Cancer

Cancer occurs when abnormal cells grow out of control. Normal cells in the body grow, divide, and die, and as we become older, our cells divide at a decreasing rate. Cancer cells, however, continue to grow and divide unchecked by the body, and they outlive normal cells. Cancers are capable of both invasion, or spreading into adjacent tissue, and metastasis, or traveling and settling into new, noncontiguous parts of the body. Cancer cells commonly form a tumor, or mass of cells, but they may also circulate in the bloodstream. Carcinogenesis describes the transformation of normal cells to neoplastic cells or abnormal cells that grow uncontrollably. Neoplasia is the process by which neoplasms develop from normal tissue. Cancers generally develop from a single neoplastic cell, commonly referred to as clonal expansion. Dysplasia refers to the early stages of neoplasia in which clonal expansion of abnormal cells occur.

Cancers are usually named for the site in which the neoplastic cells originated, even in cases of metastasis to other organs. Cancers are also usually named for their histology or morphologic features. Carcinomas are cancers of the epithelium and consist of cells found on external surfaces, internal cavity linings, and glandular linings, with cancers named squamous cell carcinoma, transitional cell carcinoma, and adenocarcinoma, respectively. Sarcomas are cancers of the mesenchyme, from which supporting tissue, including connective tissue, bone, cartilage, and muscle, is derived, andareoftenusedwithaprefixdenotingthe tissue of origin. Some cancers are given eponymous names, including Hodgkin lymphoma and Burkitt lymphoma.