissues to forms that are less toxic and water-soluble. The modification is an attempt by the host to detoxify the foreign chemicals and eliminate them by excretion. This capacity to detoxify such substances is an adaptive property of a wide spectrum of living systems, including humans, which affords them a selective advantage to exist in a hostile environment. As in most cases, however, this protective property is not foolproof, and in the process of detoxification certain forms of the carcinogen may arise that are even more toxic and active than the parent compound. In this instance the host renders a less toxic compound into one that causes injury to its cells and may eventually cause them to become cancerous. Highly reactive forms of such chemicals interact with vital macromolecules in the host's cells, causing them to be chemically altered. It is believed that when deoxyribonucleic acid (DNA), the genetic material of cells, is so altered that its expression is accompanied by uncontrolled growth, the cell undergoes a transformation that eventually leads to the development of cancer. DNA appears to be the most probable macromolecule whose alteration leads to cancer, because once cells become malignant their transformed behaviour is passed on to subsequent cell generations, indicating that it is a heritable change. There is evidence, however, that alterations of nongenetic macromolecules capable of regulating cell growth may also be involved in the causation of cancer.
The alteration of DNA is one phase of carcinogenesis. The second phase is a prolonged one during which the genetically altered cell loses its ability to grow in a regulated fashion. During this phase the altered cell, not yet expressing its malignant potential, apparently is influenced by other factors, including noncarcinogenic chemicals; dietary components, such as fat; or substances produced by the host, such as hormones. Since this phase is prolonged and subject to manipulation, it is a major focus of contemporary cancer research; it is hoped that persons at high risk of developing cancer (presumably with cells genetically altered by a chemical, but not yet cancerous) may be treated to prevent overt cancer from developing. Certain chemical substances, such as antioxidants and vitamin A, are particularly promising in this regard.
Despite the fact that cancer is a prevalent disease in modern society, quantitative experimental evidence in animals suggests that, following the exposure of cells to a carcinogenic chemical, the ultimate development of cancer is a relatively rare event. This is no doubt due to the fact that some genetic mutations lead to the death of the affected cells, so that no cancer can develop. A second, more important, reason is due to the ability of cells to "repair" altered or damaged DNA. In this process the segments of damaged DNA are excised, and identical single strands of the required segments are synthesized; these are then spliced into the defect, reestablishing its continuity. If the synthesis has accurately copied the DNA segment as it was prior to damage, there will be no adverse effects. If, on the other hand, cell replication occurs before the damage is repaired, the altered DNA is copied and the damage is amplified. When such cells undergo division the genetic defect is passed on to their descendants. It is reassuring to know that repair processes appear to be uniformly present in living cells and are very efficient, rapidly repairing damaged DNA with great fidelity.
[edit] Occupational Chemicals
The association of "substances" in the environment with the development of cancer was suggested in 1775 by Sir Percivall Pott, who reported the frequent occurrence of cancer of the scrotum in chimney sweeps chronically exposed to flue dusts. Pott suggested that this might be due to the prolonged exposure of scrotal skin to the dust. Since then, there have been numerous instances in which the development of cancer has been associated with specific occupations.
Numerous carcinogenic chemical hazards have been identified in a variety of extraction and processing industries. These include polycyclic hydrocarbons present in coal tar and its derivatives, such as pitch, tar oils, and creosote, and in products of the combustion and distillation of coal, oil, shale, lignite, and petroleum. In the past, skin cancer took the lives of many long-time workers in these industries. Fumes inhaled by workers during coke-oven operations and in refineries have been associated with high incidences of lung cancer. Benzene, a product of coal-tar distillation, may affect blood-forming tissues and is suspected of being a carcinogen capable of inducing leukemia. Various metals have been implicated as carcinogens for the lung and several other body sites among copper-ore miners and smelters of nickel and cobalt ores. Beta-naphthylamine, a chemical once widely used in the manufacture of aniline dyes, has been shown to be a carcinogen for the urinary bladder.
Asbestos has been established as a carcinogen for the lung and for the mesothelium (membrane) that lines body cavities. Workers chronically exposed to dust containing asbestos fibres have an incidence of lung cancer 10 times the normal rate. (It is noteworthy that the risk of cancer is increased 90-fold in asbestos workers if they also smoke. This is an excellent example of two agents acting synergistically to induce cancer at a higher incidence and often in a shorter time than either agent does alone.) Although a serious effort has been made to reduce industrial exposure to asbestos, in some parts of the world occupational pollution remains responsible for a debilitating and progressive form of lung disease (asbestosis), as well as cancer, in asbestos workers.
[edit] Environmental chemicals and pollution
The environment, which includes the atmosphere, land, seas, lakes, and rivers, reflects the activities of society. Environmental pollutants include the myriad effluents of daily living, industrial as well as naturally occurring. Among the greatest atmospheric pollutants are the gaseous and particulate emissions that range from the massive outputs of industry and motor transport, measuring millions of tons annually, to individual puffs of cigarette smoke.
Cigarette smoke has been shown to contain numerous compounds that are known to cause cancer in experimental animals and that appear to be strongly linked to human cancer, especially cancer of the lung. In addition, tobacco smoke has been implicated in the causation of cancer of the mouth, and to a lesser extent the esophagus, pancreas, biliary system, and urinary bladder. Cigarette smoke also has been shown to contain a number of cocarcinogens, substances that appear to enhance the effect of carcinogens when they are administered concomitantly.
Increasing evidence has accrued to suggest that the release of polychlorinated hydrocarbons and certain insecticides into the environment may pose a carcinogenic hazard. Some of these compounds have enjoyed such widespread use that significant areas of land and bodies of water have been contaminated. This, coupled with the fact that such compounds, once ingested by humans, are stored in body fat and released and metabolized very slowly, has made their removal from the environment extremely important, even though it is a difficult, prolonged, and expensive undertaking.
The effect of environmental pollution can best be illustrated by describing the phenomenon of bioconcentration and its impact on the food chain. Widely used insecticides, which metabolically degrade very slowly and are highly soluble in fat, are a case in point. After being applied on land for agricultural uses, such compounds are washed by rains into streams, rivers, and lakes, where they are ingested by microscopic life forms, which serve as food for fish that are, in turn, the major food source for larger fish and aquatic birds. Since these compounds are soluble in fat, after being ingested by an animal they are stored and concentrated in the animal's body fat. Repeated feeding eventually leads to high concentrations of the compounds in the animal's body, so that its subsequent ingestion by a larger predator, perhaps by humans, presents the predator with a significant level of the compound. This can pose a health hazard so serious that, for example, fishermen on a contaminated lake are warned not to eat the fish they catch. The magnitude of the problem is illustrated by the insecticide DDT, high levels of which persist in the environment and in the bodies of humans, despite the fact that its use was restricted worldwide by the early 1970s.
Food additives are another source of environmental chemicals that has caused concern. Although these have been the object of dispute and have given rise to the "natural food" fad, there is no evidence that food additives cause human cancer. In fact, some food additives, especially those that protect foods from becoming rancid, have actually been shown to prevent chemically induced cancer in experimental animals. An additive that remains a matter of concern, however, is sodium nitrite, which is widely used to preserve processed meats. It has been shown that nitrite can react in the stomach with amines, which arise from the digestion of meat, to form nitrosamines, a group of compounds that are potent carcinogens for certain laboratory animals. These compounds are formed in such minute amounts in the stomach that some researchers doubt that they pose a significant carcinogenic hazard for humans. Similarly, the demonstration that certain compounds formed by the burning of meat are carcinogenic for animals must be placed in proper perspective. Carcinogenesis experiments in animals usually require continuous exposure to high levels of chemicals to obtain statistically valid results in their relatively short life span of a year or two; extrapolation of these results to the effect on human health should be approached most carefully.
Problems of greater importance are the naturally occurring carcinogens, which constitute an important hazard in certain environments. One of these, aflatoxin, is formed by Aspergillus flavus, a mold that is widely distributed and is a frequent contaminant of improperly stored nuts, grains, meals, and certain other foods.
In certain areas of Africa the high incidence of liver cancer in humans appears to coincide with their ingestion of foods highly contaminated with aflatoxin. Such correlations must be interpreted conservatively, however, since these populations are also often plagued by viral hepatitis B, which also has been linked to liver cancer. In Japan and parts of China similar correlations exist, but again their interpretation must be tempered because of the presence of viral hepatitis B and other factors.
[edit] Incense smoke causes cancer
Incense burning a serious cause to be concerned in temples and homes.
- Exhaust from motor vehicles affecting our air and environment.
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- Exhaust from motor vehicles affecting our air and environment.
In Taiwan health authorities are concerned by the high concentration of toxic chemical produced by incense burning in the temples. see [[1]] and a Dr. Lin and his teams of researchers collected sample of air inside temple and compared with pollution at traffic places <inter-section> and found the air is 19 times more polluted with polycyclic aromatic hydrocarbons (PAHs), a large group of highly carcinogenic chemicals that are released when certain substances are burnt. This is a likely cause of cancer, which many researchers have yet failed <reluctance> to identify due to religious implications.
