Distorted rectal tissue on prostate needle biopsy: a mimicker of prostate cancer

Rectal tissue is often seen in needle biopsies of the prostate gland. On rare occasion distorted rectal glands can mimic prostatic adenocarcinoma, an issue not previously addressed in the peer-reviewed literature. We evaluated 16 prostate needle biopsies received in consultation where the submitting pathologist questioned whether a focus of rectal tissue was prostate cancer. In addition to the distorted architecture, features mimicking prostate cancer included: (1) blue-tinged intraluminal mucinous secretions in 10 cases (63%), (2) prominent nucleoli in 6 cases (37%), (3) mitotic activity in 6 cases (37%), (4) extracellular mucin in 5 cases (31%), and (5) adenomatous changes of the rectal tissue in 1 case (6%). Immunohistochemical results further mimicked prostate cancer with negative stains for the basal cell markers high-molecular weight cytokeratin (n=6) and p63 (n=4), and positive stains for racemase in 4 of 5 biopsies. Diagnostic clues to recognizing that these foci were distorted rectal fragments were the presence of (1) lamina propria in 12 cases (75%), (2) rectal tissue located on a detached fragment of tissue in 10 biopsies (63%), (3) associated inflammation in 10 cases (63%), (4) goblet cells in 7 cases (44%), and (5) muscularis propria in 6 cases (37%). In 2 cases, there was negative staining for prostate specific antigen (PSA) and in 1 case negative staining for cytokeratin 7 and positivity for cytokeratin 20. Rectal glands are associated with many of the classical features of prostate cancer, and immunohistochemistry may be misleading. Recognition of these features mimicking prostate cancer and awareness of other findings that are diagnostic of rectal tissue on biopsy can prevent a misdiagnosis of atypical prostate glands or prostate cancer.