Three possibilities that I know of...
1) Pathology report could be wrong
2) Some cancer cells outside the tissue examined could remain (not likely if margins were deemed negative)
3) Cancer returns later, which it does in a significant percentage of cases where the Gleason score was very high. That is why, despite a good pathology report, I am likely to have radiation later this year.
Danman Bob, Born 1951
Nerve-sparing, open prostate surgery November 13, 2007
Gleason score 9, PSA 14; Biopsy result - 9 of 12 sticks showed cancer
Post-op pathology stated that cancer was confined to the prostate
Unrelated surgery January 2008 delayed incontinence recovery, which is now showing good signs of improvement (fraction of a pad a day as of late April 2008)
100 MG Viagra 3 times a week beginning December 2007
Osbon Erec-Aid Esteem manual pump for therapy beginning mid-February 2008
30 MG papaverine/1 MG phentolamine bimix injections beginning late April 2008
Five week post-op PSA 0.2, five month post-op PSA 0.1, next test August 2008