What is cancer of unknown primary?

Cancer occurs when abnormal cells within the body grow in an uncontrolled way. Cancer can form in any of the body’s tissues.  

The first cancer that develops is called the primary cancer. Primary cancers can spread to other places in the body, such as lymph nodes or other organs . Cancer that forms from the spread of a primary cancer is called a secondary cancer or a metastasis.  

Sometimes, a secondary cancer is found but the primary cancer cannot be found. This is called cancer of unknown primary. 

There are several reasons why it may not be possible to find the primary cancer:[1] 

  • The primary cancer may be too small or slow growing to be easily detected. 
  • The body’s immune system may have killed the primary cancer. 
  • The secondary cancer may be ‘hiding’ the primary cancer on scans. 
  • The primary cancer may already have been removed by surgery for another condition. 
  • The primary cancer may not have developed a mass (tumour) – a single cancerous cell may have spread, causing the secondary cancer.