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tissue composed of cells that grow in an abnormal way. Normal tissue is growth-limited, i.e., cell reproduction is equal to cell death. Feedback controls limit cell division after a certain number of cells have developed, allowing for tissue repair but not expansion. Tumor cells are less responsive to these restraints and can proliferate to the point where they disrupt tissue architecture, distort the flow of nutrients, and otherwise do damage.

Tumors may be benign or malignant. Benign tumors remain localized as a discrete mass. They may differ appreciably from normal tissue in structure and excessive growth of cells, but are rarely fatal. However, even benign tumors may grow large enough to interfere with normal function. Some benign uterine tumors, which can weigh as much as 50 lb (22.7 kg), displace adjacent organs, causing digestive and reproductive disorders. Benign tumors are usually treated by complete surgical removal. Cells of malignant tumors, i.e., cancerscancer,
in medicine, common term for neoplasms, or tumors, that are malignant. Like benign tumors, malignant tumors do not respond to body mechanisms that limit cell growth.
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, have characteristics that differ from normal cells in other ways beside cell proliferation. For example, they may be deficient in some specialized functions of the tissues where they originate. Malignant cells are invasive, i.e., they infiltrate surrounding normal tissue; later, malignant cells metastasize, i.e., spread via blood and the lymph system to other sites.

Both benign and malignant tumors are classified according to the type of tissue in which they are found. For example, fibromas are neoplasms of fibrous connective tissue, and melanomas are abnormal growths of pigment (melanin) cells. Malignant tumors originating from epithelial tissue, e.g., in skin, bronchi, and stomach, are termed carcinomas. Malignancies of epithelial glandular tissue such as are found in the breast, prostate, and colon, are known as adenocarcinomas. Malignant growths of connective tissue, e.g., muscle, cartilage, lymph tissue, and bone, are called sarcomas. Lymphomas and leukemiasleukemia
, cancerous disorder of the blood-forming tissues (bone marrow, lymphatics, liver, spleen) characterized by excessive production of immature or mature leukocytes (white blood cells; see blood) and consequently a crowding-out of red blood cells and platelets.
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 are malignancies arising among the white blood cells. A system has been devised to classify malignant tissue according to the degree of malignancy, from grade 1, barely malignant, to grade 4, highly malignant. In practice it is not always possible to determine the degree of malignancy, and it may be difficult even to determine whether particular tumor tissue is benign or malignant.


An aberrant new growth of abnormal cells or tissues; a tumor.
References in periodicals archive ?
Because of the heterogeneity inherent in this neoplasm, the clinical and prognostic utility of epithelial subtypes is limited.
Solid pseudopapillary neoplasm of the pancreas: A single institution experience of 14 cases.
Diagnosis of spindle cell neoplasm of neural origin was established by strong, diffuse positive reactivity against S100 and negative reactivity for both CD68 and smooth muscle actin (ASMA) (Fig 1).
SPT is a rare neoplasm accounting for <3% of all pancreatic exocrine neoplasms.
In our institution, two appendiceal neoplasms have been reported among 2043 appendicectomy specimens in the last two years.
Unlike epithelial ovarian cancers, malignant germ cell neoplasms are typically diagnosed at early stages.
Of these 20 follow-up cases, 10 cases were diagnosed as follicular neoplasm, 4 cases papillary carcinoma, 2 cases medullary carcinoma, and 4 cases suspicious of malignancy on cytopathological examination.
To our knowledge, a total of 23 cases of urachal mucinous neoplasms of low malignant potential have been reported in the literature [3-5].
Solid pseudopapillary neoplasm cells positively stain for vimentin, progesterone receptor, [beta]-catenin, CD56, neuron-specific enolase (NSE), CD 10, cyclin D1 and, negative membranous E-cadherin.
The number of reported cases of this rare neoplasm has increased since the report of Ave cases by Kloppel (2) in 1981.
We selected as incident cases all children living in Cali from 1977 to 2011 with the first malignant neoplasms diagnosed in each subject, with the exception of benign tumors of the CNS that were also included.