What Is Cancer and How Does It Start?
What Is Cancer and How Does It Start?
Cancer begins when a cell develops changes that allow it to grow, survive and divide outside the body’s normal controls. Further changes may help its descendants form a tumor, invade nearby tissue or spread elsewhere. Cancer is therefore not caused by one universal defect. It develops through a series of changes that differ among cancer types and even among cells within the same tumor.
- What Is Cancer and Is It a Single Disease?
- How Do Normal Cells Control Their Growth and Division?
- How Does a Normal Cell Become a Cancer Cell?
- What Causes the Genetic Changes That Start Cancer?
- How Are Cancer Cells Different from Normal Cells?
- How Can One Cancer Cell Develop into a Tumor?
- Does Cancer Always Form a Tumor?
- What Is the Difference Between a Benign Tumor, a Precancerous Change and Cancer?
- Can Cancer Start Anywhere in the Body?
What Is Cancer and Is It a Single Disease?
Cancer is a group of diseases in which abnormal cells grow without proper control and can invade nearby tissue. Some cancer cells can also spread through the blood or lymphatic system and establish new growths in other parts of the body. This spread is called metastasis.
Cancer is not one disease. There are hundreds of cancer types and subtypes, each with its own biological behavior. A skin cancer, leukemia and brain cancer may all involve uncontrolled cell growth, but they arise from different cells and develop through different changes.
Even two cancers that begin in the same organ may not be biologically identical. They may contain different genetic changes, grow at different rates or respond differently to their surroundings. This variety is one reason cancer is better understood as a large family of related diseases rather than a single condition. (National Cancer Institute)
How Do Normal Cells Control Their Growth and Division?
Normal cells do not divide whenever they have enough food and space. Their growth is controlled by signals from neighboring cells, hormones, nutrients, the immune system and the material surrounding the tissue.
Inside each cell, a series of control points checks whether conditions are suitable for division. These checks are part of the cell cycle, the ordered process through which a cell grows, copies its DNA and divides.
How Do Healthy Cells Know When to Stop Dividing?
Cells receive chemical signals that can either encourage or prevent division. Growth factors may tell a cell that new cells are needed, while growth-inhibiting signals tell it to slow down or stop.
Cells also respond to their position within a tissue. Normal cells usually maintain contact with neighboring cells and remain within the boundaries of their tissue. Crowding, loss of proper attachment or damage to the surrounding structure can prevent further division.
Before a cell divides, control proteins check whether its DNA has been copied correctly. If a problem is found, the cell cycle may pause while repairs are attempted. These safeguards reduce the chance that damaged DNA will be passed to new cells.
Some mature cells divide regularly, while others divide only after an injury or special signal. Certain highly specialized cells, including many neurons, rarely divide after they have fully developed.
What Happens to Cells That Are Old or Damaged?
Old or mildly damaged cells may repair themselves and continue working. If repair is not possible, they may permanently stop dividing. This state is called cellular senescence.
Other cells undergo apoptosis, a controlled form of cell death. During apoptosis, the cell breaks itself into small membrane-covered pieces that can be removed without releasing large amounts of harmful material.
These processes prevent many damaged cells from multiplying. Immune cells may also recognize and remove abnormal cells. Cancer becomes possible when a cell escapes enough of these protective systems.
How Does a Normal Cell Become a Cancer Cell?
A normal cell becomes a cancer cell through changes that alter how its genes are controlled or used. These changes affect proteins involved in growth, DNA repair, cell death, immune recognition and other protective processes.
Three broad groups of genes are often involved:
- Proto-oncogenes normally help cells grow when growth is needed. When altered or overactive, they can become oncogenes that keep growth signals switched on.
- Tumor suppressor genes normally slow cell division, repair damage or trigger cell death. Cancer cells may lose these protective functions.
- DNA repair genes help correct mistakes in DNA. When they fail, additional genetic changes can build up more quickly.
Changes in gene activity can also occur without altering the DNA sequence itself. These epigenetic changes can switch genes on or off and may help cells acquire cancer-related behavior.
Does Cancer Usually Begin in One Cell?
Most cancers are thought to begin from one altered cell or a small group of closely related cells. When one of these cells gains a growth advantage, it produces descendants carrying many of the same early changes. This expanding family of cells is called a clone.
The cells do not remain identical. As they continue dividing, different descendants acquire additional changes. Natural selection within the growing cancer favors cells that survive and reproduce well in their surroundings.
A developed tumor may therefore contain several related groups of cancer cells with different properties. This is called tumor heterogeneity. In some situations, more than one altered cell may contribute to a growth, but a mainly clonal origin is common.
Why Are Several Cellular Changes Usually Needed?
One change is rarely enough to produce a fully developed cancer because cells have several overlapping safeguards. A cell that receives a strong growth signal may still be stopped by tumor suppressor proteins, DNA-damage checks, apoptosis, tissue boundaries or the immune system.
Cancer usually develops after a cell and its descendants acquire several useful abilities. These may include continued growth, resistance to cell death, access to nutrients, escape from immune attack and the ability to invade surrounding tissue.
The changes do not have to occur in one fixed order. Different cancers can reach similar biological outcomes through different genes and pathways. This gradual, multistep process also helps explain why many cancers become more common with age. (Hanahan)
What Causes the Genetic Changes That Start Cancer?
Cancer-related genetic changes can be inherited, develop through normal errors when cells copy their DNA or result from outside exposures. In many cancers, changes from several sources build up over time.
Not every DNA change affects cancer risk. Most changes are harmless, occur in unimportant parts of the DNA or do not give a cell any growth advantage.
Can Cancer-Causing Changes Be Inherited?
A person can inherit a harmful genetic change from a parent through the egg or sperm. The change is then present in nearly every cell of the body.
What is inherited is usually an increased risk of cancer, not cancer itself. Additional changes must generally occur in particular cells before a cancer develops. Some people who inherit a cancer-related change never develop cancer.
Inherited changes account for only a minority of cancers. Most cancer-related genetic changes arise in particular body cells during a person’s lifetime and are not passed to their children.
Can Mistakes Occur When Cells Copy Their DNA?
Yes. Before a cell divides, it must copy billions of DNA letters. The copying system is highly accurate, and repair proteins correct many mistakes, but it is not perfect.
A mistake that escapes repair becomes a permanent change when the cell divides. Most such changes have little effect. Occasionally, however, a change affects a gene involved in growth control, DNA repair or cell survival.
The more often a group of cells divides, the more opportunities there are for copying errors to occur. This does not mean cell division is harmful; it is essential for growth, tissue renewal and healing. It means that normal biological processes can sometimes produce changes that contribute to cancer.
How Can Carcinogens and Infections Damage DNA?
A carcinogen is an exposure that can cause cancer or increase the chance that cancer will develop. Some carcinogens directly damage DNA. Examples include chemicals in tobacco smoke and high-energy forms of radiation.
Ultraviolet radiation from sunlight can create abnormal links within DNA. Ionizing radiation can break DNA strands, while some chemicals form compounds that attach to DNA or interfere with its repair. The body can correct much of this damage, but some changes may remain.
Certain infections can also contribute to cancer. Some viruses place genetic material into cells or make proteins that interfere with growth controls. Other infections cause long-lasting inflammation, increase cell division or weaken immune protection. Infection alone does not mean that cancer will develop, because many other cellular and environmental factors are involved. (National Cancer Institute)
How Are Cancer Cells Different from Normal Cells?
Cancer cells have escaped some of the rules that allow trillions of cells to cooperate within the body. They may continue dividing, survive damage, alter their metabolism, avoid immune attack and influence nearby normal cells.
No cancer cell possesses every possible cancer-related feature from the beginning. These abilities develop and change as the cancer evolves.
Why Do Cancer Cells Ignore Signals to Stop Growing?
Some cancer cells make their own growth signals or keep growth pathways active even when no outside signal is present. Others lose receptors or proteins that normally tell them to stop dividing.
Damage to tumor suppressor genes can remove important cell-cycle brakes. For example, a cell may continue toward division even when its DNA is damaged or the tissue is already crowded.
Cancer cells may also become less dependent on normal tissue attachment. This allows them to grow in places where a healthy cell would stop dividing or die.
How Do Cancer Cells Avoid Being Destroyed?
Cancer cells may disable the pathways that trigger apoptosis. They can also increase proteins that support survival or reduce signals that mark them for removal.
Normal cells can divide only a limited number of times because the protective ends of their chromosomes, called telomeres, become shorter. Many cancer cells maintain their telomeres, allowing them to continue dividing for much longer.
The immune system can detect and destroy some abnormal cells. However, surviving cancer cells may become less visible to immune cells or produce signals that weaken the local immune response. A growing cancer may also recruit normal cells that provide support, blood vessels and protection.
How Can One Cancer Cell Develop into a Tumor?
An altered cell must divide many times before it produces a visible mass. Its first descendants form a small clone within the tissue. If they have a survival or growth advantage, they gradually replace or crowd nearby cells.
As the population grows, further changes create groups of cells with different abilities. Some may divide faster, survive with less oxygen or respond better to signals from surrounding tissue.
The cancer cells do not grow alone. They interact with blood vessels, connective-tissue cells, immune cells and the extracellular matrix that supports the tissue. Together, these components form the tumor microenvironment.
A tiny group of cells can receive oxygen and nutrients by diffusion from nearby tissue. As the mass enlarges, this supply becomes inadequate. Cancer cells may then release signals that encourage new blood vessels to grow toward the tumor. These vessels provide nutrients and routes through which cancer cells may later spread.
Does Cancer Always Form a Tumor?
No. Many cancers form solid tumors, but some begin in blood-forming cells and remain widely distributed through the bone marrow, blood or lymphatic system.
The word tumor simply means an abnormal mass of tissue. A tumor may be benign or malignant, so the presence of a lump does not by itself mean that it is cancer.
What Are Solid Tumors?
Solid tumors are abnormal masses of cells that develop within a tissue or organ. Most cancers are solid tumors.
Carcinomas begin in epithelial cells, which cover body surfaces and line organs and cavities. They include many cancers of the breast, lung, colon, prostate, kidney and skin.
Sarcomas arise from connective or supporting tissues such as bone, muscle, cartilage, fat and blood vessels. Brain tumors and many lymphomas also form solid masses, although they arise from different cell types.
Which Cancers Do Not Form Solid Tumors?
Leukemias generally do not form one main solid tumor. They begin in blood-forming cells in the bone marrow, where abnormal cells multiply and may enter the bloodstream.
Other blood cancers do not fit neatly into a single group. Lymphomas commonly form masses in lymph nodes or other tissues. Multiple myeloma develops in the bone marrow and may produce several areas of abnormal growth rather than one main tumor.
The difference between “solid” and “blood” cancer is therefore useful but simplified. It describes how the cancer is usually organized, not whether it is a true cancer. (National Cancer Institute)
What Is the Difference Between a Benign Tumor, a Precancerous Change and Cancer?
A benign tumor is an abnormal growth that does not invade nearby tissues or spread to distant organs. Some benign tumors stop growing, while others continue to enlarge. They can still cause harm if they press on an organ, produce hormones or grow in a restricted space.
A precancerous change contains abnormal cells that have some features linked to cancer but have not become invasive. Terms such as dysplasia, adenoma and intraepithelial neoplasia may be used, depending on the tissue.
Precancerous does not mean that cancer is certain to develop. Some changes remain stable or disappear, while others progress after gaining further abnormalities.
| Type of growth | Invades nearby tissue? | Can spread to distant sites? | General meaning |
|---|---|---|---|
| Benign tumor | No | No | Abnormal noncancerous growth |
| Precancerous change | No | No | Abnormal tissue with an increased chance of becoming cancer |
| Cancer | Yes, or has the ability to do so | May spread | Malignant growth |
What Does Carcinoma in Situ Mean?
Carcinoma in situ means that abnormal epithelial cells are confined to the layer in which they formed. They have not crossed the basement membrane, a thin supporting boundary separating the epithelial layer from deeper tissue.
The cells may look highly abnormal under a microscope, but they have not become invasive. If they later cross the basement membrane, the condition becomes an invasive carcinoma.
Carcinoma in situ is not one disease and does not behave the same way in every organ. Its chance of progressing depends on the tissue, the type of cellular change and other biological features. The term applies to epithelial cells and is not used for every kind of precancerous condition. (National Cancer Institute)
What Makes a Tumor Malignant?
A tumor is malignant when its cells can invade and destroy surrounding tissue. Cancer cells may break through normal tissue boundaries, produce enzymes that alter supporting material and move into nearby structures.
Some malignant cells can enter blood or lymphatic vessels, travel through the body and form secondary tumors. However, a tumor does not need to have already spread to be classified as malignant. The ability to invade is a defining difference between cancer and a benign tumor.
Malignant tumors often grow irregularly and contain cells with different genetic and biological features. Their behavior depends on the cancer type, location and changes acquired during growth.
Can Cancer Start Anywhere in the Body?
Cancer can begin in almost any tissue that contains cells capable of surviving and multiplying after harmful changes. However, cancer does not occur equally often in every cell type.
Tissues that renew themselves regularly provide more opportunities for altered cells to expand. Exposure to hormones, inflammation, infections, radiation and chemicals also differs among organs.
Some highly specialized cells rarely divide. Cancers associated with these tissues may begin in supporting cells, stem cells or less mature cells rather than in the fully developed cell itself.
Why Are Cancers Named After Where They Begin?
Cancers are usually named according to their primary site, the place where the first cancer cells developed. They may also be classified by the type of cell from which they arose.
For example, a carcinoma that begins in the lung remains lung cancer even if its cells later spread to the liver. The growth in the liver is a lung-cancer metastasis, not a new liver cancer.
This naming system matters because cells usually retain important features of their tissue of origin. A cancer’s original cell type helps explain its behavior and distinguishes it from a different cancer that happens to grow in the same organ.
Written by Chris Morais, MSc, MPhil, PhD — Making complex biology simple
Disclaimer: This article is for general educational purposes only and does not constitute medical advice, diagnosis, or treatment. Consult a qualified healthcare professional about personal health concerns.
Note on External Links: External links were checked and accessible when this article was published. Their content and availability may change over time and are outside the control of BiologyWithin.com.
