Understanding cancer development: How it starts and spreads in the body
Cancer development at a glance
- Cancer means: Cells in the body begin to grow and multiply uncontrollably. Over time, these cells can form a tumour and may affect how the body works.
- Cancer can affect the body in different ways: It can press on organs, damage healthy tissue, use the bodies energy, or stop organs from working properly.
- Cancer starts when cells change: They no longer follow the body’s normal signals that tell them when to grow, divide, or die.
- Cancer usually develops slowly: It often takes many years for a normal cell to turn into a cancer cell. During this time, the cell and its offspring gradually stop behaving like normal cells.
- Some cancers can spread: Cancer cells may move to other parts of the body and grow there. Experts call these new tumours metastases.
What is cancer?
Cancer is not one single disease. There are many different types of cancer. They all have one thing in common: Cells in the body begin to grow and behave abnormally. Over time, these cells can form a lump or growth called a tumour.
There are two main types of tumours:
- Benign tumours: These are not cancer. They usually grow slowly and remain in one place. They do not grow into nearby tissue and usually do not spread to other parts of the body.
- Malignant tumours: Malignant means cancerous. These tumours grow into nearby tissue and may spread to distant parts of the body.
Benign tumours often cause less harm. Most of them do not become cancer. Some types of benign growths can change over time and become malignant.
Malignant tumours can have serious consequences, especially if not treated. But they do not all behave in the same way. Some grow very quickly. Others grow slowly and may not cause symptoms for many years.
Different types of cancer grow in different ways.
- Some cancers grow in one place in the body and form a lump or growth. These are called solid tumours.
- Other cancers do not form a tumour. Instead, they spread immediately throughout the body. One example is leukaemia, a cancer that originates in the blood cells.
How cancer affects the body
As cancer cells grow, they can affect how organs and tissues normally work. This can happen in several ways:
- by pressing on nearby organs, nerves, blood vessels or other tissues
- by growing into and destroying healthy tissue
- by changing how the body normally works
For example, a tumour in the lungs may make breathing difficult. A tumour in the bowel can disrupt digestion. Cancer can show itself through symptoms like these.
In addition, cancer cells can produce substances that affect the whole body. And they use up energy that the body needs. This can lead to symptoms such as exhaustion, sweating, or weight loss.
How does cancer develop?
Cancer starts, when cells in your body get out of control. In a healthy body, cells grow, divide and die in a carefully controlled way. This ensures that organs and tissues stay the right size and work properly.
Cells also have built-in safety systems. If a cell is damaged, it can repair itself. If the damage is too serious, the cell can destroy itself in a controlled way. This prevents damaged cells from causing harm.
Cancer develops when these control systems fail. This can happen, when cells get the wrong instructions. The cell’s instructions are stored in its genetic material, called DNA. It tells the cell when to divide, what jobs to do, and when to stop growing or die.
If this DNA is damaged or changed, this can disturb the systems that control a cell. For example, DNA changes can cause a cell to
- divide when it should not,
- stop following the body’s control signals,
- not repair DNA damage properly,
- move into the surrounding tissue,
- stay alive even when it is badly damaged.
Usually cancer does not appear suddenly
Cancer typically develops over many years or even decades. This usually happens only after a cell’s DNA has gone through several changes.
During this long process, cells gradually lose their normal function and become cancer cells.
Because cancer develops step by step, there can be early tissue changes that are not yet cancer. Doctors may call this pre-cancer. Some of these can later become cancer. One example is bowel polyps, which are small growths in the bowel.
This gradual development is one reason why cancer screening programmes are important. They can help to find some cancers, or changes that could become cancer, before symptoms appear. This can make treatment easier and more effective.
What causes these DNA changes?
Many different factors can lead to DNA damage. Some of these causes can be avoided, others cannot. Cancer usually does not have just one cause. Several factors can work together.
- Many DNA mistakes happen by chance when cells divide or during normal processes inside cells.
- A small proportion of people inherit DNA changes from their parents. These can increase their risk to get cancer.
- Some lifestyle factors can also affect the DNA and raise cancer risk. Examples include smoking, alcohol use, and obesity.
- Factors in the environment can lead to DNA damage, such as too much sun exposure.
- Some infections, for example with certain viruses, can damage DNA and increase the risk of cancer.
Learn more
To learn what you can do to lower your cancer risk, read the text Cancer Prevention.
Can the immune system fight cancer?
The immune system plays an important role in finding and destroying cells that are not working normally. Most of the time it successfully removes cells before they can become cancer.
However, cancer cells can find ways to escape the immune system. They may “hide” by looking like normal cells, or they may weaken the immune system’s attack. This allows them to survive and continue to grow.
What are metastases?
Metastases are new tumours that grow in another part of the body, away from the tumour where the cancer started. They develop when cancer cells spread from their original place. The process of cancer spreading is called metastasis.
Metastases make a cancer more serious. They can
- damage important organs such as the lungs, liver, or brain,
- make treatment more difficult because cancer is in more than one place,
- lower the chance that treatment can get rid of the cancer or bring it under control.
However, not all cancers form metastases. And some patients with metastases can be treated successfully for long periods.
When does metastasis happen?
Not all cancers spread in the same way or at the same speed. Some stay in one area for a long time, while others spread early. This can depend on things such as
- the type of cancer,
- changes in the cancer cells,
- the conditions around the tumour.
Doctors may find metastases at different times during the illness: In some patients, metastases are already there when cancer is first diagnosed. In others, they develop during treatment. Metastases can also appear some time after a tumour has been successfully treated.
Where do metastases grow?
Metastases can grow in different parts of the body. Some grow very close to the original tumour – for example, in the lymph nodes nearby. Doctors call these local or regional metastases.
Metastases that have spread farther away in the body are called distant metastases. They can grow, for example, in other organs such as the liver, lungs, brain or bones.
Certain cancer types have preferred sites for metastasis. For example, breast cancer often spreads to the bones, while bowel cancer often spreads to the liver.
How do metastases develop?
Metastasis starts when cancer cells separate from the tumour where they first grew. It is not easy for cancer cells to leave their original place.
Normally, cells stay in their place in the body – for example, liver cells remain in the liver. However, some cancer cells do not stay where they first started growing. They can separate from the tumour, move through the body and start growing somewhere else.
Metastasis starts when cancer cells separate from the tumour where they first grew. It is not easy for cancer cells to leave their original place.
Normally, cells stay in their place in the body – for example, liver cells remain in the liver. However, some cancer cells do not stay where they first started growing. They can separate from the tumour, move through the body and start growing somewhere else.
In order to grow in another part of the body, cancer cells have to overcome several difficult hurdles:
- Growing into nearby tissue: To do so, they must separate from other cancer cells and become mobile.
- Entering blood or lymph vessels: These vessels can carry cells to other parts of the body. It is not easy for cells to cross the walls of blood and lymph vessels. Very few cancer cells manage to do this.
- Survival while travelling: Moving through the body is dangerous for cells. Many cancer cells die and only a few survive.
- Leaving the vessels: Cancer cells have to cross the vessel walls again and enter another tissue.
- Growing in a new place: Only if the cancer cells can survive there, they begin to grow and form a new tumour.
Frequently asked questions about cancer development
The longer you live, the more DNA mistakes can build up in cells. Also, exposure to risk factors is longer with aging. Over time, the body also gets less effective at repairing DNA damage and in removing abnormal cells. That makes it more likely that a cell will eventually grow out of control and develop into cancer.
Cancer itself is not contagious – you cannot “catch” cancer from someone through touching, kissing, sharing food, or being nearby.
However, some infections can increase the risk of certain cancers. These infections can spread between people. One example is Human Papillomavirus (HPV), which can lead to cancer of the cervix and other cancers.
Yes. Some cancers can come back and spread many years after the first treatment. This can happen if a few cancer cells remain in your body. These cells may stay inactive for a long time before they start to grow again.
No. Metastases come from the original cancer. For example, breast cancer that spreads to the lungs is still breast cancer, not lung cancer. This is important for finding the right diagnosis and choosing treatment.
Additional resources and reliable information
Information Websites
- Cancer Information Service (DKFZ): Topic portals on What is cancer?, cancer development and metastasis.
Professional articles
Gerstberger S, Jiang Q, Ganesh K. Metastasis. Cell. 2023 Apr 13;186(8):1564-1579. doi: 10.1016/j.cell.2023.03.003.
Hanahan D. Hallmarks of cancer-Then and now, and beyond. Cell. 2026 Apr 16;189(8):2254-2277. doi: 10.1016/j.cell.2025.12.049.
Specialist books
Alberts B, Heald R, Johnson A, Morgan D, Raff M, Roberts K, Walter P. Molecular Biology of the Cell. 7th edition. W. W. Norton & Company, 2022.
Weinberg RA. The Biology of Cancer. 3rd edition. W. W. Norton & Company, 2023.
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Dr. Anke Ernst is part of our Team.