Causes

In general, cancer forms when cells develop changes (mutations) in their DNA. The DNA contains instructions that tell cells what to do. Certain mutations can cause a cell to multiply uncontrollably and to continue living when normal cells would die. When this happens, the abnormal cells accumulate and form a tumor. The tumor cells can break away and spread (metastasize) to other parts of the body.

In carcinoma of unknown primary, the cancer cells that spread to other parts of the body are found. But the original tumor isn't found.

This can happen if:

• The original cancer is too small to be detected by imaging tests
• The original cancer was killed by the body's immune system
• The original cancer was removed in an operation for another condition