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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 ?
In the present clinical study, there were three tumor cases which affected third eyelid, upper eyelid and temporal limbus and tentative diagnosis was made as sarcoid and squamous cell carcinoma based on gross appearance and clinical examination.
The data included patients' age clinical presentation operative findings size of tumor gross appearance and microscopic features.
The location of the omentum (present or not around the appendix) and its gross appearance were noted (thickenend / indurated or edematous).
When the results of TLC were compared with gross appearance we observed that in 86 patients the diagnosis of TLC was correct while in 14 patients the diagnosis was incorrect.
Due to unavailability of intra operative monitoring of PTH, gamma probe or frozen section the adenomatous parathyroid gland was identified per operatively in these cases by the gross appearance and large size of the gland compared to the other three.
The overall gross appearance of EGC and AGC in male or female patients was consistent with those of the total population for both Chinese and Japanese patients.
Our results indicate that a normal liver ultrasound, liver function tests, and gross appearance does not exclude the presence of significant liver disease," the researchers, led by Dr.
One of the newer, relatively rare variants, the cotyledonoid leiomyoma, (3) has been the subject of a number of recent case reports, (4,5) a result, no doubt, of its dramatic placental-like gross appearance which belies its otherwise benign nature.
The gross appearance of chordoma is an expansile soft-tissue mass with a lobulated appearance and cystic spaces filled with gelatinous material.
The gross appearance of the hairy leukoplakia lesion may be variable, resulting in possible misdiagnosis.
The gross appearance of Microciona adult summer sponge and of dormant or winter sponge is shown in Figure 1A and 1B, respectively.