Cancer is a disease in which some of the body’s cells grow out of control and can spread to other parts of the body. It can start almost anywhere. Normal cells divide only when needed and die when damaged or old. In cancer, that orderly process breaks down and abnormal cells may form tumors or circulate in blood.
Cancer cells grow without the usual “start” signals and ignore “stop” or self-destruct signals. They invade nearby tissues and can move through blood or lymph to seed new tumors. They recruit blood vessels for fuel, hide from or trick the immune system, accumulate chromosome errors, and shift how they make energy. Many modern treatments target these abnormal features.
Cancer is a genetic disease caused by changes in DNA that control cell growth and division. Changes arise from mistakes as cells divide, from environmental damage such as tobacco chemicals or ultraviolet light, or are inherited. Key gene classes are involved: proto-oncogenes that can become oncogenes, tumor suppressor genes that normally restrain growth, and DNA repair genes that fix damage. With age, damaged cells are less often eliminated, which raises risk. Before invasive cancer, tissues may pass through hyperplasia, dysplasia, or carcinoma in situ.
Metastasis occurs when cancer cells break away from the primary site, travel via blood or lymph, exit vessels, and form new tumors elsewhere. Metastatic tumors keep the identity of the original cancer and often share its molecular traits. Metastatic disease causes most cancer deaths; treatment may prolong life, control growth, or relieve symptoms.
Stages of cancer
- Stage 0 (carcinoma in situ): Abnormal cells are confined to the layer where they arose. This is not yet invasive cancer, but some lesions can become cancer and are usually treated to prevent progression.
- Stage I: A small, localized cancer that has not spread to nearby tissues or has minimal local involvement. Many stage I cancers can be removed or controlled effectively with local therapies and have favorable outcomes.
- Stage II: A larger tumor or deeper local growth. It may involve nearby structures or local lymph nodes. Additional treatments beyond surgery, such as radiation or drugs, are often considered to reduce the risk of spread.
- Stage III: More extensive local spread and more frequent lymph-node involvement. Multimodal therapy is common. The risk of recurrence is higher because cancer cells have moved beyond the original site.
- Stage IV (metastatic): Cancer has spread to distant sites. Goals may include controlling growth, easing symptoms, and extending life. Some people benefit from targeted therapy, immunotherapy, radiation, or surgery to specific sites.
Risk factors
Cancer risk reflects a mix of genes, exposures, and time. Important factors include inherited mutations, aging, errors during cell division, tobacco smoke, ultraviolet radiation, certain chemicals, radiation exposure, some viruses, and environmental toxins such as asbestos, pesticides, or radon. Diet high in fat or sugar, limited physical activity, and some hormone therapies can also raise risk.
Major types of cancer
- Carcinoma (including adenocarcinoma, basal cell, squamous cell, transitional cell): Arises from epithelial cells that cover surfaces and line organs. Adenocarcinomas form in gland-like tissues such as breast, colon, and prostate. Basal and squamous cell cancers start in skin layers. Transitional cell cancers begin in the lining of the bladder, ureters, or renal pelvis.
- Sarcoma: Forms in bone and soft tissues such as muscle, fat, blood and lymph vessels, tendons, and ligaments. Osteosarcoma is the most common bone cancer. Soft tissue sarcomas include leiomyosarcoma, Kaposi sarcoma, liposarcoma, and related tumors. They often present as deep masses and may spread through blood.
- Leukemia: Starts in blood-forming tissues of bone marrow. Instead of solid tumors, abnormal white blood cells crowd blood and marrow, reducing normal cells that carry oxygen, fight infection, and control bleeding. Acute leukemias progress quickly; chronic types grow more slowly. Treatment depends on cell type and pace.
- Lymphoma: Begins in lymphocytes, a type of white blood cell. Hodgkin lymphoma features Reed-Sternberg cells and usually arises from B cells. Non-Hodgkin lymphoma is a large group that can start in B or T cells and may grow slowly or rapidly. Disease often involves lymph nodes and immune organs.
- Multiple myeloma: A cancer of plasma cells that accumulate in bone marrow and form tumors in bones. It can weaken bones and affect blood counts and kidney function. Also called plasma cell myeloma or Kahler disease. Therapies aim to control abnormal plasma cells and their effects.
- Melanoma: Arises from melanocytes, the pigment-producing cells. Most melanomas form on the skin but can also occur in pigmented tissues like the eye. Early detection is critical because melanoma can spread if not treated promptly.
- Brain and spinal cord tumors: Classified by the cell type and location within the central nervous system. Examples include astrocytic tumors from astrocytes. These tumors can be benign or malignant; their behavior relates to grade and location, which guide treatment choices.
- Germ cell tumors: Originate from the cells that give rise to sperm or eggs and can occur in many body sites. They may be benign or malignant. Location, age, and tumor markers help define the subtype and approach to therapy.
- Neuroendocrine tumors (including carcinoid): Develop from hormone-releasing cells. They may overproduce hormones and cause varied symptoms. Carcinoid tumors are typically slow-growing, often in the gastrointestinal tract, and may cause carcinoid syndrome when they secrete certain substances.
There is no single cure for cancer. Outcomes depend on the cancer’s type, its stage, and how it responds to treatment. Some cancers can be cured, especially when found early. Others can be placed into remission or controlled for long periods. Metastatic disease often leads to serious complications and is responsible for most cancer deaths, though treatment can still extend life and improve quality of life.
Why knowing the type and stage matters
Different cancers behave differently. Some grow fast while others are slow. Some depend on hormones. Some are best treated with surgery, and others respond better to radiation, chemotherapy, targeted therapy, or immunotherapy. Identifying the exact type and stage helps clinicians choose the most effective treatment plan and estimate outlook.
