Maybe he was not monitoring it, and maybe he is one of the patients that do not produce high levels of PSA even with aggressive cancer.

It doesn't seem to be the case that Biden was not being screened or that his doctors missed something. From today's coverage: "Otis Brawley, a medical oncologist at Johns Hopkins University who treats patients with metastatic prostate cancer, said it would not be surprising if previous prostate exams or screenings while Biden was president failed to turn up indications of cancer. “We need a better test,” he said."

It’s possible that he didn’t have regular PSA years. Current USPSTF guidelines recommend against PSA screening after age 70 (https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/prostate-cancer-screening). So, it would be interesting to know if his doctors strictly followed current USPSTF guidelines. Did he even have a PSA screening test last year (or even the past decade) as part of his annual health exams?