Comprehensive Overview of Cancer
Cancer is a group of diseases in which abnormal cells grow out of control and may spread to other parts of the body. Normally, the body carefully controls how cells grow, divide, repair damage, and die. Cancer develops when these controls break down and damaged cells continue to multiply.
Cancer is not a single disease. There are more than 200 types of cancer, and each type behaves differently depending on where it begins and what genetic changes are involved.
Cancer can begin in almost any organ or tissue, including the colon, lungs, breast, prostate, blood, skin, brain, and lymphatic system.
This guide explains what cancer is, how cancer develops, common cancer symptoms, major types of cancer, causes, diagnosis, staging, treatments, and prevention.
This information is educational only and not medical advice.
What Cancer Is
Cancer develops when cells begin dividing without normal limits. Healthy cells divide only when needed and die when they become damaged or old. Cancer cells continue to grow even when they should stop.
Healthy cells:
- Grow when needed
- Repair damage
- Die when worn out
- Stay in their proper location
Cancer cells:
- Divide continuously
- Avoid cell death
- Accumulate mutations
- Can spread into nearby tissues
- Can travel to distant organs
Cancer cells may form solid tumors or may circulate in the blood, as seen in leukemia.
Not all tumors are cancer. Some tumors are benign and do not spread. Cancerous tumors are called malignant tumors.
How Cancer Develops
Cancer usually develops slowly over many years. It begins when changes in DNA disrupt the normal controls that regulate cell growth.
DNA contains instructions that control how cells behave. When DNA becomes damaged, cells may begin to behave abnormally.
These genetic changes can cause cells to:
- Grow too quickly
- Survive longer than normal
- Ignore signals to stop dividing
- Avoid programmed cell death
- Accumulate additional mutations
Most cancers develop after multiple genetic changes occur over time.
DNA Mutations and Cancer
Cancer is fundamentally a genetic disease caused by DNA mutations. These mutations may be:
Inherited Mutations
Some people inherit gene mutations that increase cancer risk.
Examples include:
- BRCA1 and BRCA2 mutations
- Lynch syndrome
- Familial colon cancer syndromes
Inherited mutations account for a small percentage of cancers.
Acquired Mutations
Most cancers develop from mutations acquired during life.
These mutations may be caused by:
- Tobacco smoke
- Radiation exposure
- Chemical exposures
- Infections
- Chronic inflammation
- Random DNA-copying errors
Cells divide trillions of times during a lifetime, and occasional errors occur naturally.
Differences Between Cancer Cells and Normal Cells
Cancer cells behave differently from normal cells in several important ways.
Cancer cells:
- Grow without normal signals
- Ignore stop signals
- Resist programmed cell death
- Form new blood vessels
- Invade nearby tissues
- Spread through the body
- Avoid immune detection
- Accumulate genetic damage
Normal cells do not display these behaviors.
Hallmarks of Cancer
Researchers describe common features shared by many cancers.
These include:
Uncontrolled Growth
Cancer cells divide continuously without normal regulation.
Resistance to Cell Death
Cancer cells avoid apoptosis, which is the normal process of programmed cell death.
Genetic Instability
Cancer cells accumulate additional mutations as they grow.
Angiogenesis
Tumors stimulate the growth of new blood vessels to supply oxygen and nutrients.
Invasion
Cancer cells invade nearby tissues.
Metastasis
Cancer cells spread to distant parts of the body.
These characteristics help explain why cancer behaves differently from normal tissue.
Major Types of Cancer
Cancer is classified according to where it begins.
Carcinomas
Carcinomas develop in epithelial cells that line organs and surfaces.
These are the most common cancers.
Examples include:
- breast cancer
- lung cancer
- colon cancer
- prostate cancer
- skin cancer
Sarcomas
Sarcomas develop in connective tissues.
Examples include:
- bone cancer
- muscle cancer
- fat tissue tumors
- cartilage tumors
Sarcomas are less common than carcinomas.
Leukemia
Leukemia is a cancer of blood-forming tissue.
It usually does not form solid tumors.
Leukemia affects white blood cells and bone marrow.
Lymphoma
Lymphoma develops in immune cells called lymphocytes.
Lymphomas affect lymph nodes and the lymphatic system.
Types include:
- Hodgkin lymphoma
- Non-Hodgkin lymphoma
Multiple Myeloma
Multiple myeloma is a cancer of plasma cells in the bone marrow.
Plasma cells normally produce antibodies.
Brain and Nervous System Cancers
These cancers begin in the brain or spinal cord.
Examples include:
- gliomas
- meningiomas
- astrocytomas
Tumor Growth
Tumors develop as cancer cells continue dividing.
Tumors may:
- Grow slowly or rapidly
- Invade nearby tissues
- Press on surrounding organs
Tumor growth rates vary widely between cancers.
Benign vs Malignant Tumors
Benign Tumors
Benign tumors:
- Grow locally
- Do not spread
- Usually grow slowly
They may still cause problems if they press on organs.
Malignant Tumors
Malignant tumors:
- Invade nearby tissue
- Spread to other organs
- Can be life-threatening
Malignant tumors are cancer.
Metastasis (How Cancer Spreads)
Metastasis occurs when cancer spreads from the original tumor to other parts of the body.
Cancer cells may travel through:
- the bloodstream
- the lymphatic system
Common metastatic sites include:
- liver
- lungs
- bones
- brain
Metastasis is responsible for most cancer-related deaths.
Metastatic cancer keeps the name of the original cancer.
Example:
Colon cancer that spreads to the liver is still colon cancer.
Causes and Risk Factors
Cancer develops from a combination of genetic and environmental factors.
Tobacco
Tobacco is the leading preventable cause of cancer.
It is linked to:
- lung cancer
- throat cancer
- bladder cancer
- pancreatic cancer
Diet and Lifestyle
Risk factors include:
- obesity
- physical inactivity
- poor diet
- excess alcohol use
Radiation
Radiation damages DNA.
Sources include:
- sunlight (UV radiation)
- medical imaging
- environmental radiation
Infections
Some infections increase cancer risk.
Examples include:
- HPV and cervical cancer
- Hepatitis B and liver cancer
- Hepatitis C and liver cancer
- H. pylori and stomach cancer
Environmental Exposures
Some chemicals increase cancer risk.
Examples include:
- asbestos
- benzene
- industrial chemicals
Age
Cancer risk increases with age because mutations accumulate over time.
Genetics
Some cancers run in families.
Inherited mutations increase risk but do not guarantee cancer will develop.
Cancer Symptoms
Cancer symptoms vary depending on the type and location of cancer.
Common cancer symptoms include:
- unexplained weight loss
- persistent fatigue
- lumps or swelling
- persistent pain
- changes in bowel habits
- persistent cough
- unexplained bleeding
- skin changes
For a full list see:
Cancer Symptoms and Warning Signs
How Cancer Is Diagnosed
Cancer diagnosis usually involves several steps.
Medical History and Physical Exam
Doctors review symptoms and risk factors.
Imaging Tests
Imaging may include:
- CT scans
- MRI scans
- PET scans
- ultrasound
- X-rays
Blood Tests
Blood tests may show abnormalities.
Some cancers produce tumor markers.
Biopsy
A biopsy removes tissue for examination.
Biopsy is usually required to confirm cancer.
Cancer Staging
Cancer staging describes how far cancer has spread.
Stage 0
Very early cancer.
Stage 1
Small localized tumor.
Stage 2
Larger tumor or limited spread.
Stage 3
Regional spread to lymph nodes.
Stage 4
Cancer has spread to distant organs.
Cancer Grading
Grading describes how abnormal cancer cells look under a microscope.
High-grade cancers tend to grow faster.
Low-grade cancers tend to grow more slowly.
Cancer Treatments
Treatment depends on cancer type and stage.
Many patients receive more than one treatment.
Surgery
Surgery removes tumors when possible.
Radiation Therapy
Radiation uses high-energy beams to destroy cancer cells.
Chemotherapy
Chemotherapy uses drugs that kill rapidly dividing cells.
Immunotherapy
Immunotherapy helps the immune system attack cancer.
Targeted Therapy
Targeted therapy attacks specific molecular changes.
Hormone Therapy
Hormone therapy treats cancers that depend on hormones.
Examples include breast and prostate cancer.
Cancer Prevention
Some cancers can be prevented.
Important steps include:
- avoiding tobacco
- limiting alcohol
- maintaining a healthy weight
- regular exercise
- healthy diet
- sun protection
- vaccinations
- screening tests
Experts estimate that many cancers are preventable.
How Common Cancer Is
Cancer is one of the leading causes of death worldwide.
Millions of people are diagnosed each year.
Many people survive cancer with treatment.
Early detection improves survival.
Medical References
National Cancer Institute
https://www.cancer.gov/about-cancer/understanding/what-is-cancer
American Cancer Society
https://www.cancer.org/cancer.html
Mayo Clinic
https://www.mayoclinic.org/diseases-conditions/cancer/symptoms-causes/syc-20370588
World Health Organization
https://www.who.int/news-room/fact-sheets/detail/cancer
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