@diverjer Well...rather than write yet another thesis, and to get a short, specific answer, I "asked Siri", who said succinctly:
"Intraductal carcinoma (IDC) of the prostate is a type of prostate cancer where the cancer cells grow within the ducts or "acini" of the prostate gland. This type of cancer is usually associated with a high-grade Gleason Score, large tumor volume, and advanced stage." The ducts themselves are responsible for transporting/moving prostatic secretions to the urethra. The ducts connect the glandular acini to the urethra allowing for the release of seminal fluid during ejaculation. The acini themselves produce the prostatic fluid which is the liquid component of semen. The fluid contains enzymes and citric acid. The acini themselves are lined with epithelial cells that secret the fluid into the ducts. IDC happens because of genetic mutations to the key regulatory genes that control that epithelial cell growth. Of note: A person can be initially diagnosed as low as a Gleason 3+3=6 on biopsy, but upon surgical dissection and microscopic examination of the entire tumorous areas of the prostate, will be reclassified to a Gleason 8 or 9 if IDC is detected. Yet another example of how, as I have coined:" The Gleason Score is just the tip of a large and looming iceberg, most of which is not seen or known until it is all examined on post-RP examination of the entire prostate.
Makes me wonder how many men with a Gleason 3+3=6 or 3+4=7 are, with misguided, overly-confident assumption, put on Active Surveillance, only to discover further down that two-year journey, that they have suddenly doubled, tripled, or quadrupled their PSA and have become a Gleason 8 or 9 after a second biopsy, and now have fewer viable options for successful treatment and longevity.