What Is Cancer? Understanding How Cancer Cells Develop

What Is Cancer and How Does It Develop?

Cancer is a disease where cells in the body undergo genetic changes (mutations) that cause them to grow and divide uncontrollably. Unlike normal cells that follow orderly patterns of growth and death, cancer cells continue multiplying indefinitely, forming masses called tumors that can invade surrounding tissues and spread throughout the body.

To understand cancer, it helps to first understand how normal cells work. Your body is made up of trillions of cells that constantly grow, divide, and die in a carefully controlled process. This allows your body to replace old or damaged cells and maintain healthy tissues and organs. The instructions for this process are contained in your DNA, which acts as a cellular blueprint.

Cancer develops when something goes wrong with this orderly process. Changes in a cell's DNA—called mutations—can cause the cell to ignore the normal signals that tell it when to stop dividing and when to die. Instead of following the body's instructions, these mutated cells continue to grow and divide, creating more and more abnormal cells. Over time, these cells can accumulate into a mass of tissue called a tumor.

The word "cancer" comes from the Greek word "karkinos," meaning crab, because the ancient Greek physician Hippocrates thought that tumors with swollen blood vessels around them looked like crabs. Today, cancer refers to a large group of diseases—more than 200 different types—that share this fundamental characteristic of uncontrolled cell growth.

Cancer is not a single disease but rather an umbrella term for many related conditions. What they all have in common is this breakdown in the normal controls that regulate cell growth. However, different types of cancer can behave very differently depending on where they start, how fast they grow, and whether they spread to other parts of the body.

The Biology of Cancer Cells

Cancer cells differ from normal cells in several important ways. Understanding these differences helps explain why cancer behaves the way it does and how treatments work to fight it.

Normal cells follow a strict life cycle: they grow, divide a limited number of times, perform their specialized functions, and eventually die through a process called apoptosis (programmed cell death). They also stay where they belong, respecting the boundaries between different tissues. When normal cells become damaged beyond repair, they trigger their own death to prevent problems.

Cancer cells break these rules. They ignore signals telling them to stop dividing, avoid apoptosis, and can continue dividing indefinitely. They may also lose their specialized functions, essentially becoming "immature" cells that don't contribute usefully to the body. Perhaps most dangerously, they can develop the ability to invade other tissues and spread to distant parts of the body.

How Tumors Form and Grow

The development of cancer is typically a gradual process that occurs over many years. It usually takes multiple genetic mutations accumulating in a single cell before that cell becomes cancerous. This is why cancer is more common in older adults—they've had more time for mutations to accumulate.

The journey from a normal cell to a cancerous tumor generally follows these stages:

• Initiation: The first mutation occurs in a cell's DNA, often affecting genes that control cell growth
• Promotion: Additional mutations accumulate, and the cell begins to divide more rapidly than normal
• Progression: The abnormal cells continue to acquire more mutations, becoming increasingly aggressive
• Tumor formation: The cancerous cells multiply enough to form a detectable mass
• Invasion: The tumor begins to grow into surrounding tissues
• Metastasis: Cancer cells break away and spread to distant parts of the body

As tumors grow, they require nutrients and oxygen to survive. Cancer cells can stimulate the growth of new blood vessels (a process called angiogenesis) to supply themselves with what they need. This blood supply also provides a route through which cancer cells can potentially spread to other parts of the body.

What Is the Difference Between Benign and Malignant Tumors?

Benign tumors are not cancer—they grow slowly, do not invade surrounding tissues, and do not spread to other parts of the body. Malignant tumors (cancer) can grow faster, invade nearby tissues, and spread (metastasize) through the bloodstream or lymphatic system to form new tumors in distant organs.

Not all tumors are cancerous. The critical distinction lies in whether the tumor is benign (not cancer) or malignant (cancer). Understanding this difference is essential for knowing what to expect from a diagnosis and what treatment might be needed.

Benign tumors are masses of cells that grow in a contained, organized manner. They are surrounded by a fibrous capsule that keeps them localized. While they can grow large and sometimes cause problems by pressing on nearby organs or nerves, they don't have the ability to invade other tissues or spread through the body. Examples include lipomas (fatty tumors), fibroids (uterine growths), and most moles.

Benign tumors typically grow slowly and stop growing once they reach a certain size. When removed surgically, they rarely come back. They usually don't cause serious health problems unless they're located in a critical area (like the brain) where even a non-cancerous growth can cause significant issues.

Malignant tumors—what we call cancer—behave very differently. They don't respect the normal boundaries between tissues and can invade into surrounding structures. Their cells are often less organized and more irregular in appearance. Most importantly, they can metastasize, sending cells through the blood or lymphatic system to establish new tumors in other parts of the body.

It's worth noting that there's also a category called pre-cancerous or pre-malignant conditions. These are abnormal cells that aren't cancer yet but have a higher than normal chance of becoming cancerous. Examples include certain types of polyps in the colon or abnormal cells detected during a cervical screening. Identifying and treating pre-cancerous conditions can prevent cancer from developing.

How Does Cancer Spread (Metastasis)?

Metastasis is the process by which cancer cells break away from the original (primary) tumor, travel through the bloodstream or lymphatic system, and form new tumors in other parts of the body. Common sites for metastasis include the liver, lungs, bones, and brain. Metastatic cancer is named after where it started, not where it spreads.

One of the most dangerous aspects of cancer is its ability to spread from where it started to other parts of the body. This process, called metastasis, is responsible for most cancer-related deaths. Understanding how metastasis works helps explain why early detection is so important and why treatment approaches may change for cancer that has spread.

Metastasis is a complex, multi-step process. For cancer cells to successfully spread, they must:

1. Break away from the primary tumor: Cancer cells must first become detached from the mass where they formed. They may produce enzymes that break down the surrounding tissue, creating pathways for escape.

2. Enter the bloodstream or lymphatic system: Once free from the primary tumor, cancer cells can penetrate the walls of nearby blood vessels or lymphatic channels. The lymphatic system—a network of tissues and organs that help remove toxins and transport immune cells—often serves as a highway for cancer spread.

3. Survive the journey: The bloodstream is a hostile environment for cells. Cancer cells must survive being attacked by the immune system and the physical stresses of circulation. Only a small percentage of cells that enter the bloodstream survive this journey.

4. Exit into a new location: Cancer cells must then exit the bloodstream or lymphatic vessel and enter a new tissue. This often happens in capillaries, the smallest blood vessels, where cancer cells can get trapped and then migrate through the vessel wall.

5. Establish a new tumor: Finally, the cancer cells must survive in their new environment and begin growing. This requires adapting to a new tissue environment, which isn't always possible—many cancer cells that reach distant sites fail to establish new tumors.

Where Does Cancer Commonly Spread?

Different types of cancer tend to spread to different parts of the body, though several organs are common destinations for metastasis:

• Liver: The liver filters blood from the digestive system, making it a common site for metastasis from colorectal, pancreatic, and other abdominal cancers
• Lungs: The lungs receive all the blood returning from the body, making them vulnerable to metastasis from many cancer types
• Bones: Breast, prostate, lung, kidney, and thyroid cancers commonly spread to bones
• Brain: Lung cancer, breast cancer, and melanoma frequently spread to the brain
• Lymph nodes: Often the first place cancer spreads, as the lymphatic system drains many tissues

What Causes Cancer to Develop?

Cancer is caused by changes (mutations) in cellular DNA that disrupt normal cell growth controls. These mutations can be inherited, caused by environmental factors (tobacco, radiation, chemicals), lifestyle factors (diet, obesity), certain infections (HPV, Hepatitis B), or occur randomly during cell division. Usually, multiple mutations must accumulate before cancer develops.

Cancer doesn't have a single cause. Instead, it results from a combination of factors that lead to the genetic changes necessary for cancer to develop. Understanding these causes helps us know how to reduce cancer risk and why some people develop cancer while others don't.

Genetic factors play a role in some cancers. About 5-10% of all cancers are linked to inherited genetic mutations passed down through families. Examples include BRCA1 and BRCA2 mutations that increase the risk of breast and ovarian cancer, and Lynch syndrome that increases colorectal cancer risk. Having a family history of cancer doesn't mean you'll definitely get cancer, but it may increase your risk.

Environmental and lifestyle factors are responsible for the majority of cancers. The most significant include:

• Tobacco use: The leading preventable cause of cancer, linked to lung, mouth, throat, bladder, and many other cancers
• Ultraviolet radiation: Sun exposure and tanning beds significantly increase skin cancer risk
• Obesity: Associated with at least 13 types of cancer including breast, colorectal, and endometrial cancer
• Alcohol: Increases risk of liver, breast, colorectal, and other cancers
• Poor diet: Diets high in processed meat and low in fruits/vegetables may increase certain cancer risks
• Physical inactivity: Lack of exercise is linked to increased risk of several cancer types
• Occupational exposures: Asbestos, benzene, and other workplace chemicals can cause cancer

Infections cause approximately 15-20% of cancers worldwide. Key cancer-causing infections include:

• Human papillomavirus (HPV): Causes cervical cancer and some head/neck cancers—preventable with vaccination
• Hepatitis B and C viruses: Can cause liver cancer—Hepatitis B is preventable with vaccination
• Helicobacter pylori: A bacterium that can cause stomach cancer
• HIV: Increases risk of several cancers by weakening the immune system

Random mutations also play a role. Every time a cell divides, its DNA must be copied. Occasionally, errors occur during this copying process. While cells have mechanisms to catch and repair most errors, some slip through. Over a lifetime of cell divisions, these random errors can accumulate. This partly explains why cancer risk increases with age—more time means more opportunities for random mutations to occur.

What Are the Main Types of Cancer?

Cancer is classified by where it starts in the body and the type of cell involved. The main categories are carcinomas (starting in skin or tissue lining organs), sarcomas (starting in bone or soft tissue), leukemias (blood cancers), lymphomas (lymphatic system cancers), and brain/spinal cord tumors. The most common cancers worldwide are lung, breast, colorectal, and prostate cancer.

There are more than 200 different types of cancer, but they can be grouped into several main categories based on the type of cell where they begin:

Carcinomas are the most common type of cancer, accounting for 80-90% of all cancer diagnoses. They begin in the epithelial cells that line the inner and outer surfaces of the body. This includes the skin, as well as the tissues that line the digestive tract, respiratory tract, and internal organs. Subtypes include:

• Adenocarcinoma: Develops in gland cells that produce fluids or mucus (breast, prostate, lung, colon)
• Squamous cell carcinoma: Develops in flat cells that line many organs (skin, lung, esophagus)
• Basal cell carcinoma: The most common type of skin cancer

Sarcomas are cancers that begin in bone or soft tissues like muscle, fat, blood vessels, cartilage, or connective tissue. They are relatively rare, accounting for less than 1% of all cancers. Examples include osteosarcoma (bone cancer) and liposarcoma (fat tissue cancer).

Leukemias are cancers of the blood-forming tissues, primarily the bone marrow. Unlike most cancers, leukemias don't typically form solid tumors. Instead, abnormal white blood cells accumulate in the blood and bone marrow. Types include acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), chronic lymphocytic leukemia (CLL), and chronic myeloid leukemia (CML).

Lymphomas are cancers that begin in the lymphatic system—the network of vessels and glands that help fight infection and disease. The two main types are Hodgkin lymphoma and non-Hodgkin lymphoma, which behave differently and require different treatments.

Brain and spinal cord tumors (central nervous system cancers) develop in the brain or spinal cord. They are named based on the type of cell in which they form and the location. Some brain tumors are benign, while others are malignant.

Other types include melanoma (skin cancer from pigment cells), multiple myeloma (cancer of plasma cells), and neuroendocrine tumors.

Most Common Cancers Worldwide

According to the latest global cancer statistics:

• Lung cancer: The most commonly diagnosed cancer and leading cause of cancer death
• Breast cancer: Most common cancer in women globally
• Colorectal cancer: Third most common cancer overall
• Prostate cancer: Most common cancer in men in many countries
• Stomach cancer: Particularly common in Asia

Can Cancer Be Prevented?

While not all cancers are preventable, 30-50% of cancer cases could be avoided by eliminating known risk factors. Key prevention strategies include avoiding tobacco, maintaining a healthy weight, eating a balanced diet, exercising regularly, limiting alcohol, protecting against UV radiation, getting vaccinated (HPV, Hepatitis B), and participating in cancer screening programs.

Cancer prevention is a powerful approach to reducing the global burden of this disease. The World Health Organization estimates that between 30-50% of all cancers could be prevented by avoiding known risk factors and implementing evidence-based prevention strategies. While we can't eliminate all cancer risk, there's much we can do to lower it significantly.

Avoid tobacco in all forms. This is the single most important step you can take to prevent cancer. Tobacco use is linked to at least 15 types of cancer and accounts for about 22% of cancer deaths worldwide. If you smoke, quitting at any age provides health benefits. Even long-term smokers can significantly reduce their cancer risk by stopping.

Maintain a healthy body weight. Obesity is associated with increased risk of at least 13 types of cancer, including breast, colorectal, endometrial, kidney, and pancreatic cancer. The link between obesity and cancer involves complex mechanisms including chronic inflammation, hormonal changes, and altered immune function. Even modest weight loss can reduce cancer risk.

Eat a healthy diet. While no food can prevent or cause cancer on its own, your overall dietary pattern matters. Evidence supports eating plenty of fruits, vegetables, and whole grains while limiting processed meat, red meat, and highly processed foods. A healthy diet also helps maintain a healthy weight.

Be physically active. Regular exercise reduces the risk of several cancer types, including colorectal, breast, and endometrial cancer. Aim for at least 150 minutes of moderate aerobic activity or 75 minutes of vigorous activity per week. Physical activity helps by controlling weight, reducing inflammation, and improving immune function.

Limit alcohol. Alcohol increases the risk of cancers of the mouth, throat, esophagus, liver, colon, rectum, and breast. The risk increases with the amount consumed. For cancer prevention, not drinking alcohol is best, but if you do drink, limit intake.

Protect yourself from the sun. Skin cancer is one of the most common cancers, but it's also one of the most preventable. Use broad-spectrum sunscreen with SPF 30 or higher, seek shade during peak sun hours (10 AM - 4 PM), wear protective clothing, and avoid tanning beds.

Get vaccinated. Vaccines can prevent cancer-causing infections:

• HPV vaccine: Protects against cervical cancer and other HPV-related cancers—recommended for boys and girls ages 11-12
• Hepatitis B vaccine: Reduces liver cancer risk—given as part of routine childhood vaccination

Participate in screening programs. Regular cancer screening can detect cancer early or even prevent it by finding pre-cancerous changes. Recommended screenings include:

How Is Cancer Treated?

Cancer treatment depends on the type, location, stage, and individual factors. Main treatments include surgery (removing tumors), chemotherapy (drugs that kill cancer cells), radiation therapy (using high-energy rays), immunotherapy (boosting the immune system), targeted therapy (drugs targeting specific cancer cell features), and hormone therapy (for hormone-sensitive cancers). Many patients receive combination treatments.

Cancer treatment has advanced dramatically in recent decades, offering more effective options with fewer side effects than ever before. The choice of treatment depends on many factors, including the type of cancer, how advanced it is (stage), where it's located, and the patient's overall health and preferences.

Surgery remains one of the most effective treatments for solid tumors that haven't spread. The goal is to remove the cancer along with a margin of healthy tissue to ensure all cancer cells are eliminated. Advances in surgical techniques, including minimally invasive and robotic surgery, have made many procedures less traumatic with faster recovery times. For some cancers, surgery may be the only treatment needed.

Chemotherapy uses powerful drugs to kill cancer cells or stop them from dividing. These drugs travel through the bloodstream, making chemotherapy effective against cancers that have spread. Different drugs work in different ways, and combinations are often used. While chemotherapy can cause significant side effects (because it also affects rapidly dividing normal cells), supportive care has improved considerably to manage these effects.

Radiation therapy uses high-energy beams, typically X-rays, to damage cancer cell DNA and kill them. It can be delivered externally (from a machine outside the body) or internally (brachytherapy, where radioactive material is placed near the tumor). Modern techniques like intensity-modulated radiation therapy (IMRT) and proton therapy can target tumors more precisely while sparing healthy tissue.

Immunotherapy represents one of the most exciting advances in cancer treatment. Instead of directly attacking cancer cells, it helps the immune system recognize and fight cancer. Types include:

• Checkpoint inhibitors: Remove the "brakes" that normally prevent immune cells from attacking
• CAR-T cell therapy: Engineers a patient's own immune cells to attack cancer
• Cancer vaccines: Stimulate the immune system to attack cancer cells

Targeted therapy uses drugs that target specific molecules involved in cancer growth and spread. Unlike traditional chemotherapy, targeted therapies are designed to affect only cancer cells while minimizing harm to normal cells. Examples include drugs targeting specific proteins that help cancer cells grow or spread.

Hormone therapy is used for cancers that are hormone-sensitive, particularly breast and prostate cancer. It works by blocking the body's ability to produce hormones or interfering with hormone action. This can slow or stop cancer growth in tumors that depend on hormones to grow.

Combination therapy uses two or more treatment types together. This approach is often more effective than single treatments because it attacks cancer in multiple ways and can help prevent resistance.

How Is Cancer Diagnosed?

Cancer diagnosis typically involves multiple steps: medical history and physical exam, imaging tests (X-rays, CT scans, MRI, PET scans), laboratory tests (blood tests, tumor markers), and biopsy (examining tissue samples under a microscope). A biopsy is usually required to confirm a cancer diagnosis. Once cancer is confirmed, additional tests determine the cancer's stage—how far it has spread.

Diagnosing cancer involves several steps to not only identify whether cancer is present, but also to determine its type, location, and extent. This information is crucial for determining the best treatment approach and predicting outcomes.

Medical history and physical examination are often the first steps. Your doctor will ask about symptoms, risk factors, and family history. A physical exam may reveal signs such as lumps, enlarged lymph nodes, or changes in the skin or organs.

Imaging tests help visualize internal structures and identify abnormal growths:

• X-rays: Can reveal tumors in some areas
• CT (computed tomography) scans: Provide detailed cross-sectional images
• MRI (magnetic resonance imaging): Particularly useful for soft tissue imaging
• PET (positron emission tomography) scans: Show areas of high metabolic activity, often indicating cancer
• Ultrasound: Uses sound waves to create images

Laboratory tests including blood tests can provide valuable information. Tumor markers are substances produced by cancer or by the body in response to cancer. While not definitive on their own, they can help with diagnosis and monitoring treatment. Examples include PSA (prostate cancer), CA-125 (ovarian cancer), and CEA (colorectal cancer).

Biopsy is typically required to definitively diagnose cancer. A biopsy involves removing a sample of tissue to examine under a microscope. Types include:

• Needle biopsy: Using a needle to extract tissue
• Surgical biopsy: Removing tissue during surgery
• Endoscopic biopsy: Using a thin tube with a camera to reach internal areas

Pathologists examine the biopsy sample to determine if cancer is present and, if so, what type. They also look at characteristics that can guide treatment, such as hormone receptor status in breast cancer or genetic mutations that might make the cancer responsive to specific therapies.

Staging determines how far the cancer has spread. The most common system is the TNM system:

• T (Tumor): Size and extent of the primary tumor
• N (Nodes): Whether cancer has spread to nearby lymph nodes
• M (Metastasis): Whether cancer has spread to distant parts of the body

Stage ranges from I (localized, early cancer) to IV (cancer has spread to distant organs). Staging is crucial because it guides treatment decisions and helps predict outcomes.