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Invasion means that instead of remaining contained within its own area of the body the cancer is capable of extending outside this area. A cancer of the lungs is capable of growing into other sites in the chest. A cancer in the intestine is capable of growing through the wall of the intestine and sticking to other organs. This process of invasion may carry the cancer beyond the realm of a single surgical operation and prevent a surgeon from carrying out an operation that might lead to a cure. If extensive invasion has occurred, successful curative surgery is usually not possible.

The final feature of a cancer, which is the one that makes it so difficult to cure in so many cases, is its ability to spread to distant sites of the body. This process is called metastasis and metastasis occurs by the spread of cancer cells through the fluid circulations of the body, either tissue fluids called lymph or in the bloodstream. Once a cancer has spread in this way it cannot usually be cured by an operation because so many different sites within the body will be involved. In these situations the potential for cure rests upon the much less fully developed treatments with drugs of either a chemical or biological kind.

Growth, invasion and spread do not occur to the same extent in all cancers. Cancers that arise in different organs of the body have very different patterns of spread and for each individual site from which a cancer can originate there will be a different chance of spread. For instance, lung cancers have a very high probability of spread. At the time of their diagnosis about three quarters of patients will have evidence of spread when they are assessed by their doctors and when tests are carried out. Therefore, only about one quarter of lung cancer patients will have any chance of cure as a result of an operation. Spread may have occurred even though it is not detected by the tests. On the other hand, cancers that arise in the brain cause damage by local growth but very rarely spread outside the brain. The chance of spread for the other commoner cancers such as breast, bowel, bladder and gynaecological cancers is intermediate between these two extremes and spread has generally occurred in between one third and two thirds of all the patients who develop the disease.

These statistics allow plans to be made for assessing and treating most patients. However, even when the diagnosis is made, the doctor will be unable to tell an individual patient with certainty what the outcome of his or her cancer is going to be. If there appears to have been no spread of the cancer and it has been completely removed by an operation, cautious optimism will be due in many cases. If, however, after the tests are done and an operation has been carried out, it is discovered that some of the cancer remains, the outcome will depend upon the chance of treating the remaining cancer by means other than surgery. For some cancers, such as Hodgkin’s disease or testicular cancer in men, the chances of effecting a cure in these circumstances will remain high.

Even when cures are not achieved, there remains great uncertainty about the outcome for an individual patient. The progression of a cancer varies tremendously between individual patients. Some cancers grow rapidly and others very slowly. In some cases little change may appear over a period of several years whereas in others rapid changes will occur over a few months. Although examination of the cancer in the laboratory can give important guidance, the information obtained in this way is, as yet, not very precise and there will often be a good deal of uncertainty in the information that is given to a patient.