After heart disease, cancer is the most common cause of death in developed nations. Most deadly cancers result from the proliferation of cells in a body called a tumor. Cells within the tumor gradually mutate away from their proper function, taking up space in the body without providing any benefit. If this occurs in certain tissues, such as the brain, the growth of the tumor can itself be deadly, but more frequently tumors are dangerous primarily because the cells within the tumor continue to mutate, ultimately detaching from the tumor and spreading through the body in a process called metastasis.
Prior to metastasis, surgical removal of a tumor will generally eliminate the cancer, although depending on its characteristics removing a tumor may require that some surrounding tissue be removed and that the body undergo onerous radiation treatments or chemotherapy in order to remove every potentially tumor-forming cell. After metastasis, large numbers of tumors can grow throughout the body, making removal unlikely. Many chemicals can kill cancer cells throughout the body, but all kill some fraction of non-cancerous cells as well. The same is true of radiation. For this reason, treatment of cancer after metastasis consists primarily of full body treatments that shrink tumors and delay its progress but which also kill many of the body's cells, causing the sickness and misery typically associated with dying from cancer.
Some cancers follow very different patterns from the one discussed above. For instance, leukemia is cancer of the bone marrow. Because the cells in question start out in a liquid they never form tumors. As a result there is no metastasis, but also no benign period. Treatment must be entirely via the chemotherapeutic model. Fortunately, for most varieties of leukemia such treatments are substantially more effective than they are in most other types of cancer.
Over the last 40 years, cancer research has been the single best-funded field within science. Expenditures for the treatment of cancer have increased rapidly and exponentially, and now come to over one percent of the US GDP. It is typically asserted that this has lead to substantial progress in cancer treatment. Evidence for this comes from declining total cancer mortality and increased five-year survival rates for most forms of cancer. Unfortunately, a more careful analysis of the evidence shows that both of these statistics are misleading. Although prognoses have definitely improved for a few types of cancer, especially leukemia, and in recent years breast cancer, the vast majority of the decline in cancer mortality appears to be due to reduced smoking leading to a reduction in the rate at which lung cancer occurs.
The vast majority of the observed increase in five year survival seems to come from earlier diagnosis, due partly to improved detection technologies but primarily to more aggressive screening. Earlier diagnosis leads to the removal of tumors prior to metastasis, contributing to the decline in breast cancer mortality and presumably saving some people from other cancers, but it has a much larger impact on five year survival than on human well being, as it leads to survival being measured from an earlier point in the disease's progression. It also has serious costs, such as the huge number of invasive biopsies and surgeries that occur due in the process of testing for pathology or reducing risk of future pathology, and in suffering and incapacitation due to chemotherapy and radiation therapy undertaken by elderly people who would have died in any even prior to the cancer becoming malignant. Improvements in chemotherapies have reduced the degree of incapacitation they bring about, but have far from eliminated it, and even the placebo effect of a cancer diagnosis can greatly impair a person's quality of life for years if not permanently. It is extremely plausible that they are, on the net, not worth it, especially in the light of the fact that for all of the effort that has been put into cancer research and treatment, the age adjusted mortality due to adult cancers other than lung cancer has not changed substantially in the United States over the last 50 years.