Distinguishing Prostatic From Colorectal Adenocarcinoma on Biopsy Samples: The Role of Morphology and Immunohistochemistry.

Context.—Poorly differentiated carcinoma on prostate or colorectal biopsy can occasionally present a diagnostic challenge in determining tumor source especially in locally advanced colorectal carcinoma (CRCa) or prostate carcinoma (PCa). Such determination can affect prognosis and therapy. Objective.—To evaluate the role of morphology and immunohistochemistry in the previously mentioned setting. Design.—Surgical pathology and consultation records. Hematoxylin-eosin sections were reviewed in 16 cases (11 PCa, 5 CRCa). Immunohistochemistry for 9 markers was performed in 15 cases. Results.—Dirty necrosis, seen in 5 (100%) of 5 CRCa and 2 (18%) of 11 PCa cases, and the presence of columnar cells with basal nuclei, seen in 5 (100%) of 5 CRCa and 1 (9%) of 11 PCa cases, appear to be the most useful morphologic parameters. Immunohistochemistry confirmed the value of prostate-specific antigen (PSA), CDX2, cytokeratin (CK) 20, and β-catenin in the differential of CRCa (0% PSA+, 60% CDX2+, 80% CK20+, and 100% β-catenin+) versus PCa (80% PSA+, 0% CDX2+, 10% CK20+, and 0% β-catenin+). P501S had a similar sensitivity as PSA in detecting PCa (80%). Two (20%) of 10 PCa cases were positive for 1 of the 2 markers but not the other. P501S was negative in all 5 cases of CRCa. Conclusions.—P501S is a useful marker in this setting when included together with PSA, CDX2, CK20, and β-catenin. P501S labels a subset of PCa cases that are negative for PSA. Dirty necrosis and/or columnar cells with basal nuclei could also be of help. [ABSTRACT FROM AUTHOR]