What do you use for post-treatment follow up for prostate cancer patients whose cancers make little to no PSA, such as very high Gleason grade/neuroendocrine tumors?

We can probably separate the question into two categories: the rare prostate cancers that make absolutely no PSA and those that make relatively little PSA. For those that make zero PSA, then I've generally followed them with imaging similar to how one might follow a small cell cancer of any primary ...