Do We Know What Causes a Cancer of Unknown Primary?

Cancers of unknown primary (CUP) include a variety of cancers, which may each have a number of different causes. This is why it’s hard to assign a particular cause to CUP.

Cancer is the result of changes in a cell’s DNA. DNA is the chemical in each of our cells that makes up our genes – the instructions for how our cells function. It’s packaged in chromosomes – long strands of DNA in each cell. We normally have 23 pairs of chromosomes in each cell in our body (one set of chromosomes comes from each parent). We usually look like our parents because they are the source of our DNA. But DNA affects more than how we look.

Some genes control when our cells grow, divide, and die. Certain genes that promote cell division or make cells live longer than they should are called oncogenes. Others that slow down cell division, or make cells die at the right time, are called tumor suppressor genes. Cancers can be caused by DNA mutations (changes) that “turn on” oncogenes or “turn off” tumor suppressor genes. In most cases, a number of mutations are needed before a cell becomes cancerous.

Most of the DNA mutations related to CUP probably occur during a person’s lifetime rather than having been inherited before birth. These acquired mutations may sometimes result from known exposures such as tobacco smoke, ultraviolet light, radiation, or certain cancer-causing chemicals, but often they occur for no apparent reason.

As scientists learn more about how cancers develop, they are also beginning to understand why some cancers tend to grow and spread so quickly that they are diagnosed as cancers of unknown primary.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master's-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: July 2, 2014 Last Revised: January 27, 2016

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