The general term used to refer to a malignant TUMOUR, irrespective of the tissue of origin. ‘Malignancy’ indicates that (i) the tumour is capable of progressive growth, unrestrained by the capsule of the parent organ, and/or (ii) that it is capable of distant spread via LYMPHATICS or the bloodstream, resulting in development of secondary deposits of tumour known as ‘metastases’. (See METASTASIS.) Microscopically, cancer cells appear different from the equivalent normal cells in the affected tissue. In particular they may show a lesser degree of differentiation (i.e. they are more ‘primitive’), features showing they are replicating more rapidly than normal and are aligned in a disorganised way in relationship to other cells or blood vessels. The diagnosis of cancer usually depends upon the observation of these features by examining biopsies (tissue removed surgically) under a microscope.
Cancers are classified according to the type of cell from which they are derived as well as the organ of origin. Hence cancers arising within the bronchi, often collectively referred to as ‘lung cancer’, include both ADENOCARCINOMAS, derived from EPITHELIUM (surface tissue), and carcinomas from glandular tissue. Sarcomas are cancers of connective tissue, including bone and cartilage. The behaviour of cancers and their response to therapy vary widely depending on this classification as well as on numerous other factors such as how large the cancer is, how fast the cells grow and how well defined they are. It is entirely wrong to see cancer as a single disease entity with a universally poor prognosis. For example, fewer than one-half of women in whom breast cancer (see BREASTS, DISEASES OF) is discovered will die from the disease, and 80 per cent of children with lymphoblastic LEUKAEMIA can be cured.
In most Western countries, cancer is the second most important cause of death after heart disease and accounts for 20–25 per cent of all deaths. There is wide international variation in the most frequently encountered types of cancer, reflecting the importance of environmental factors in the development of the disease. In the UK as well as the US, carcinoma of the BRONCHUS is the most common. Since it is usually inoperable at the time of diagnosis, it is even more strikingly the leading cause of cancer deaths. In women, breast cancer is the most common malignant disease, accounting for a quarter of all cancers. Other common sites are: colon, rectum, prostate and bladder in males; colon, rectum, uterus, ovary and pancreas in females.
In 2013, of the more than 163,000 people in the UK who died of cancer, nearly 36,000 had the disease in their respiratory system, over 11,400 in the breast, a similar number in the prostate and nearly 16,000 in the bowel. The incidence of cancer varies with age; the older a person is, the more likely it is that he or she will develop the disease. The over-85s have an incidence about nine times greater than those in the 25–44 age group. There are also differences in incidence between sexes: for example, more men than women develop lung cancer, though the incidence in women is rising as the effects of smoking become apparent. The death rate from cancer is falling in people under 75 in the UK, a trend largely determined by the cancers which cause the most deaths: lung, breast, colorectal, stomach and prostate.
The clearest link is that between lung cancer and tobacco smoking, with over 80% of deaths preventable if those individuals had not smoked. In most cases the causes of cancer remains poorly defined, though a genetic cause may be inferred where there is a family history of cancer. Even where there is no family history, research shows that cancer results from acquired changes in the genetic make-up of a particular cell or group of cells, which ultimately lead to a failure of the normal mechanisms regulating their growth. It appears that in most cases a cascade of changes is required for cells to behave in a truly malignant fashion; the critical changes affect specific key GENES, known as oncogenes, which are involved in growth regulation. (See APOPTOSIS.)
Since small genetic errors occur within cells at all times – most but not all of which are repaired – it follows that some cancers may develop as a result of an accumulation of random changes which cannot be attributed to environmental or other causes. The environmental factors known to cause cancer, such as radiation and chemicals (including tar from tobacco, asbestos, etc.), do so by increasing the overall rate of acquired genetic damage. Certain viral infections can induce specific cancers (e.g. HEPATITIS B virus and HEPATOMA, EPSTEIN BARR virus and LYMPHOMA) probably by inducing alterations in specific genes. HORMONES may also be a factor in the development of certain cancers such as those of the prostate and breast. Where there is a particular family tendency to certain types of cancer, it now appears that one or more of the critical genetic abnormalities required for development of that cancer may have been inherited. Where environmental factors such as tobacco smoking or asbestos are known to cause cancer, then health education and preventive measures can reduce the incidence of the relevant cancer. Cancer can also affect the white cells in the blood and is called LEUKAEMIA.
Many cancers can be cured by surgical removal if they are detected early, before there has been spread of significant numbers of tumour cells to distant sites. Important within this group are breast, colon and skin cancer (melanoma). The probability of early detection of certain cancers can be increased by screening programmes in which (ideally) all people at particular risk of development of such cancers are examined at regular intervals. Routine screening for CERVICAL CANCER and breast cancer (see BREASTS, DISEASES OF) is currently practised in the UK. The effectiveness of screening people for cancer is, however, controversial. Apart from questions surrounding the reliability of screening tests, they undoubtedly create anxieties among the subjects being screened and false positive results may result in overtreatment.
If complete surgical removal of the tumour is not possible because of its location or because spread from the primary site has occurred, an operation may nevertheless be helpful to relieve symptoms (e.g. pain), and to reduce the bulk of the tumour remaining to be dealt with by alternative means such as RADIOTHERAPY or CHEMOTHERAPY. In some cases radiotherapy is preferable to surgery and may be curative, for example, in the management of tumours of the larynx or of the uterine cervix. Certain tumours are highly sensitive to chemotherapy and may be cured by the use of chemotherapeutic drugs alone. These include testicular tumours, LEUKAEMIAS, LYMPHOMAS and a variety of tumours occurring in childhood. These tend to be rapidly growing tumours composed of primitive cells which are much more vulnerable to the toxic effects of the chemotherapeutic agents than the normal cells within the body.
Unfortunately neither radiotherapy nor currently available chemotherapy provides a curative option for the majority of common cancers if surgical excision is not feasible. New effective treatments in these conditions are needed and this is most likely to come about from methods of IMMUNOTHERAPY such as MONOCLONAL ANTIBODY DRUGS specifically designed to combat particular malignant cell lines; ‘immune checkpoint inhibitors’, BIOLOGICS designed to ‘take the brakes off’ the immune system to help it recognise and combat malignant cells; and cancer vaccines. Where cure is not possible, there often remains much that can be done for the cancer-sufferer in terms of control of unpleasant symptoms such as pain. Many of the most important recent advances in cancer care relate to such ‘palliative’ treatment, and include the establishment in the UK of palliative care hospices.