Cancer is a disease characterized by disorderly division of cells, combined with the malignant behavior of these cells. Malignant cancer cells tend to spread, either by direct growth into adjacent tissue through invasion, or by implantation into distant sites by metastasis (the process whereby cancer cells can move through the bloodstream or lymphatic system to distant locations). Cancer may affect people at all ages, but risk tends to increase with age. It is one of the principal causes of death in developed countries.

There are many theories about the cause of cancer. At least six important factors contribute to cancer pathogenesis, including failure of apoptosis, overactivation of oncogenes, inactivation of tumor suppressor genes, cell cycle activation of quiescent cells, acquisition of metastatic behavior by malignant cells, disordered responses to cellular growth factors, and immune system surveillance failure. Epigenetic changes are increasingly recognized as critical to cancer pathogenesis; these events, such as DNA methylation, can profoundly influence the activation and deactivation of critical genes. microRNAs are increasingly being studied as important contributors to cancer etiology.

Environmental stimuli, or carcinogens, such as tobacco smoke, radiation, chemicals, or infectious agents, can cause cancer. The interaction between carcinogens and the host genome, to explain why some patients get cancer after exposure to a known carcinogen, while others don’t, is the subject of immense recent research.

Cancer is usually classified according to the tissue of origin more than the location of manifestations. Prognosis in most cases depends on the original staging of the disease, though increasing use of molecular markers is leading to individually tailored treatments. A definitive diagnosis usually requires the histologic examination of a tissue biopsy specimen by a pathologist, though the initial indication of malignancy can be symptoms or radiographic imaging abnormalities. Most cancers can be treated and some cured, depending on the specific type, location, and stage. Once diagnosed, cancer is usually treated with a combination of surgery, chemotherapy and radiotherapy. As research develops, treatments are becoming more specific for the type of cancer pathology. Drugs that target markers specific to a particular form of cancer, thereby minimizing damage to normal tissue, already exist for several types of malignancies.

When normal cells are damaged beyond repair, they are eliminated by apoptosis (A). Cancer cells avoid apoptosis and continue to multiply in an unregulated manner (B).

DiseasesDB 28843

MedlinePlus 001289

MeSH C04