Primary and secondary cancers
Cancer can sometimes spread from where it first started to grow (primary cancer) to form cancers in other parts of the body (secondary cancers).
CUP is when a secondary cancer is diagnosed, but even after tests have been carried out, doctors can’t tell where the cancer first started. The primary cancer is unknown. To understand CUP it helps to know more about cancer and the difference between a primary and a secondary cancer.
The organs and tissues of the body are made up of tiny building blocks called cells. Cancer is a disease of these cells. Cancer is not a single disease with a single cause and a single type of treatment. There are more than 200 different types of cancer, each with its own name and treatment.
Although cells in different parts of the body may look and work differently, most repair and reproduce themselves in the same way. Normally, cells divide in an orderly and controlled way. But if for some reason the process gets out of control, the cells carry on dividing and develop into a lump called a tumour.
Tumours are either benign (non-cancerous) or malignant (cancerous). Doctors can tell if a tumour is benign or malignant by removing a piece of tissue (biopsy) and examining a small sample of cells under a microscope.
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 break away from the original (primary) cancer. They may spread to other parts of the body through the bloodstream 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. But the cancer is still named and treated 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. So it’s treated differently to a cancer that starts in the liver (primary liver cancer).