There is no known cause of cancer of unknown primary (CUP). Find out about the general causes of cancer.
General causes of cancer
Cancer is not one single disease. There are more than 200 different types of primary cancer. We can't be specific about the possible causes or risk factors of CUP, because we don't know which type of cancer it is.
Different types of cancer have different causes. For example, smoking most often causes lung cancer and overexposure to ultraviolet light (from the sun or sun beds) is the main cause of skin cancer.
Generally speaking, cancer is a disease caused by many different factors coming together, rather than one single thing. Our risk of cancer depends on a combination of our genes, our environment and our lifestyle.
How you may feel
People diagnosed with cancer often feel strongly that they must find out what caused it. Unfortunately, for many types of cancer there is no obvious cause.
Cancer can just happen because of bad luck. Over years, a cell gathers changes to its genetic code (DNA) that eventually make it become cancerous.
Cancer causing chemicals, such as those in cigarettes, may trigger one or two of the changes. But often, something just goes wrong in the DNA of the cell by itself – a bit like a spelling mistake in the code. The damaged cell then begins to grow and divide much more quickly than normal cells. Or it doesn't die off as a normal cell would. The overgrowth of cells forms a tumour.
Being diagnosed with any type of cancer is difficult to accept and you may find yourself spending a lot of energy trying to work out what caused it. But often the cause is never found.
Everyone copes with this in their own way. You may find it helpful to focus your energy on getting through your treatment and staying as well as you can.