What are primary and secondary cancers?
Cancer can sometimes spread from where it first started to grow (primary cancer) to form a new cancer in another part of the body (secondary cancer).
Cancer of unknown primary (CUP) is when a secondary cancer is diagnosed, but doctors can’t tell where the primary cancer started. Your doctors may arrange tests to try to find out where the cancer first started. But in many cases, even after these tests, the primary cancer cannot be found.
To understand CUP, it helps to know more about cancer and the difference between primary and secondary cancers.
Cancer starts in cells in our body. Cells are tiny building blocks that make up the organs and tissues of our bodies. They divide to make new cells in a controlled way. This is how our bodies grow, heal and repair. Cells receive signals from the body telling them when to divide and grow and when to stop growing. When a cell is no longer needed or can’t be repaired, it gets a signal to stop working and die.
Cancer develops when the normal workings of a cell go wrong and the cell becomes abnormal. The abnormal cell keeps dividing making more and more abnormal cells. These eventually form a lump (tumour). Not all lumps are cancerous. Doctors can tell if a lump is cancerous by removing a small sample of tissue or cells from it. This is called a biopsy. The doctors examine the sample under a microscope to look for cancer cells.
A lump that is not cancerous (benign) may grow but cannot spread to anywhere else in the body. It usually only causes problems if it puts pressure on nearby organs. A lump that is cancerous (malignant) can grow into nearby tissue.
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A cancer is usually named after the part of the body where it first started to grow. This is known as the primary site or the primary cancer. For example, a cancer that starts in the bowel is known as a bowel cancer and a cancer that started in the lung is called a lung cancer.
Sometimes cancer cells spread to other parts of the body. They can travel through the blood or lymphatic system.
The lymphatic system is part of the immune system - the body’s natural defence against infection and disease. Lymph nodes (glands) are part of this system. They exist throughout the body and are connected together by a network of tiny tubes (ducts) that carry a fluid called lymph.
If cancer cells spread from the primary cancer to another part of the body, they may go on dividing and form a new cancer called a secondary cancer or a metastasis.
The secondary cancer is named according to the primary cancer. For example, a cancer that starts in the lungs and spreads to the liver is still a lung cancer. The secondary cancer in the liver is made up of lung cancer cells and not liver cells.
Secondary cancers are also usually treated according to the primary cancer. Knowing where the cancer started helps the doctors to know what types of treatment to use for the secondary cancer. For example, a lung cancer that has spread to the liver will be treated using lung cancer treatments. It’s treated differently to a cancer that starts in the liver (primary liver cancer).
With CUP, the primary cancer isn’t known. This means that treatment choices are often more difficult to make.