Abstract
Cancers of unknown primary site are metastatic cancers for which primary tumors are not found after detailed investigations. In many cases, the site of origin is not identified even on postmortem examination. These cancers are the fourth most common cause of cancer death. The biological events involved in the development of this type of cancers remain unknown. This manuscript discusses that, like metastatic cells, stem cells have a natural ability to migrate. A cancer of unknown primary site would form when deregulated, premalignant or cancerous stem cells migrated away from their natural tissue and gave rise to a cancer in a new site before or without generating a tumor in their original tissue. It is important to realize that forming a tumor in a tissue is not a prerequisite for stem cells to migrate away from that tissue. This view is in accordance with recent observations that strongly support the tumorigenesis model in which cancer arises from normal stem cells. Evidence has accumulated that cancer stem cells may play a key role in cancer progression and resistance to therapy. Successful treatment of cancer, including that of unknown primary site, may therefore require the development of therapies against cancer stem cells.