Tumour
Also spelled
tumor, also called neoplasm, a mass of abnormal tissue that arises without
obvious cause from preexisting body cells, has no purposeful function, and is
characterized by a tendency to autonomous and unrestrained growth. Tumours are
quite different from inflammatory or other swellings because the cells in
tumours are abnormal in their appearance and other characteristics. Abnormal
cells--the kind that generally make up tumours--differ from normal cells in
having undergone one or more of the following alterations: (1) hypertrophy, or an increase
in the size of individual cells; this feature is occasionally encountered in
tumours but occurs commonly in other conditions; (2) hyperplasia, or an increase
in the number of cells within a given zone; in some instances it may constitute
the only criterion of tumour formation; (3) anaplasia, or a regression
of the physical characteristics of a cell toward a more primitive or
undifferentiated type; this is an almost constant feature of malignant tumours,
though it occurs in other instances both in health and in disease.
In some instances
the cells of a tumour are normal in appearance, faithful reproductions of their
parent types; 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 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 may that tumour be expected to grow.
Malignancy refers
to the ability of a tumour to ultimately 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 masses of
cells (new tumours) in new and distant sites, precludes 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 assume physical characteristics other than that of lumps. They may appear
as ulcers, indurated 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 nerve trunks or the
destruction of bone.
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, however.
All benign
tumours tend to remain localized at the site of origin. Many benign tumours are
encapsulated. The capsule consists 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
accretion, pushing aside the adjacent tissues without involving them
intimately. Malignant tumours, by contrast, do not usually possess a capsule;
more important for the affected individual, they invade the surrounding
tissues, thereby becoming fixed to them and making surgical removal more
difficult or risky.
Among the major
types of benign tumours are the following: lipomas, which are composed of fat
cells; angiomas, which are composed of blood or lymphatic vessels; osteomas,
which arise from bone; chondromas, which arise from cartilage; and adenomas,
which arise from glands. For malignant tumours, see cancer. For plant tumours,
see gall.