Cancer: Types, Causes, Symptoms, Diagnosis, Treatment and Prevention

Cancer is a group of more than 100 diseases in which some of the body’s cells grow out of control and spread into surrounding tissues. Normally, cells grow, divide and die in an orderly way. In cancer, this process breaks down. Damaged cells survive when they should die, and new abnormal cells form when the body does not need them.

Cancer can start almost anywhere in the body. It is named after the organ or cell type where it begins, such as lung cancer, breast cancer or leukemia. According to the World Health Organization, there were an estimated 20 million new cancer cases and 9.7 million cancer deaths worldwide in 2022. Roughly one in five people develops cancer during their lifetime.

Many cancers can be treated successfully, especially when found early. Knowing the warning signs, risk factors and screening options can make a real difference.

The body is made of trillions of cells. Genes inside each cell act as instructions that control when a cell grows, divides and dies. Cancer develops when changes (mutations) in these genes accumulate. Three main types of genes are involved:

• Oncogenes normally help cells grow. When mutated, they act like a stuck accelerator and drive constant growth.
• Tumor suppressor genes normally slow division and repair DNA errors. When mutated, they act like failed brakes.
• DNA repair genes fix mistakes made when DNA is copied. When they fail, further mutations pile up faster.

These changes usually build up over many years, which is why cancer becomes more common with age. A single mutation rarely causes cancer. It typically takes several.

Over time, the abnormal cells may form a lump called a tumor. Not all tumors are cancer:

• Benign tumors do not invade nearby tissue or spread to other parts of the body. They can still cause problems if they press on organs, nerves or blood vessels.
• Malignant tumors are cancerous. They can invade nearby tissue and spread.

Some cancers, such as leukemias, do not form solid tumors. They affect the blood and bone marrow.

Metastasis

Metastasis is the spread of cancer from where it started (the primary site) to other parts of the body. Cancer cells can break away and travel through the bloodstream or lymphatic system. When they settle and grow in a new organ, the new tumor is still named after the original cancer. For example, breast cancer that spreads to the bones is called metastatic breast cancer, not bone cancer. Metastasis is the main reason cancer becomes harder to treat and is responsible for most cancer deaths.

Cancers are grouped by the type of cell or tissue where they begin:

• Carcinoma: Starts in the skin or in the tissue lining or covering organs. This is the most common group and includes most breast, lung, prostate and colorectal cancers.
• Sarcoma: Starts in bone, cartilage, fat, muscle, blood vessels or other connective tissue.
• Leukemia: Starts in blood-forming tissue such as the bone marrow. It leads to large numbers of abnormal blood cells entering the blood.
• Lymphoma and myeloma: Start in the cells of the immune system.
• Central nervous system cancers: Start in the brain or spinal cord.

Most Common Cancers Worldwide

Based on WHO and GLOBOCAN data for 2022, the most commonly diagnosed cancers were:

1. Lung cancer
2. Female breast cancer
3. Colorectal cancer
4. Prostate cancer
5. Stomach cancer

The leading causes of cancer death were lung, colorectal, liver, breast and stomach cancers. Patterns differ by country, sex, age and access to screening and treatment.

Cancer rarely has a single cause. A risk factor is anything that raises the chance of developing the disease. Having a risk factor does not mean a person will get cancer, and many people with cancer have no obvious risk factor.

Lifestyle and environmental factors

• Tobacco use: The single largest preventable cause of cancer. Smoking is linked to cancers of the lung, mouth, throat, esophagus, bladder, kidney, pancreas, stomach, liver, cervix and others. Smokeless tobacco and secondhand smoke also raise risk.
• Alcohol: Increases the risk of cancers of the mouth, throat, esophagus, liver, breast and colon. Risk rises with the amount consumed.
• Excess body weight and physical inactivity: Linked to several cancers, including breast (after menopause), colorectal, endometrial, kidney and pancreatic cancers.
• Unhealthy diet: Diets low in fruit, vegetables and fiber, and high in processed meat, are linked to higher risk of some cancers. The International Agency for Research on Cancer classifies processed meat as carcinogenic to humans.
• Ultraviolet (UV) radiation: From sunlight and tanning beds. It is the main cause of skin cancer.
• Ionizing radiation: Including radon gas, X-rays and gamma rays at high doses.
• Workplace and environmental exposures: Such as asbestos, benzene, certain dyes, and air pollution.

Infections

Some infections raise cancer risk:

• Human papillomavirus (HPV): Cervical and other anogenital cancers, and some cancers of the throat.
• Hepatitis B and hepatitis C viruses: Liver cancer.
• Helicobacter pylori: Stomach cancer.
• Epstein–Barr virus: Certain lymphomas and nasopharyngeal cancer.
• HIV: Raises the risk of several cancers by weakening the immune system.

Inherited and personal factors

• Age: The risk of most cancers increases with age.
• Family history and inherited gene changes: Only about 5% to 10% of cancers are strongly linked to inherited mutations. Examples include BRCA1 and BRCA2 (breast, ovarian, prostate and pancreatic cancers) and Lynch syndrome (colorectal and endometrial cancers).
• Weakened immune system: For example after an organ transplant or with long-term immunosuppressive treatment.
• Previous cancer or radiation therapy: Can raise the risk of a new cancer later in life.
• Chronic inflammation: Such as long-standing ulcerative colitis or Barrett’s esophagus.

The WHO estimates that between 30% and 50% of cancers can be prevented by avoiding risk factors and applying existing prevention strategies.

Symptoms depend on the type of cancer, its size and where it is. Early cancers often cause no symptoms at all, which is why screening matters. Many of the symptoms below are far more often caused by non-cancer conditions, but they should be checked if they are new, unexplained or persistent.

General symptoms

• Unexplained weight loss
• Persistent fatigue that does not improve with rest
• Fever or night sweats with no clear cause
• Persistent pain with no obvious cause
• Skin changes, such as yellowing, darkening or redness

Specific warning signs

• A new lump or thickening in the breast, testicle, neck, armpit or elsewhere
• Changes in bowel or bladder habits that last
• Blood in the stool, urine, sputum or vomit
• Abnormal vaginal bleeding, including bleeding after menopause or between periods
• A sore that does not heal, including in the mouth
• A mole or skin spot that changes in size, shape or color, or bleeds
• A cough or hoarseness that lasts more than three weeks
• Difficulty swallowing or ongoing indigestion
• Persistent bloating or abdominal discomfort
• Frequent infections, easy bruising or bleeding

When to see a doctor

See a doctor if you notice any of the changes above and they do not go away within a few weeks, or sooner if they are severe. Do not wait for symptoms to worsen. Seek urgent medical care for heavy bleeding, severe pain, sudden breathlessness, or sudden neurological symptoms such as severe headache with confusion, weakness or seizures.

Diagnosis usually involves several steps.

• Medical history and physical examination: The doctor asks about symptoms, risk factors and family history, and examines the body for lumps or changes.
• Laboratory tests: Blood, urine or other tests can show abnormalities. Blood tests alone usually cannot confirm cancer. Tumor markers, such as PSA or CA-125, can support diagnosis or monitoring but are not reliable on their own for diagnosis.
• Imaging tests: X-ray, ultrasound, CT scan, MRI, PET scan, mammography and nuclear medicine scans help find tumors and show their size and spread.
• Biopsy: A sample of tissue or cells is removed and examined under a microscope by a pathologist. A biopsy is the only way to confirm most cancers.
• Molecular and genetic testing: The tumor may be tested for specific gene changes or proteins (such as HER2, EGFR or microsatellite instability). Results help doctors choose targeted or immune-based treatments.

Staging and grading

Stage describes how far the cancer has spread. Many cancers use the TNM system:

• T: size and extent of the primary tumor
• N: whether nearby lymph nodes are involved
• M: whether the cancer has metastasized

Stages are usually grouped from 0 to IV. Stage 0 is cancer in its earliest form, confined to where it began. Stage IV generally means the cancer has spread to distant parts of the body. Some cancers, such as blood cancers, are staged differently.

Grade describes how abnormal the cancer cells look under the microscope. Higher-grade cells tend to grow and spread faster.

Treatment depends on the type of cancer, its stage and grade, molecular features, the person’s overall health, and their preferences. Many people receive more than one type of treatment. Care is usually planned by a team of specialists.

Common side effects

Side effects vary by treatment. They may include fatigue, nausea, vomiting, hair loss, mouth sores, loss of appetite, low blood counts and a higher risk of infection, skin changes, nerve problems, and emotional distress. Many can be prevented or managed, so tell your care team about any symptoms early.

Survival has improved for many cancers thanks to earlier detection and better treatment. A person who has finished treatment is often called a cancer survivor. Follow-up care usually includes regular check-ups and tests to watch for recurrence and long-term effects of treatment.

Helpful steps include:

• Keeping all follow-up appointments
• Staying physically active as advised by your doctor
• Eating a balanced diet
• Not smoking and limiting alcohol
• Seeking support for anxiety, depression or fear of recurrence
• Asking your care team about fertility, work, finances and sexual health concerns

Cancer affects family and friends too. Support groups, counseling and patient organizations can help both patients and caregivers.

Reducing your risk

• Do not use tobacco in any form, and avoid secondhand smoke.
• Limit alcohol. Less is better for cancer risk.
• Maintain a healthy weight and stay physically active.
• Eat a diet rich in vegetables, fruit, whole grains and legumes. Limit processed meat and ultra-processed foods.
• Protect your skin from the sun, use sunscreen, and avoid tanning beds.
• Get vaccinated. HPV vaccine helps prevent cervical and other HPV-related cancers. Hepatitis B vaccine helps prevent liver cancer.
• Practice safe sex and avoid sharing needles to reduce the risk of HPV, hepatitis and HIV.
• Reduce workplace and environmental exposures to known carcinogens where possible.
• Know your family history and discuss it with a doctor, who may suggest genetic counseling or earlier screening.

Screening

Screening looks for cancer before symptoms start. Recommendations vary by country, age, sex and personal risk, so ask your doctor what applies to you. Commonly screened cancers include:

• Breast cancer: mammography
• Cervical cancer: HPV testing or Pap test
• Colorectal cancer: stool-based tests, colonoscopy or sigmoidoscopy
• Lung cancer: low-dose CT scan for certain high-risk current or former heavy smokers
• Prostate cancer: PSA testing after discussing benefits and risks with a doctor

Screening has benefits and limits. It can find cancer early, but it can also cause false alarms or detect slow-growing cancers that would never have caused harm.

• World Health Organization. Cancer fact sheet: https://www.who.int/news-room/fact-sheets/detail/cancer
• National Cancer Institute. What Is Cancer?: https://www.cancer.gov/about-cancer/understanding/what-is-cancer
• American Cancer Society: https://www.cancer.org
• International Agency for Research on Cancer, GLOBOCAN 2022: https://gco.iarc.who.int