Cancer,types of cancer
Cancer,types of cancer
Cancer: canceris a class ofdiseases in which a group ofcells display uncontrolled growth, invasion that intrudes upon and destroys adjacent tissues, and sometimesmetastasis, or spreading to other locations in the body vialymph orblood. These three malignant properties of cancers differentiate them frombenign tumors, which do not invade or metastasize.Researchers divide the causes of cancer into two groups: those with an environmental cause and those with ahereditary genetic cause. Cancer is primarily anenvironmental disease, though genetics influence the risk of some cancers.
Classification:Cancers are classified by thetype of cell that the tumor resembles and is therefore presumed to be the origin of the tumor. These types include:
Carcinoma: Cancer derived fromepithelial cells.This group includes many of the most common cancers, including those of thebreast,prostate,lung andcolon.
Sarcoma: Cancer derived fromconnective tissue, ormesenchymal cells.
Lymphoma andleukemia: Cancer derived from hematopoietic (blood-forming) cells
Germ cell tumor: Cancer derived frompluripotent cells. In adults these are most often found in thetesticle andovary, but are more common in babies and young children.
Blastoma: Cancer derived from immature "precursor" or embryonic tissue. These are also commonest in children.
Cancers are usually named using -carcinoma, -sarcoma or -blastoma as a suffix, with the Latin or Greek word for theorgan or tissue of origin as the root. For example, a cancer of the liver is calledhepatocarcinoma; a cancer of fat cells is called aliposarcoma. For some common cancers, the English organ name is used. For example, the most common type ofbreast cancer is calledductal carcinoma of the breast.
Signs and symptoms:
Local symptoms: are restricted to the site of the primary cancer. They can include lumps or swelling (tumor),hemorrhage (bleeding from the skin, mouth or anus),ulceration andpain. Although local pain commonly occurs in advanced cancer, the initial swelling is often painless.
Metastatic symptoms: are due to the spread of cancer to other locations in the body. They can include enlargedlymph nodes (which can be felt or sometimes seen under the skin),hepatomegaly (enlarged liver) orsplenomegaly (enlarged spleen) which can be felt in theabdomen, pain orfracture of affected bones, andneurological symptoms.
Mystemic symptoms: occur due to distant effects of the cancer that are not related to direct or metastatic spread. Some of these effects can includeweight loss (poor appetite andcachexia),fatigue, excessivesweating (especiallynight sweats),anemia (low blood count) and other specific conditions termedparaneoplastic phenomena. These may be mediated byimmunological orhormonal signals from the cancer cells.
Causes:Cancers are primarily an environmental disease with 90-95% of cases attributed to environmental factors and 5-10% due to genetics.Environmental, as used by cancer researchers, means any cause that is notgenetic.
Diet and exercise:Diet,physical inactivity, andobesity are related to approximately 30-35% of cancer cases.In the United States excess body weight is associated with the development of many types of cancer and is a factor in 14-20% of all cancer death.Physical inactivity is believed to contribute to cancer risk not only through its effect on body weight but also through negative effects onimmune system andendocrine system.
Diets that are low in vegetables, fruits and whole grains, and high in processed or red meats are linked with a number of cancers.A highsalt diet is linked togastric cancer,aflatoxin B1, a frequent food contaminate, with liver cancer, andBetel nut chewing with oral cancer.This may partly explain differences in cancer incidence in different countries for examplegastric cancer is more common in Japan with its high salt dietandcolon cancer is more common in the United States. Immigrants develop the risk of their new country, often within one generation, suggesting a substantial link between diet and cancer.
Infection:Worldwide approximately 18% of cancers are related toinfectious diseases.This proportion varies in different regions of the world from a high of 25% in Africa to less than 10% in the developed world.Viruses are usual infectious agents that cause cancer butbacteria andparasites may also have an effect.
A virus that can cause cancer is called anoncovirus. These includehuman papillomavirus (cervical carcinoma),Epstein-Barr virus (B-cell lymphoproliferative disease andnasopharyngeal carcinoma),Kaposi's sarcoma herpesvirus (Kaposi's Sarcoma and primary effusion lymphomas),hepatitis B andhepatitis C viruses (hepatocellular carcinoma), andHuman T-cell leukemia virus-1 (T-cell leukemias). Bacterial infection may also increase the risk of cancer, as seen inHelicobacter pylori-inducedgastric carcinoma.
Radiation:Up to 10% of cancers are related to radiation exposure either ionizing or nonionizing.Sources ofionizing radiation, includemedical imaging, andradon gas. Radiation can cause cancer in most parts of the body, in all animals, and at any age, although radiation-induced solid tumors usually take 1015 years, and up to 40 years, to become clinically manifest, and radiation-induced leukemias typically require 210 years to appear.Some people, such as those withnevoid basal cell carcinoma syndrome orretinoblastoma, are more susceptible than average to developing cancer from radiation exposure.Children and adolescents are twice as likely to develop radiation-induced leukemia as adults; radiation exposure before birth has ten times the effect.Ionizing radiation is not a particularly strong mutagen.Residential exposure to radon gas, for example, has similar cancer risks aspassive smoking.
Prolonged exposure toultraviolet radiation from thesun can lead tomelanoma and other skin malignancies.Clear evidence establishes ultraviolet radiation, especially the medium waveUVB, as the cause of most non-melanomaskin cancers, which are the most common forms of cancer in the world.Non-ionizingradio frequency radiation frommobile phones,electric power transmission, and other similar sources has also been proposed as a cause of cancer, but there is currently little established evidence of such a link.
Heredity:Less than 0.3% of the population are carriers of a genetic mutation which has a large effect on cancer risk.They cause less than 3-10% of all cancer.Some of thesesyndromes include:
certain inherited mutations in the genesBRCA1 andBRCA2 with a more than 75% risk ofbreast cancer andovarian cancer.
tumors of various endocrine organs inmultiple endocrine neoplasia (MEN types 1, 2a, 2b)
Li-Fraumeni syndrome (various tumors such asosteosarcoma, breast cancer,soft tissue sarcoma,brain tumors) due to mutations ofp53
Turcot syndrome (brain tumors and colonic polyposis)
Familial adenomatous polyposis an inherited mutation of theAPC gene that leads to early onset ofcolon carcinoma.
Hereditary nonpolyposis colorectal cancer (HNPCC, also known as Lynch syndrome) can include familial cases ofcolon cancer, uterine cancer,gastric cancer, andovarian cancer, without a preponderance ofcolon polyps.
Retinoblastoma, when occurring in young children, is due to a hereditary mutation in the retinoblastoma gene.
Down syndrome patients, who have an extrachromosome 21, are known to develop malignancies such asleukemia andtesticular cancer, though the reasons for this difference are not well understood.
Physical agents:Some substances cause cancer primarily through their physical, rather than chemical, effects on cells.A prominent example of this is prolonged exposure toasbestos fibers. Asbestos is a naturally occurring, fibrous rock that causesmesothelioma, a type of lung cancer.Other substances in this category include both naturally occurring and synthetic asbestos-like fibers, such aswollastonite,attapulgite,glass wool, androck wool, are believed to have similar effects.Nonfibrous particulate materials that cause cancer include powdered metalliccobalt andnickel, andcrystalline silica (quartz,cristobalite, andtridymite).Usually, physical carcinogens must get inside the body (such as through inhaling tiny pieces) and require years of exposure to develop cancer.
Physical trauma and inflammation:Physical trauma resulting in cancer is relatively rare.Claims that breaking bone resulted in bone cancer, for example, have never been proven.Similarly, physical trauma is not accepted as a cause for cervical cancer, breast cancer, or brain cancer.One accepted source is frequent, long-term application of hot objects to the body. It is possible that repeated burns on the same part of the body, such as those produced bykanger and kairo heaters (charcoalhand warmers), may produce skin cancer, especially if carcinogenic chemicals are also present.Frequently drinking scalding hot tea may produce esophageal cancer.
Generally, it is believed that the cancer arises, or a pre-existing cancer is encouraged, during the process of repairing the trauma, rather than the cancer being caused directly by the trauma.However, repeated injuries to the same tissues might promote excessive cell proliferation, which could then increase the odds of a cancerous mutation. There is no evidence thatinflammation itself causes cancer.
Hormones:Somehormones cause cancer, primarily by encouragingcell proliferation.Hormones are an important cause of sex-related cancers such as cancer of the breast,endometrium, prostate, ovary, andtestis, and also ofthyroid cancer andbone cancer.An individual's hormone levels are mostly determined genetically, so this may at least partly explains the presence of some cancers that run in families that do not seem to have any cancer-causing genes.For example, the daughters of women who have breast cancer have significantly higher levels ofestrogen andprogesterone than the daughters of women without breast cancer. These higher hormone levels may explain why these women have higher risk of breast cancer, even in the absence of a breast-cancer gene.
However, non-genetic factors are also relevant: Obese people have higher levels of some hormones associated with cancer, and a higher rate of those cancers.Women who takehormone replacement therapy have a higher risk of developing cancers associated with those hormones.On the other hand, people who exercise far more than average have lower levels of these hormones, and lower risk of cancer.Osteosarcoma may be caused bygrowth hormones.Some treatments and prevention approaches leverage this cause by artificially reducing hormone levels, and thus discouraging hormone-sensitive cancers.
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