Stem cells

Almost all cancers arise in organs and tissues containing cells that have the ability, for the lifetime of the organism to replicate in order to maintain and replace cells that are lost because of aging or  damage.

The entire lining of gut is replaced every 7 days.

The skin epithelium is replaced once a month.

Healing damaged tissue with use of large number of un damaged cells seems to be he most efficient mechanism of repair.

However tissues in which cancers usually arise eg skin,mammary glands,gut,brain and blood ,only a minority population of stem cells maintains and repairs the tissue,where as the most replication competent cells have a limited life span.