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."