Jen,
"Who grades" are those as described in the World Health Organization reports on cancers. The current edition of WHO Guidelines is dated 2016. Most of the various cancer guidelines have been updated.
As far as I can tell, there is no direct free access to the expensive WHO guides.
There is a ZORA link to a full text download of an article titled, "The 2016 WHO Classification of Tumours of the Urinary System and Male Genital Organs-Part B: Prostate and Bladder Tumours" at:
www.ncbi.nlm.nih.gov/pubmed/26996659
More common is the TNM description, see:
www.cancer.gov/about-cancer/diagnosis-staging/staging
Best,
Jack