Cancer: cancer is a class of diseases in which a group of cells display uncontrolled growth, invasion that intrudes upon and destroys adjacent tissues, and sometimes metastasis, or spreading to other locations in the body via lymph orblood. These three malignant properties of cancers differentiate them from benign tumors, which do not invade or metastasize.Researchers divide the causes of cancer into two groups: those with an environmental cause and those with a hereditary genetic cause. Cancer is primarily an environmental disease, though genetics influence the risk of some cancers.
Classification:Cancers are classified by the type of cell that the tumor resembles and is therefore presumed to be the origin of the tumor. These types include:
- Carcinoma: Cancer derived from epithelial cells.This group includes many of the most common cancers, including those of the breast,prostate, lung and colon.
- Sarcoma: Cancer derived from connective tissue, or mesenchymal cells.
- Lymphoma and leukemia: Cancer derived from hematopoietic (blood-forming) cells
- Germ cell tumor: Cancer derived from pluripotent cells. In adults these are most often found in the testicle and ovary, 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 the organ or tissue of origin as the root. For example, a cancer of the liver is called hepatocarcinoma; a cancer of fat cells is called a liposarcoma. For some common cancers, the English organ name is used. For example, the most common type of breast cancer is called ductal 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 and pain. 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 enlarged lymph nodes (which can be felt or sometimes seen under the skin),hepatomegaly (enlarged liver) or splenomegaly (enlarged spleen) which can be felt in theabdomen, pain or fracture of affected bones, and neurological 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 include weight loss (poor appetite and cachexia),fatigue, excessive sweating (especially night sweats), anemia (low blood count) and other specific conditions termed paraneoplastic phenomena. These may be mediated byimmunological or hormonal 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 not genetic.
Diet and exercise:Diet, physical inactivity, and obesity 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 on immune system and endocrine 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 to gastric cancer, aflatoxin B1, a frequent food contaminate, with liver cancer, and Betel nut chewing with oral cancer.This may partly explain differences in cancer incidence in different countries for example gastric cancer is more common in Japan with its high salt diet and colon 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 to infectious 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 but bacteria andparasites may also have an effect.
A virus that can cause cancer is called an oncovirus. These include human papillomavirus (cervical carcinoma), Epstein-Barr virus (B-cell lymphoproliferative disease and nasopharyngeal carcinoma), Kaposi’s sarcoma herpesvirus (Kaposi’s Sarcoma and primary effusion lymphomas), hepatitis B and hepatitis C viruses (hepatocellular carcinoma), and Human T-cell leukemia virus-1 (T-cell leukemias). Bacterial infection may also increase the risk of cancer, as seen in Helicobacter pylori-induced gastric carcinoma.
Radiation:Up to 10% of cancers are related to radiation exposure either ionizing or nonionizing.Sources of ionizing radiation, include medical imaging, and radon 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 10–15 years, and up to 40 years, to become clinically manifest, and radiation-induced leukemias typically require 2–10 years to appear.Some people, such as those with nevoid basal cell carcinoma syndrome or retinoblastoma, 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 as passive smoking.
Prolonged exposure to ultraviolet radiation from the sun can lead to melanoma and other skin malignancies.Clear evidence establishes ultraviolet radiation, especially the medium wave UVB, as the cause of most non-melanoma skin cancers, which are the most common forms of cancer in the world.Non-ionizing radio frequency radiation from mobile 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 these syndromes include:
- certain inherited mutations in the genes BRCA1 and BRCA2 with a more than 75% risk of breast cancer and ovarian cancer.
- tumors of various endocrine organs in multiple endocrine neoplasia (MEN types 1, 2a, 2b)
- Li-Fraumeni syndrome (various tumors such as osteosarcoma, breast cancer, soft tissue sarcoma, brain tumors) due to mutations of p53
- Turcot syndrome (brain tumors and colonic polyposis)
- Familial adenomatous polyposis an inherited mutation of the APC gene that leads to early onset of colon carcinoma.
- Hereditary nonpolyposis colorectal cancer (HNPCC, also known as Lynch syndrome) can include familial cases of colon cancer, uterine cancer, gastric cancer, and ovarian cancer, without a preponderance of colon polyps.
- Retinoblastoma, when occurring in young children, is due to a hereditary mutation in the retinoblastoma gene.
- Down syndrome patients, who have an extra chromosome 21, are known to develop malignancies such as leukemia and testicular 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 to asbestos 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 as wollastonite, attapulgite, glass wool, and rock wool, are believed to have similar effects.Nonfibrous particulate materials that cause cancer include powdered metallic cobalt and nickel, and crystalline 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 by kanger and kairo heaters (charcoal hand 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 that inflammation itself causes cancer.
Hormones:Some hormones cause cancer, primarily by encouraging cell proliferation.Hormones are an important cause of sex-related cancers such as cancer of the breast, endometrium, prostate, ovary, and testis, and also of thyroid cancer and bone 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 of estrogen and progesterone 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 take hormone 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 by growth hormones.Some treatments and prevention approaches leverage this cause by artificially reducing hormone levels, and thus discouraging hormone-sensitive cancers.