A high Gleason score means cells that are very abnormal and unpredictable. Any rogue cells whose abnormality leads to poor reproductive success will die fast; but any rogue cells whose abnormality includes excellent ability to reproduce will multiply rapidly. Guys with high Gleason scores hope to have a lot of the first and none of the second, but we have to prepare for the worst, watching for signs of change, which can come rapidly as the cells evolve.
A low PSA doubling time means either that the prostate cells are multiplying rapidly (a situation that will likely continue unabated in the absence of intervention), or that they are progressively leaking more and more PSA into the bloodstream (a situation that is self-limiting).
By definition, a high-Gleason cell is unpredictable: It might evolve into an ever-more-rapidly type of cell (super-exponential rate of increase), or it might not (exponential or sub-exponential rate of increase.
Even for those high-Gleason cell lines that multiply very rapidly, there is eventually some competition for resources, thus slowing what would otherwise be a strictly exponential rise.
/Paul in Los Angeles