Answer: Colonic mucosa

Histology: On three separate cores there is a detached fragment of tissue containing somewhat crowded basophilic glands. These glands are associated with fibrotic connective tissue. The glands are lined by columnar-appearing nuclei, some with hyperchromasia and visible nucleoli. Cytoplasm is amphophilic. Scattered mitotic figures are identified.

Immunohistochemically, a stain combining antibodies to high molecular weight cytokeratin, p63, and AMACR (racemase) showed racemase staining without evidence of high molecular weight cytokeratin or p63 immunoreactivity.

Discussion: There are several features both architecturally and cytologically that are worrisome for adenocarcinoma. First, the glands are crowded which is one of the architectural patterns seen within adenocarcinoma of the prostate. At higher magnification, the finding of enlarged nuclei, some visible nucleoli, and the mitotic figures further compound the resemblance to adenocarcinoma. If this were to be adenocarcinoma, it would be more consistent with ductal adenocarcinoma given the pseudostratified columnar nature of the nuclei. However, the vast majority of ductal adenocarcinomas have either a papillary or cribriform architectural pattern whereas the single glandular pattern, which the current case would have to represent if it were to be ductal adenocarcinoma, is relatively rare. The single glandular pattern of ductal adenocarcinoma would show a more infiltrative growth pattern as compared to the case illustrated. The features seen in this case are those of distorted colonic mucosa showing regenerative changes. Normally, small fragments of colon sampled during the process of needle biopsy of the prostate are more readily recognizable of being of colonic origin. What makes this case difficult is the lack of goblet cells and the reactive nuclear atypia due to regenerative findings within the colonic tissue samples. The key feature to the diagnosis is the relatively uniform architecture of glands that are evenly spaced. The fact that the tissue is on a small fragment dissociated from the rest of the benign prostate tissue is also helpful. Furthermore, rather than the glands being situated within smooth muscle as would be expected in prostate adenocarcinoma, the glands are situated within a fibrotic connective tissue corresponding to lamina propria. In the current case to help verify the diagnosis, stains for prostate specific antigen were performed, which were negative. In addition, the stains for cytokeratin 7 and 20 were done, showing strong staining for cytokeratin 20 and negative for cytokeratin 7, consistent with colonic mucosa as well. We have seen several cases of normal distorted colonic mucosa obtained on needle biopsy, which have caused difficulty in interpretation by contributing pathologists.

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