Answer: Mucinous metaplasia

Histology: This section of tissue is taken from a radical prostatectomy specimen. Within the transition zone there is a focus of mucinous glands. These glandular structures are seen adjacent to skeletal muscle and are associated with benign prostate glands. These glands consist of cells with abundant mucinous cytoplasm and small dark nuclei without atypia.

Discussion: Mucinous metaplasia of prostate glands is seen in approximately 1% of prostates. The cells are positive for PAS, diastase resistent, and mucicarmine. They are usually negative for PSA and PSAP. Mucinous metaplasia is seen in benign processes, including atrophy, transitional cell metaplasia, basal cell hyperplasia, prostatrophic hyperplasia, and nodular hyperplasia. It’s finding is not associated with adenocarcinoma of the prostate. Rarely it may be seen in high grade PIN.

Mucinous metaplasia of the prostate may be mistaken for Cowper’s or bulbourethral glands, which are benign structures with mucinous epithelium. Cowper’s glands are anatomically located at the proximal portion of the membraneous urethra and not in the prostate proper. Their location outside of the prostate gland proper helps to differentiate it from mucinous metaplasia of prostate glands. Foamy gland adenocarcinoma of the prostate will show cytologic and architectural atypia diagnostic for cancer, while mucinous metaplasia lacks cytologic atypia and are seen in association with benign glands. High grade PIN would show cytologic atypia, and although mucinous metaplasia may rarely be seen in high grade PIN, cells with abundant mucinous cytoplasm is not a common feature. Foamy PIN is a rare entity that has been reported. Mucinous metaplasia can be distinguished from foamy PIN by seeing focal partial involvement of small benign glands, as seen in this case, in contrast to large glands with papillary infolding composed entirely of foamy cells in foamy PIN.