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Prostate cancer gets rated with something called a Gleason score. It measures how mutated and aggressive the cancer looks after a biopsy. Typically if someone has a low Gleason score, the odds of the cancer spreading are low and even if it does it'll take a while to get to that point. When you're 73 there are so many other things that will kill you before you get to that point so treatment usually isn't pursued.

Having cancer in your prostate at his age is super common and probably was just an incidental finding.