Malignant tumours arise from cells that have transformed into cancer cells. Cancer cells no longer perform the functions that are characteristic of the tissue from which they have developed. They often consist of rapidly growing, undifferentiated tissues with irregular cell structure and infiltrate and destroy their environment through diffuse growth. They also have the ability to settle in other parts of the body. The uninhibited spread of malignant tumours usually leads to the destruction of the surrounding tissue and to adverse side effects.