Performance of different prostate specific antibodies in the cytological diagnosis of metastatic prostate adenocarcinoma

Background: Diagnosis of metastatic prostate adenocarcinoma (metPA) in cytology specimens can be challenging and frequently requires the use of immunohistochemistry (IHC). Prostate specific membrane antigen (PSMA) and NKX3.1 have emerged as promising IHC markers to determine prostatic origin of metPA in surgical specimens. Our goal is to evaluate the performance of PSMA and NKX3.1 and compare them with those of prostate-specific antigen (PSA) and prostate specific alkaline phosphatase (PSAP) in the cytological diagnosis of metPA. Materials: Cytology specimens from patients with a history of prostate adenocarcinoma at our institution between January 01, 2005 and December 31, 2015 were retrieved. IHC stains were performed on the cell blocks. In addition to the staining pattern and intensity, the sensitivity, and specificity of PSMA and NKX3.1 were assessed and compared to those of PSA and PSAP markers. Results: A total of 56 cytology cases were retrieved with the following diagnoses: 13 metPA, 37 metastatic carcinomas from other origins, 4 rare atypical cells, and 2 benign. Additional 9 cases were re-classified as metPA based on positive PSMA and/or NKX3.1 immunostains. In our cohort of 22 cases of metPA, 18 were positive for PSMA (82%), 15 for NKX3.1 (68%), 9 for PSA (41%), and 9 for PSAP (41%). PSMA and NKX3.1 were negative in all 6 cases of metastatic carcinoma of nonprostate origin (specificity 100%). PSMA demonstrated strong membranous staining pattern, and NKX3.1 exhibited moderate nuclear staining pattern. Conclusion: Because of their higher sensitivity and specificity, PSMA and NKX3.1 are valuable surrogate markers for metPA in cytology specimens, when compared with PSA and PSAP markers.