In some instances the cells of a tumour are normal in appearance; the differences between them and normal body cells can be discerned only with some difficulty. Such tumours are more often benign than not. Other tumours are composed of cells that appear different from normal adult types in size, shape, and structure; they usually belong to tumours that are malignant. Such cells may be bizarre in form or may be arranged in a distorted manner. In more extreme cases, the cells of malignant tumours are described as primitive, or undifferentiated, because they have lost the appearance and functions of the particular type of (normal) specialized cell that was their predecessor. As a rule, the less differentiated a malignant tumour’s cells are, the more quickly the tumour may be expected to grow.
Malignancy refers to the ability of a tumour ultimately to cause death. Any tumour, either benign or malignant in type, may produce death by local effects if it is appropriately situated. The common and more specific definition of malignancy implies an inherent tendency of the tumour’s cells to metastasize (invade the body widely and become disseminated by subtle means) and eventually to kill the patient unless all the malignant cells can be eradicated.
Metastasis is thus the outstanding characteristic of malignancy. Metastasis is the tendency of tumour cells to be carried from their site of origin by way of the circulatory system and other channels, which may eventually establish these cells in almost every tissue and organ of the body. In contrast, the cells of a benign tumour invariably remain in contact with each other in one solid mass centred on the site of origin. Because of the physical continuity of benign tumour cells, they may be removed completely by surgery if the location is suitable. But the dissemination of malignant cells, each one individually possessing (through cell division) the ability to give rise to new tumours in new and distant sites, requires complete eradication by a single surgical procedure in all but the earliest period of growth.
A mass of tumour cells usually constitutes a definite localized swelling that, if it occurs on or near the surface of the body, can be felt as a lump. Deeply placed tumours, however, may not be palpable. Some tumours, and particularly malignant ones, may appear as ulcers, hardened cracks or fissures, wartlike projections, or a diffuse, ill-defined infiltration of what appears to be an otherwise normal organ or tissue.
Pain is a variable symptom with tumours. It most commonly results from the growing tumour pressing on adjacent nerve tracts. In their early stages all tumours tend to be painless, and those that grow to a large size without interfering with local functions may remain painless. Eventually, however, most malignant tumours cause pain by the direct invasion of nerves or the destruction of bone.
All benign tumours tend to remain localized at the site of origin. Many benign tumours are enclosed by a capsule consisting of connective tissue derived from the structures immediately surrounding the tumour. Well-encapsulated tumours are not anchored to their surrounding tissues. These benign tumours enlarge by a gradual buildup, pushing aside the adjacent tissues without involving them intimately. Malignant tumours, by contrast, do not usually possess a capsule; they invade the surrounding tissues, making surgical removal more difficult or risky.
A benign tumour may undergo malignant transformation, but the cause of such change is unknown. It is also possible for a malignant tumour to remain quiescent, mimicking a benign one clinically, for a long time. The regression of a malignant tumour to benign is unknown.
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