HOW DOES A TUMOUR GROW TO A CANCER?
Most aspects of normal cellular behaviour are subverted in the development of tumours. These changes include a switch to aberrant signalling in both pro- and anti-proliferative pathways, the acquisition of the capacity to avoid cell death and to replicate indefinitely, and perturbation of the normal metabolic profile. In addition, dynamic interactions between tumours and normal cells in their environment can progressively co-opt inflammatory and immune responses so that they support rather than inhibit tumour growth and can recruit host endothelium to provide a blood supply. The defining feature of malignant tumours is the ability of cells to migrate through adjacent tissue and eventually to colonise distant sites. This process of metastasis is poorly understood at the molecular level. It remains essentially untreatable and is the major cause of cancer death.
Cancer is a group of diseases characterised by abnormal cell growth through which cells may acquire the potential to disperse (metastasise) from the site of origin (a primary tumour) to other sites in the body (secondary tumours). A word tumour comes from the Latin ‘tumour’ referring to the swelling that occurs as a consequence of these abnormal growths and is now used interchangeably with ‘neoplasm’, meaning new or abnormal cell growth. This definition of neoplasm leads to a major division of cancers into malignant and benign. The terms ‘tumour’ and ‘cancer’ have also come to be used synonymously but a distinction might be made in that metastatic cancer occurs because a tumour has acquired the capacity to invade its surroundings, the first step in spreading to secondary sites. This involves the destruction of other cells, critically some that make up the vessels of the circulatory (blood and lymphatic) systems. Once the tumour cell can get into the circulation it can be carried to other locations: it has become malignant. The implication, of course, is that there are tumours that are not malignant.
Because the term neoplasm simply means new, abnormal growth it refers to both malignant tumours and benign tumours. This second category lacks the distinguishing features of malignancy – that is, the cells can’t invade surrounding tissues and therefore they do not metastasise. Benign tumours are usually encapsulated, that is, surrounded by a membrane that restrains their invasive capacity. They can arise in any tissue, their cells generally resemble those of the tissue of origin, in contrast to malignant cells, and for the most part, they are pretty harmless. Despite being slow growing, they can reach a considerable size (as big as a grapefruit) they compress other tissues (e.g. blood vessels or the brain) can have serious effects that require surgical treatment. In addition, some benign tumours can have harmful, indirect effects if they occur in endocrine tissues and result in abnormal levels of hormone production, for example, adenomas in the thyroid, adrenocortex or pituitary glands. This effect suggests that the neoplasms are of cell types that normally make up the tissues in which they arise. This is often a feature of benign tumours to the extent that usually they do not give rise to any symptoms and are only detected by chance. The general name for such tumours is hamartoma, the counterpart of which is choristoma when comprised of normal cells growing in an abnormal location.