Barry is spot on! The MRI indicates areas of interest and the biopsy, which uses a predefined plan of where to extract the cores and how many, is based on the MRI. These cores are then examined and an interim diagnosis provided which guides the next stage of treatment. But even the biopsy is based on samples of the gland and does not investigate the seminal vessels etc and so only when the prostate (and lymph nodes) are sliced and examined on the lab table can they be definitive.
For example, my MRI showed a single lesion and a G7(3+4) but the lab changed this to G9(4+5) after examining it.
However, they are experienced in what they do and even though the diagnosis are 'guesstimates' they are pretty accurate in most cases.