AS protocols are intentionally flexible; in other words, each doctor or institution has some general guidelines they seek to follow, but may modify on a case-by-case basis as needed. No "one-size-fits-all" approach.
Many of the protocols you will find online don't even (yet) include much reference to mpMRIs, although in contemporary practice at major institutions they are becoming more widely used...often on an alternating basis with needle biopsies.
I have a link to a paper published last year from a set of AS-experienced doctors (including Klotz) who documented their own recommendations/guidelines, including this:
RECOMMENDATION 3: The AS protocol should include the following tests:
- PSA test every 3 to 6 months.
- DRE every year.
- 12- to 14-core confirmatory transrectal ultrasound (TRUS) biopsy (including anterior directed cores) within 6 to 12 months, then serial biopsy a minimum of every 3 to 5 years thereafter.
The AS protocol may include the following test:
- mpMRI is indicated when a patient’s clinical findings are discordant with the pathologic findings and it is useful in identifying occult cancers or changes indicative of tumour progression in patients at risk.
www.ncbi.nlm.nih.gov/pmc/articles/PMC4479637/