Cancer tissue has a distinctive appearance under the microscope. Among the distinguishing traits are a large number of dividing cells, variation in nuclear size and shape, variation in cell size and shape, loss of specialized cell features, loss of normal tissue organization, and a poorly defined tumor boundary. Immunohistochemistry and other molecular methods may characterise specific markers on tumor cells, which may aid in diagnosis and prognosis.

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Biopsy and microscopical examination can also distinguish between malignancy and hyperplasia, which refers to tissue growth based on an excessive rate of cell division, leading to a larger than usual number of cells but with a normal orderly arrangement of cells within the tissue. This process is considered reversible. Hyperplasia can be a normal tissue response to an irritating stimulus, for example callus.

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Dysplasia is an abnormal type of excessive cell proliferation characterized by loss of normal tissue arrangement and cell structure. Often such cells revert to normal behavior, but occasionally, they gradually become malignant.

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The most severe cases of dysplasia are referred to as “carcinoma in situ.” In Latin, the term “in situ” means “in place”, so carcinoma in situ refers to an uncontrolled growth of cells that remains in the original location and shows no propensity to invade other tissues. Nevertheless, carcinoma in situ may develop into an invasive malignancy and is usually removed surgically, if possible.

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Heredity

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Most forms of cancer are “sporadic”, and have no basis in heredity. There are, however, a number of recognised syndromes of cancer with a hereditary component, often a defective tumor suppressor allele. Examples are:

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   * certain inherited mutations in the genes BRCA1 and BRCA2 are associated with an elevated 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.

   * Retinoblastoma in young children is an inherited cancer

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Lifestyle factors

The incidence of lung cancer is highly correlated with smoking. Source:NIH.

The incidence of lung cancer is highly correlated with smoking. Source:NIH.

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The most consistent finding, over decades of research, is the strong association between tobacco use and cancers of many sites. Hundreds of epidemiological studies have confirmed this association. Further support comes from the fact that lung cancer death rates in the United States have mirrored smoking patterns, with increases in smoking followed by dramatic increases in lung cancer death rates and, more recently, decreases in smoking followed by decreases in lung cancer death rates in men. Lifestyle choices cause cancer: tobacco, diet, exercise, alcohol, tanning choices, and certain sexually transmitted diseases are the major risks. “Most cancers are related to known lifestyle factors.”[13]

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There is also a growing body of research that correlates cancer incidence with the lower levels of melatonin produced in the body when people spend more time in bright-light conditions, as happens typically in the well-lit nighttime environments of the more developed countries.[14] This effect is compounded in people who sleep fewer hours and in people who work at night, two groups that are known to have higher cancer rates.[15]