Tumour (Lat. tumor, 'a swelling') is the term applied in medicine to new formations characterised generally by their 'independent growth and almost independent life, so little do they appear to concern themselves with the interest of the body as a whole; 'their tendency to continuous growth; and 'the tenacity with which they maintain their hold upon the individual, rarely disappearing or even diminishing in size.' They are thus separated on the one hand from simple hypertrophy of organs normally present, and on the other from inflammatory swellings, and the enlargements associated with syphilis, tubercle, glands, &c. Many cysts, however, though grouped with tumours, are merely enlargements of normal cavities (see CYST).
The most important division of tumours is that into innocent or benign and malignant tumours. The malignant tumours tend (1) to grow continuously into the tissues surrounding them, irrespective of the natural boundary lines between different structures; (2) to recur after removal; (3) to cause secondary growths of the same nature in the neighbouring lymphatic glands, (4) and in distant parts of the body. Malignant tumours of different structures and in different organs possess these characters in varying degrees, and may not manifest them all; but if malignant at all they are dangerous, and certain sooner or later, if unchecked, to destroy life. Innocent tumours, on the other hand, are sharply marked off from the surrounding tissues, tend to grow by pushing aside, rather than by invading adjacent structures, do not return after removal, and do not lead to the occurrence of similar growths in other parts, though several or even many of them may be present in the same patient. They may cause death by interfering, on account of their situation, with the function of important organs, by becoming the seat of ulceration and hæmorrhage, &c.; but they have not the intrinsic dangers of the other group.
With regard to the causes of tumours little is known. In some cases they can be traced to the effects of injury or of long-continued irritation of a part, though the reason why they should so arise in a particular individual is often inexplicable; in some cases heredity seems to be an important factor in their occurrence. But 'in the large majority of cases they are formed without any apparent cause.' The organs most frequently affected are the womb and the female breast; hence they are on the whole more common in the female sex. But some organs common to both sexes are affected almost exclusively in males (e.g. the lip by cancer).
With regard to treatment, simple tumours demand removal, generally speaking, only when they give rise to discomfort or disfigurement; malignant tumours should be removed, if removal is practicable, at the earliest possible moment. With few exceptions removal by the knife, éraseur, or cautery is the only method of dealing with them.
The solid simple tumours resemble more or less closely the fully developed structures of the body, generally of the part in which they grow, and are classified accordingly. The most important groups are the fatty tumours (lipoma), the most frequent of all; the fibrous (fibroma); the cartilaginous (enchondroma); the bony (exostosis); the vascular (Nævus, q.v.); the warty (papilloma); and the glandular (adenoma). The malignant tumours are either aberrant developments of epithelium (carcinoma; see CANCER), or resemble imperfectly developed tissues (see SARCOMA).