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What is cancer?

What is cancer?

Our bodies are made up of trillions of cells. Usually every cell has a certain function in the body. Cancer is caused by cells that no longer work properly. Uncontrolled cell growth can crowd out other cells and become a tumour (Fig. 1).

Fig. 1: Cancer cells crowding out healthy cells.

Types of tumours

Benign tumours are not cancer. They usually grow slowly and push away healthy tissue. They do not spread to other parts of the body. If a benign tumour is large, it can press on other body organs or structures and cause bleeding or pain.

Malignant tumours are cancer. A malignant tumour is able to invade and destroy surrounding tissue.

Metastatic tumours grow from cancer cells that have travelled to another part of the body (Fig. 2). The original tumour, called the primary tumour, is the source of the cancer cells. The new tumour is called the secondary tumour. Cancer cells can spread through the blood (haematogenous metastasis) or through the lymph vessels to the lymph nodes (lymph node metastasis).

 Example—Metastasis in prostate cancer 

 Prostate cancer can sometimes spread to the bones and cause another tumour to grow there. When this happens, the cancer cells from the prostate (primary cancer) have travelled with the blood to the bones. The new tumour (secondary cancer) is called a bone metastasis of prostate cancer.
Fig. 2: Malignant cancer cells can travel to other parts of the body and become metastatic tumours.

Different cancers spread to common sites or organs, usually in a typical pattern. Cancer can spread to any part of the body, but this is more rare. The process of metastasis is complex, and many factors have to be just right. Of a few thousand cancer cells, it is thought that only few survive and are able to grow in another part of the body.


Micrometastases contain very few cells and can only be seen in a tissue sample under a microscope.

Macrometastases are large enough to be seen with an imaging test such as CT scan or MRI.

If the doctor thinks micrometastases are likely, drug therapy can be given to try to kill the remaining cancer cells before they can grow into macrometastases (adjuvant therapy).

What causes cancer?

Cancer happens in the body’s cells. Errors called mutations can occur in the genes as a cell reproduces. If the cell cannot repair the mutation, the cell’s components and finally the cell can malfunction. It is commonly thought that if there are more than six mutations in critical genes, a healthy cell can turn into a cancer cell.

Not every mutation leads to cancer. Mutations can be inherited, which means they are in the genes already at birth, or they can occur during your lifetime. Mutations can occur by chance, without a known cause. Sometimes they are caused by harmful substances in the environment, such as sunlight, air pollution, cigarette smoke, or alcohol. Exposure to harmful substances can increase the risk of getting cancer.

  FAQ—Why does cancer sometimes come back?      

Doctors do their best to destroy all cancer cells, but sometimes cancer comes back even after a long time. Cancer cells may have spread or gone into a state of rest somewhere in the body and eventually grow there.

Sometimes not all cancer cells are removed during surgery or killed by chemotherapy or radiation; these cells can grow and produce a new tumour of the original cancer at the same place (relapse). In addition, cancer cells are very effective at adapting to new conditions and can become resistant to certain types of treatment.

After cancer treatment, you will be scheduled for follow-up visits with your doctor. It is important to attend these visits. Your doctor can suggest additional treatment if there are signs of tumour cells remaining in the body.

Types of cancer

Cancer can arise from cells in different tissues and organs in the body (Table 1). For example, a primary cancer that develops in the breast is called breast cancer. Cancer will have different names depending on the tissue of origin.

The main groups of cancer

Carcinoma This cancer begins in epithelial cells. These cells line the surface of the skin, inner organs, and glands. It is the most common type of cancer and usually grows as a tumour. There are different types of carcinomas. For example, adenocarcinomas originate from the glands, squamous cell carcinomas from the skin, and urothelial carcinomas from the lining of the urinary system.
Melanoma This cancer begins in pigmented cells. Melanomas usually develop in the skin but also can begin in inner organs.
Sarcoma This cancer begins in different types of tissues in the body, such as bone (osteosarcoma) or “soft tissues” such as muscles, nerves, vessels, cartilage, and connective or fat tissue (soft tissue sarcomas).
Lymphoma This cancer begins in the white blood cells in the lymphatic system, for example, in a lymph node.
Leukaemia This cancer begins in blood-forming tissue such as the bone marrow and causes the tissue to produce abnormal blood cells. This type of cancer usually does not form a solid tumour.
Brain and spinal cord tumours These tumours arise in the brain and spinal cord, called the central nervous system, and usually are limited to this site.

This information was produced by the European Association (EAU) Patient Information Working Group.
– Dr. H. Reis, Essen (DE)