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neoplasm or tumor, 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., cancers, 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 leukemias 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.

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A malignant epithelial tumor.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


1. any malignant tumour derived from epithelial tissue
2. another name for cancer
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
Adenocystic Carcinoma: CECT Scan Shows Heterogeneous Enhancing Irregular Mass Lesion with Non-Enhancing Necrotic/Cystic Areas Within Mass in Left Parotid Giand.
Moderately differentiated 71 50.71 squamous cell carcinoma Adenocarcinoma 2 1.43 Invasive carcinoma 2 1.43 Olfactory Neuroblastoma 2 1.43 Adenocystic carcinoma 1 0.71 Verrucous carcinoma 1 0.71 Keratinising squamous 1 0.71 cell carcinoma Premalignant Leukoplakia 9 6.43 Dysplasia 2 1.43 Erythroplakia 1 0.71 Hyperplastic epithelium 1 0.71 Benign Angiofibroma 2 1.43 Spindle cell tumours 2 1.43 Papilloma 2 1.43 Cavernous haemangioma 1 0.71 Capillary haemangioma 1 0.71 Haemangioendothelioma 1 0.71 Pleomorphic adenoma 1 0.71 Leiomyoma 1 0.71 Table 3.
Of the 218 epidermoid carcinomas, 146 (66.9%) were squamous cell carcinomas (which accounted for 50.2% of the total of 291 cases), 43 (19.7%) were undifferentiated carcinomas (14.8% of the total), 17 (7.8%) were adenocystic carcinomas (5.8% of the total), and 12 (5.5%) were adenocarcinomas (4.1% of the total).