I was watching a PBS special on cancer last week. One of the physicians/scientists/researchers made a comment specifically about prostate cancer. He said they did a study and found during autopsies that a very high percentage (don't remember the number, but it was high, because it caught my attention) of men have prostate cancer cells. The remainder of his comment was the clincher... most prostate cancer cells do not develop into full blown cancer.

Later in the show, one of the gentlemen stated we all have cancer cells growing in our bodies constantly and that some mutate, sometimes growing into what we label "cancer" cells (even they are initially cancer cells), but others die off... but they are there constantly.

It was the Charlie Rose show. Here's a list of his guests:

SIR PAUL NURSE President of The Rockefeller University

HAROLD VARMUS President, Memorial Sloan-Kettering Cancer Center

DAVID NATHAN President Emeritus, Dana-Farber Cancer Institute

DEB SCHRAG Memorial Sloan-Kettering Cancer Center

MARTIN ABELOFF Director, The Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins

Very interesting show... don't know if this is it in its entirety Charlie Rose Show PBS, but you can find at least some of it through this link (find Wednesday, April 18th, 3rd down center column as of this posting):
http://www.charlierose.com/
David Nathan was the one who made the comment about cancer cells growing in himself as he spoke (not verbatim).