Small glandular proliferations on needle biopsies: most common benign mimickers of prostatic adenocarcinoma sent in for expert second opinion.

The current study aimed to determine the incidence of various benign mimickers of prostatic adenocarcinoma most commonly encountered in a busy consultation practice. All prostate needle biopsies from the consult service of one of the authors were prospectively evaluated over a 7-month period. Only cases with foci where the contributor questioned malignancy and which upon expert review the entire case was determined to be benign were included in this study. A total of 567 separate suspected atypical foci from 345 patients of a total of 4,046 patients (8.5%) received in consultation were identified. Of these, 281 foci (49.5%) had immunohistochemical (IHC) studies performed by the outside institution, which included high molecular weight cytokeratin (HMWCK) (n = 280), alpha-methylacyl-CoA racemase (AMACR) (P504s) (n = 45), and p63 (n = 34). The most common mimicker was partial atrophy (203 of 567; 35.8%). Technically adequate IHC for basal cells was performed in 117 cases of partial atrophy with patchy or patchy/negative staining seen in 102 of 117 (87%), with the remaining 13% of cases completely negative. A total of 15 of 19 (79%) cases of partial atrophy were positive with AMACR. Crowded benign glands, insufficiently crowded or numerous to warrant a diagnosis of adenosis, was the second most common mimicker (146 of 567; 25.7%). Crowded benign glands had patchy or patchy/negative IHC for basal cells in 66 of 81 (81%) cases with the remaining 19% of cases completely negative. A total of 7 of 11 (64%) cases of crowded glands were positive for AMACR. In the past, complete atrophy, adenosis, seminal vesicle, and granulomatous prostatitis were considered common mimickers of prostate cancer on prostatic needle biopsies. Our study shows that currently partial atrophy and crowded benign glands are the most common benign changes causing diagnostic difficulty and prompting consultation. Negative or patchy staining for basal cells and positive staining for AMACR may contribute to diagnostic difficulty in these entities.