Malignant tumours grow more rapidly than innocent tumours, and tend to infiltrate their surroundings by sending out prolongations or offshoots; they are therefore liable to recur after an operation which is restricted to the removal of the main tumour.  They are not encapsulated, although they may appear to be circumscribed by condensation of the surrounding tissues; they are rarely multiple at the outset, but show a marked tendency to spread to other parts of the body.  Fragments of the parent tumour may become separated and be carried off in the lymph or blood-stream and deposited in other parts of the body, where they give rise to secondary growths.  Malignant tumours tend to invade and destroy the overlying skin or mucous membrane, and thus give rise to bleeding ulcers; if the tumour tissue protrudes through the gap in the skin, it is said to fungate.  In course of time they give rise to a condition of ill-health or cachexia, the patient becoming pale, sallow, feverish, and emaciated, probably as a result of chronic poisoning from the absorption of toxic products from the tumour.  They ultimately destroy life, it may be by their local effects, such as ulceration and haemorrhage, by favouring the entrance of septic infection, by interfering with the function of organs which are essential to life, by cachexia, or by a combination of these effects.

The situation of a malignant tumour exercises considerable influence on the rapidity, as well as on the mode, in which it causes death.  Some cancers, such as that known as “rodent,” show malignant features which are entirely local, while others, such as melanotic cancer, exhibit a malignancy characterised by rapid generalisation of growths throughout the body.  Tumours that are structurally alike may show variations in malignancy, according to their situation and to the age of the patient, as well as to other factors which are as yet unknown.

In attempting to arrive at a conclusion as to the innocence or malignancy of any tumour, too much reliance must not be placed on its histological features; its situation, rate of growth, and other clinical features must also be taken into consideration.  It cannot be too emphatically stated that there is no hard-and-fast line between innocent and malignant growths; there is an indefinite transition from one to the other.  The possibility of the transformation of a benign into a malignant tumour must be admitted.  Such a transformation implies a change in the structure of the growth, and has been observed especially in fibrous and cartilaginous tumours, in tumours of the thyreoid gland, and in uterine fibroids.  The alteration in character may take place under the influence of injury, prolonged or repeated irritation, incomplete removal of the benign tumour by operation, or the altered physiological conditions of the tissues which attend upon advancing years.

After a tumour has been removed by operation it should as a routine measure be subjected to microscopical examination; the results are often instructive and sometimes other than what was expected.