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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
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(also neoplasm), the excessive, pathological growth of plant, animal, or human tissues in which the cells qualitatively change and lose their capacity for differentiation. Tumor cells transmit their pathological properties to their offspring and continue to multiply even after the causative factors cease to act. In contrast to pseudotumors, which result from injury, inflammation, or circulatory disturbance, true tumors enlarge as a result of multiplication of their own cells. True tumors include leu-kemias. Oncology, which is the study of tumors, has devoted most attention to tumors in man, mice, rats, hamsters, and dogs.
For purposes of clinical and morphological description, a distinction is made between benign and malignant tumors. Benign tumors enlarge to distend and sometimes also compress the surrounding tissues, whereas malignant tumors invade and destroy the surrounding tissues while injuring the blood and lymphatic vessels. Tumor cells can be carried throughout the body in the bloodstream or in the lymph and can settle in various organs and tissues, forming metastases. Benign tumors do not metastasize, but they may be dangerous because of their location, for example, they can compress vital tissues when they develop in the brain. The occurrence, rate, and degree of metastasis depend on the immunobiological state of the organism.
The first sign of tumor growth in a tissue is the appearance of a small number of cells that exhibit a tendency toward uncontrolled division. Tumor growth proceeds through stages of irregular hyperplasia, or a disorderly increase in the number of cells, focal growth, benign growth, and malignant growth; the stages immediately preceding malignancy—focal and benign growth—are called precancerous. The evidence of many clinical observations and animal experiments confirms that every cancer develops through a precancerous stage. Tumor development is progressive, that is, it proceeds through stages of increasing malignancy. The autonomy of the tumor increases in that the tumor becomes more independent of the bodily systems that normally control cell division.
The parenchyma of a tumor consists of tumorous tissue, while the stroma forms from surrounding connective tissue. A tumor is named by adding the Greek suffix -oma to the name of the tissue from which it originated. For example, a tumor from cartilage is a chondroma and a tumor from muscle a myoma. Some tumors have special names. For instance, a malignant tumor from connective tissue is called a sarcoma (from Greek sarx, gen. case sarkos, “meat”) because in cross section its tissue resembles fish meat. A malignant epithelioma is called a carcinoma (from Greek karkinos, “cancer”). In many countries the word “cancer” is applied to all malignant tumors regardless of the source. Soviet oncologists use “cancer” to refer only to malignant tumors of epithelial origin. Some tumors are named after the organ or organ part from which they originate; for example, an insuloma is a tumor from the islands of Langerhans, in the pancreas.
Experimental studies played a major role in elucidating the nature and causes of tumors. The autonomous growth of tumors was demonstrated by transplantation experiments in which tumors from one animal survived and grew for many years on secondary animal hosts. Tumor strains, consisting of tumors that are repeatedly transplanted over a long period of time, are used to study the properties of tumors and to develop and test methods of treating tumors, especially chemotherapeutic methods. Modern experimental oncology also makes use of explanation studies, in which tumorous tissues and cells are excised for cultivation outside the organism. Experimental studies showed that many tumors can be caused by viruses. However, cancer and most other tumors are not considered infectious in the ordinary sense of the word.
Evidence shows that individuals who work at jobs that involve prolonged contact with certain substances may develop cancer; for example, chimney sweeps are prone to skin cancer, and uranium miners tend to develop cancer of the lungs. These observations led to experiments that showed that several substances belonging to different classes of chemical compounds may cause cancer and other tumors. These substances are called carcinogens. Tumors can be caused not only by exogenous substances, that is, by those that appear in the environment, but also by endogenous carcinogens, which originate in the organism itself. Endogenous carcinogens can form, for example, as a result of disturbances of protein metabolism in which derivatives of the amino acid tryptophan or tyrosine are present and as a result of disturbances in steroid metabolism, especially disturbances in the metabolism of steroid sex hormones. Many human tumors have a dyshormonal origin, including those of the breast, prostate, uterine body, uterine muscles, and, occasionally, ovaries and testicles. In addition to being caused by viruses and chemicals, tumors can be caused by radiation. Thus, tumors may result from a variety of physical, chemical, and biological factors. The internal mechanisms by which these factors operate have not been fully explained.
The systemic reaction of the body strongly influences the origin and development of tumors. This reaction depends on both genotype and environmental factors. Some rare forms of tumors and precancerous states are definitely inheritable in man, for example, retinoblastoma and xeroderma pigmentosum. The latter readily becomes malignant after exposure to ultraviolet radiation. However, hereditary transmission of most tumors has not been demonstrated. Hereditary factors mainly predispose an organism to tumor growth, by altering the body’s response to tumorigenic influences. The general condition of the body, which is dependent on such factors as nutrition, plays a decisive role in the realization of the specific effect of these influences.
Tumors are clinically diagnosed by instrumental methods, including roentgenography and endoscopy, and by morphological, immunological, and chemical methods. Early diagnosis is essential because treatment is most effective in the early stages of the disease. A conclusive diagnosis is often based on biopsy results.
Tumors are treated with surgery, radiation therapy, and chemotherapy, and combinations of these methods. The location, structure, and stage of development of the tumors are taken into account when selecting a method. Surgery is the most important method in the treatment of tumors of the alimentary canal; radiation is used against tumors of the lower lip, skin, and uterine cervix; a combination of surgery, radiation, and chemotherapy is used with breast cancer. Chemotherapy, the newest approach, is effective as an independent method in, for example, chorionepithelioma, or choriocarcinoma, a malignant tumor of the uterine body in young women.
Epidemiological studies are of value in oncology and tumor prevention. Statistics on cancer morbidity and mortality rates are gathered in many countries. In the USSR, patients with malignant tumors must be registered. This information is very helpful in studying the prevalence of tumors in different parts of the country and in detecting possible etiological relationships between a particular oncological disease and natural or ethnographic factors.
Malignant tumors are the second leading cause of death in the economically developed countries, including the USSR, the USA, Great Britain, France, and Sweden. In descending order of frequency of occurrence, the most common cancers in the majority of countries are those of the lung, uterus, female breast, and male esophagus. Malignant tumors mostly affect older persons. An increase in the average life expectancy and improved methods of diagnosis may explain the apparent increase in tumor morbidity and mortality that has been observed. Special correction factors in the form of standardized indexes are used in the statistical analysis of oncological data. Worldwide statistics revealed significant unevenness in the distribution of the various types of tumors among different nations, different peoples, and different isolated populations. Skin cancer, especially on exposed parts of the body, was found to occur most frequently in hot countries, as a result of excessive Ultraviolet radiation. Cancer of the mouth, tongue, and gums is prevalent among Indians, Pakistanis, and inhabitants of some other Asian countries as a result of these people’s harmful habit of chewing betel. Penile cancer is common in some Asian and South American countries, probably because of careless personal hygiene.
Epidemiological studies have shown that the incidence of cancer in a particular geographical location changes when the living conditions of the people change. For example, among Englishmen who migrate to Australia, the USA, or South Africa, lung cancer is more common than among the indigenous populations of these countries but less common than in the general population of Great Britain itself. Stomach cancer is more widespread in Japan than in the United States. The Japanese who permanently reside in the United States, for example, in San Francisco, are more prone to stomach cancer than other American citizens but are less prone to stomach cancer and at a later age than their fellow countrymen in Japan.
Tumors can either be prevented from occurring (hygienic prevention) or from developing (clinical prevention). To prevent the occurrence of tumors, environmental conditions can be improved by completely or partially eliminating the carcinogens that constitute a potential threat to man. It has been well established that cancer develops more slowly and less frequently in environments that contain smaller amounts of carcinogen.
Early diagnosis and healing of precancers is essential in curtailing tumor development. The principal method of clinical prevention is systematic mass screening of the entire population and clinical observation of individual groups. Special methods are employed to prevent certain tumors. For example, regular cytological examination of vaginal smears permits prompt detection of cancer of the uterine cervix, and careful self-examination is an effective means of preventing breast cancer.
REFERENCESUspekhi v izuchenii raka, vols. 1–10. Edited by L. M. Shabad. Moscow, 1955–71. (Translated from English.)
Rukovodstvo po obshchei onkologii. Edited by N. N. Petrov. Leningrad, 1961.
Zil’ber, L. A. Virusogeneticheskaia teoriia vozniknoveniia opukholei. Moscow, 1968.
Shabad, L. M. Endogennye blastomogennye veshchestva. Moscow, 1969.
Shabad, L. M. O tsirkuliatsii kantserogenov v okruzhaiushchei srede. Moscow, 1973.
Klinicheskaia onkologiia, vols. 1–2. Edited by N. N. Blokhin and B. E. Peterson. Moscow, 1971.
L. M. SHABAD
In animals. Tumors are commonly found in all species. Their frequency increases with age and is under the influence of such parameters as species, breed, and climate. Tumors are especially common in dogs and chickens. Similar tumors in different animals are morphologically and clinically identical, but the frequency with which individual organs are affected varies. For example, mammary tumors are extremely rare in cows but are quite common in dogs, accounting for 35–40 percent of all canine tumors. Pulmonary fibromatosis frequently arises in sheep, and tumors of the eye are frequent in cattle. Stomach cancer is exceptionally rare in animals. Animal tumors are diagnosed by clinical examination, which includes cytological and roentgenological studies. The principal method of treatment is surgery, which is sometimes combined with chemotherapy and the administration of hormones.
P. F. TEREKHOV
In plants. Plant tumors may be caused by fungi, bacteria, viruses, insects, nematodes, or low temperature. They are biologically distinct from animal tumors and arise as a result of hyperplasia (intensified division) or hypertrophy (intensified growth) of plant cells. Plant tumors generally appear on roots, tubers, stems, and, less commonly, leaves. Typical examples are root canker of fruit trees, potato canker, and beet tuberculosis. Tumors impede the flow of nutrient solutions, especially when the main root or root collar is affected. Diseased plants often die prematurely. Control measures are aimed at eliminating the underlying causes of the tumors.