Cancer is a group of diseases in which abnormal cells grow without normal regulation and can invade surrounding tissues or spread to distant parts of the body. It comprises more than 100 distinct diseases rather than a single condition. Many cancers form solid masses called tumors, whereas some blood cancers generally do not. Malignant tumors differ from benign tumors, which do not invade nearby tissues or spread to other organs, although benign growths can still cause serious harm. (cancer.gov)
Biological basis
Normal tissue maintenance depends on regulated cell division and the removal of damaged or unnecessary cells. Cancer develops when changes in DNA disrupt genes controlling these processes. Such changes can arise through errors during cell division, environmental damage, or inherited variants. Calling cancer a genetic disease therefore does not mean that every cancer is inherited. Different cells within the same tumor can carry different genetic alterations. (cancer.gov)
Cancer-associated alterations commonly affect growth-promoting genes, growth-restraining genes, and genes responsible for DNA repair. Cancer cells may ignore signals that normally stop growth or initiate apoptosis, evade the immune system, and stimulate blood-vessel growth that supplies a tumor with nutrients. These abnormalities vary between cancers and provide targets for some treatments. (cancer.gov)
Classification and spread
Cancers are classified by their original anatomical site and by the tissue or cell type from which they arise. Major categories include:
- Carcinomas, which originate in epithelial tissues covering body surfaces and lining organs.
- Sarcomas, which arise in connective or supportive tissues, including bone, muscle, and fat.
- Leukemias, which originate in blood-forming tissues.
- Lymphomas, which arise from lymphocytes.
- Multiple myeloma, which originates in plasma cells. (training.seer.cancer.gov)
Metastasis is the spread of cancer from its original site to a distant location. Metastatic disease retains the identity of the primary cancer: breast cancer that spreads to the lungs is metastatic breast cancer, not a new primary lung cancer. This distinction matters because cancers arising in different tissues have different biological characteristics and treatment approaches. (training.seer.cancer.gov)
Causes and risk factors
Cancer usually develops through a multistage process involving interactions between genetic susceptibility and external exposures. Established cancer-causing agents include tobacco smoke, asbestos, alcohol, ultraviolet radiation, and ionizing radiation. Certain infections also contribute, including human papillomavirus, hepatitis B and C viruses, and Helicobacter pylori. Obesity, physical inactivity, and air pollution are associated with increased risk of particular cancers. (who.int)
Risk generally rises with age as exposures and cellular alterations accumulate and repair mechanisms become less effective. A risk factor increases the likelihood of disease but does not establish why a particular individual developed cancer. Preventive measures studied in public health include tobacco control, reduction of carcinogenic exposures, and vaccination against cancer-associated infections. These measures address specific causes; they do not prevent every cancer. (who.int)
Diagnosis, stage, and grade
Cancer diagnosis combines clinical history, examination, laboratory investigations, and imaging. No single test detects all cancers. Imaging methods such as computed tomography and magnetic resonance imaging help locate abnormalities and assess their extent. A biopsy, involving the collection of cells or tissue, is often necessary to establish the diagnosis. Examination through pathology identifies the nature of the abnormal cells. Abnormal blood-test results alone do not establish cancer, and normal results do not always exclude it. (cancer.gov)
Stage describes how extensive a cancer is. The widely used TNM system records the primary tumor’s size and extent, regional lymph-node involvement, and distant metastasis. Many cancers are grouped into stages I–IV, but blood cancers and some nervous-system tumors use other systems. Grade is different: it describes how abnormal tumor cells appear microscopically and provides information about likely behavior. (cancer.gov)
Treatment and supportive care
Treatment depends on cancer type, extent, biological characteristics, and the individual’s health. Surgery removes cancerous tissue, while radiation therapy uses radiation to damage cancer cells. Chemotherapy kills or inhibits cancer cells and other rapidly growing cells. Hormone therapy interferes with hormonal stimulation in susceptible cancers. Treatments are frequently combined rather than used individually. (cancer.gov)
Targeted therapy acts on specific molecular features involved in cancer growth. Cancer immunotherapy helps the immune system attack cancer. Testing a biomarker may help identify an appropriate treatment, although the usefulness of a particular marker depends on the cancer and therapy. Palliative care addresses symptoms and physical, psychosocial, and other needs; it can accompany treatment aimed at controlling the disease. (cancer.gov)
Screening, outcomes, and population burden
Screening tests people without symptoms for selected cancers or precancerous changes. Some established screening methods reduce cancer-specific mortality, and certain cervical and colorectal tests identify abnormalities that can be treated before invasive cancer develops. Screening can also produce false-positive results, false-negative results, overdiagnosis, and unnecessary treatment. Its benefits and harms depend on the test and population. (cancer.gov)
Outcomes vary widely by cancer type, stage, biological features, and treatment response. Remission means that signs and symptoms have decreased or disappeared; it is not necessarily equivalent to cure. Survival estimates describe groups and cannot predict an individual outcome precisely. The International Agency for Research on Cancer estimated 20 million new cases and 9.7 million cancer deaths worldwide in 2022, illustrating the disease’s substantial population burden. (cancer.gov)