Cancer is a group of diseases in which cells grow and divide without respect to normal boundaries or limits. Cancer may affect people at all ages, but the risk of acquiring the more common varieties tends to increase with age. Cancer is a diverse class of diseases that differ widely in their causes and biology. The common thread in all known cancers is the acquisition of abnormalities in the genetic material of the cancer cell and its progeny. Research into the pathogenesis of cancer can be divided into three broad areas of focus. The first area of research focuses on the agents and events that cause or facilitate genetic changes in cells destined to become cancer. Second, genes or mutations that may cause it, and the third focus is on the consequences of those genetic changes on the biology of the cell, both in generating the defining properties of a cancer cell, and in facilitating additional genetic events, leading to further progression of the cancer. Some researched areas of interest are:
Nearly all cancers are caused by abnormalities in the altered cells. These abnormalities may be due to the effects of carcinogens, such as tobacco smoke, radiation, chemicals, or infectious agents. Other cancer-promoting abnormalities may be acquired through errors in DNA replication, or maybe inherited, and thus present in all cells from birth.
Genetic abnormalities found in cancer typically affect two general classes of genes. Cancer-promoting oncogenes are often activated in cancer cells, giving those cells new properties, such as hyperactive growth and division, protection against programmed cell death, loss of respect for normal tissue boundaries, and the ability to become established in diverse tissue environments. Tumor suppressor genes are often inactivated in cancer cells, resulting in the loss of normal functions in those cells, such as accurate DNA replication, control over the cell cycle, orientation and adhesion within tissues, and interaction with protective cells of the immune system.
Cancer is usually classified according to the tissue from which the cancerous cells originate, as well as the normal cell type they most resemble. Once diagnosed, cancer is usually treated with a combination of surgery, chemotherapy and radiotherapy. The prognosis of cancer patients is most influenced by the type of cancer, as well as the stage, or extent of the disease.
The following terms may be used to designate forms of cancer:
Neoplasm: an abnormal proliferation of genetically altered cells.
Malignant neoplasm: interchangeable with cancer.
Tumor: can be any swelling or mass. However, the vast majority of 'tumors' in common usage are in fact called neoplasms.
Benign tumor:a tumor that has self-limiting growth and does not invade other tissues nor metastasize and is usually not cancerous.
Pre-malignancy or pre-cancer: A non-invasive neoplasm that may not form an obvious mass or lesion but has the potential to progress to cancer if left untreated.
Cancers maybe classified by the type of cell that resembles the tumor and, therefore, the tissue presumed to be the origin of the tumor. Examples of general categories include:
Carcinoma: Malignant tumors derived from epithelial cells. This group represents the most common cancers, including the common forms of breast, prostate, lung and colon cancer.
Sarcoma: Malignant tumors derived from connective tissue, or mesenchymal cells.
Lymphoma and leukemia: Malignancies derived from hematopoetic (blood-forming) cells
Adult forms of cancer:
In the U.S. and other developed countries, cancer is presently responsible for about 25% of all deaths. On a yearly basis about 0.5% of the population is diagnosed with cancer. Statistics shown below are for adults in the United States, and vary in other countries:
Diagnosing Cancer
Most cancers are initially recognized because known signs or symptoms appear.
Cancer symptoms can be divided into three main groups: