Cancer

Cancer is not a single illness but a large group of diseases defined by one shared feature: cells that grow and divide out of control and can invade other parts of the body. Because cancers arise from many different tissues and behave in different ways, they are treated as dozens of distinct conditions, each with its own patterns and options.

Cancer is a major cause of illness and death worldwide. Understanding its biology has transformed how it is detected and treated, and research continues to improve survival for many types.

How cancer develops

The body's cells normally follow instructions that control when to grow, divide and die. Cancer begins when changes, or mutations, accumulate in the genes that govern these processes. Some mutations switch on genes that drive growth, while others disable genes that normally restrain it or repair damaged DNA.

As faulty cells multiply, they can form a mass called a tumour. Tumours that stay in place are described as benign; those that invade nearby tissue and can spread elsewhere are malignant. The spread of cancer to distant sites through the blood or lymph is called metastasis, and it is a leading reason cancer becomes life-threatening.

Types of cancer

Cancers are commonly named for the tissue or organ where they begin. Carcinomas start in the skin or in the linings of organs and are the most common type. Sarcomas arise in bone, muscle or connective tissue. Leukaemias and lymphomas affect blood-forming tissue and the immune system. Cancers of the brain and spinal cord form a further group.

Breast, lung, colorectal and prostate cancers are among the most frequently diagnosed in many countries, though patterns vary with population, environment and lifestyle.

Causes and risk factors

Cancer usually results from a combination of factors rather than a single cause. Age is a major factor, since mutations accumulate over a lifetime. Tobacco use is linked to many cancers, including those of the lung, mouth and bladder. Other important influences include obesity, alcohol, poor diet, lack of exercise, exposure to certain chemicals and radiation, and some infections such as certain human papillomaviruses and hepatitis viruses.

Inherited genetic changes raise risk for some people and families, but most cancers are not strongly inherited. Ultraviolet light from the sun or tanning devices increases the risk of skin cancer. Reducing avoidable exposures can lower the chance of developing several common cancers.

Detection and diagnosis

Cancer is often easier to treat when found early. Screening programmes aim to detect some cancers before symptoms appear; examples include tests for cervical, breast and colorectal cancer. Warning signs such as unexplained weight loss, persistent lumps, unusual bleeding or changes in a mole should be checked by a doctor.

Diagnosis typically involves imaging, laboratory tests and a biopsy, in which a small sample of tissue is examined under a microscope. Further tests may determine the stage of the cancer, meaning how far it has spread, which guides treatment choices.

Treatment

Treatment depends on the type and stage of the cancer and on the person's overall health. Surgery can remove a localised tumour. Radiation therapy uses targeted high-energy beams to damage cancer cells. Chemotherapy uses drugs to attack fast-dividing cells throughout the body. Newer approaches include targeted therapies that act on specific molecular features of a tumour and immunotherapies that help the immune system recognise and attack cancer cells.

These treatments are often combined. They can cause side effects because they may also harm healthy tissue, and managing those effects is an important part of care. Supportive and palliative care focuses on controlling symptoms and improving quality of life at every stage.

Prevention

Many cancers are linked to factors people can influence. Not smoking, limiting alcohol, maintaining a healthy weight, staying active, eating a varied diet, protecting skin from strong sunlight and taking part in recommended screening and vaccination programmes can reduce risk. Public health measures such as tobacco control and vaccination against cancer-related infections have had a measurable effect in some populations.

Staging and outlook

Once cancer is diagnosed, doctors assess its stage, which describes the size of the tumour and how far it has spread. Staging helps predict how the disease may behave and guides which treatments make sense. Broadly, cancers caught while still local are easier to treat than those that have spread to distant organs.

Outlook varies enormously by type. Some cancers, such as many skin cancers and some childhood cancers, have high survival rates when treated early, while others, including some lung and pancreatic cancers, are often found late and remain difficult to cure. Survival for many cancers has improved over recent decades thanks to earlier detection, better surgery, more precise radiation and effective new drugs.

Research continues through clinical trials, which test new treatments in controlled settings. Taking part in a trial can offer access to promising options, though it also carries uncertainty.

Key points