According to my urologist, malignant cells can escape during surgery.
According to my radiation oncologist, malignant cells are sometimes left behind during surgery, especially if the cancer is at the bottom of the prostate, as this is a difficult spot to get everything out.
It depends on what you mean by 'bottom'. Prostate language is confusing with the top (closest to the bladder, seminal vesicles etc) being named the 'base' and the bottom (closest to the penis, away somewhat from other glands) being named the apex. Therefore most spread is actually at the top (base) where it often attacks the bladder wall, vesicles etc.