Mesonephric remnant hyperplasia involving prostate and periprostatic tissue: findings at radical prostatectomy.

Mesonephric remnant hyperplasia is a very rare benign mimicker of prostate adenocarcinoma. As most reported cases are from transurethral resection specimens, the anatomic location and histologic spectrum of this entity have not been fully elucidated. Its immunohistochemical profile using current prostatic diagnostic markers has also not been well studied. In this study, we retrospectively characterized 10 cases of mesonephric remnant hyperplasia involving the prostate and periprostatic tissue, including 8 cases seen in radical prostatectomy specimens, with emphasis on the histopathologic and immunohistochemical features. Patients ranged in age from 48 to 70 years (average, 60 y). Seven of them had concurrent prostatic adenocarcinoma and underwent radical prostatectomy; one patient underwent prostatectomy because of the misdiagnosis of mesonephric remnant hyperplasia on transurethral resection as carcinoma; 2 patients had transurethral resection for urinary obstruction. The distribution of prostatic mesonephric hyperplasia was concentrated in 2 areas: one was in the anterior fibromuscular stroma and adjacent anterolateral periprostatic tissue (n=6 of 8); the other was located toward the base posteriorly and posterolaterally either within or exterior to the prostate and around the seminal vesicle (n=4 of 8). Histologic patterns observed included the following: small-to-medium-sized acini or tubules with a lobular distribution (n=10 of 10); cysts either in clusters or scattered containing secretions (n=8 of 10); small or ill-formed glands with an infiltrative growth (n=7 of 10); glands with papillary infoldings or micropapillary tufts (n=4 of 10); and 2 cases exceptionally displayed nodules of ill-formed small glands intermixed with spindle cells, mimicking sclerosing adenosis or Gleason pattern 5 prostate cancer. Most cases (7 of 10) had florid hyperplasia and harbored 3 or more growth patterns. All cases were negative for prostate-specific antigen. Cytokeratin 34βE12 was diffusely positive in 4 of 9 cases, and showed focal immunoreactivity in the remaining 5 cases. Except for focal positivity seen in 4 of 7 cases, p63 was largely negative. Racemase was focally positive in 4 of 7 cases. Small glands with an infiltrative growth pattern, the most difficult to distinguish from cancer, were negative (n=3 of 6) or only focally positive (n=3 of 6) for 34βE12, negative for p63 (n=6 of 6), and focally positive for racemase (n=4 of 6). All cases examined in the study were diffusely positive for PAX8. In conclusion, mesonephric remnant hyperplasia not only involves the bladder neck and base of the prostate as previously described, but may also present as a florid growth in the anterior fibromuscular stroma from the apex to the base, closely mimicking prostate cancer. Although basal cell marker and racemase expression overlaps with prostate cancer, mesonephric hyperplasia's unique morphology along with distinctive immunohistochemical expression of PAX8 and lack of prostate-specific antigen can help in distinguishing this benign entity from prostatic adenocarcinoma.