Cancer of unknown primary (CUP)

To diagnose secondary cancer, a specialist doctor called a pathologist examines the cancer cells under a microscope. The pathologist can see that the cancer cells do not belong to or originate in the surrounding tissue, and this can be confirmed by further laboratory tests. With CUP, the cancer cells could possibly have come from a number of places but no primary cancer can be found.

There could be several reasons why the primary cancer cannot be found. It may be that:

• the secondary cancer has grown and spread quickly, but the primary cancer is still too small to be seen on scans
• your immune system has destroyed the primary cancer, but not the secondary cancer
• the primary cancer cannot be seen on x-rays, scans or endoscopies because it is hidden by a secondary cancer that has grown close to it
• the primary cancer was removed during surgery for another condition without doctors realising the cancer was there, so some cancer cells remained behind and have since spread
• there may never have been a detectable primary cancer mass – a single cell may have changed into cancerous cells and then spread through the body.

Finding the primary cancer helps doctors decide what treatment to recommend. If it can’t be found, the treatment path can be less clear but treatment can still be given. To help plan the treatment, your doctor will try to learn as much as possible about the spread of the cancer, the cells involved, your symptoms and your medical history.

Different cases of CUP will have different causes, but without knowing where the cancer started, it’s difficult to be specific. Cancer is a group of more than 200 different diseases. Each type of cancer has different risk factors, such as getting older, eating an unhealthy diet, smoking, drinking too much alcohol, being overweight or certain infections. These risk factors may play a role in some cases of CUP.

CUP is the 11th most common cancer in Australian women and the 12th most common cancer in Australian men.3 There are over 2500 new cases of CUP diagnosed each year in Australia. For both women and men, CUP is more likely to occur over the age of 60.

Even if tests can’t find where the cancer started, your doctor will try to work out what type of cell the cancer developed from. Knowing the type of cell helps doctors work out what sort of treatment is most likely to be helpful. Most cancers are cancers of the epithelial cells. These are found in the cells lining the skin and internal organs. Cancers that start in epithelial tissue are called carcinomas. In most people with CUP, doctors can tell that they have some sort of carcinoma. There are different types of carcinoma depending on which type of epithelial cell is affected. See table below for more information.