Diagnostic utility of calretinin immunohistochemistry in cytologic cell block preparations

Calretinin (CR) is a valuable marker in the immunohistochemical distinction between malignant mesothelioma (MM) and adenocarcinoma (ACA) in tissue sections. However, there is limited and conflicting data regarding the utility of CR in this differential diagnosis on cytologic material, especially cell block preparations. Also, the possible role of CR in the distinction of papillary serous borderline tumor (SBT) cells from reactive mesothelial cells in peritoneal washings has not been examined.

Formalin-fixed paraffin-embedded cell block specimens of cytologic fluids, washings, and aspirates with a suspicious or positive cytologic diagnosis and a confirmed diagnosis of MM (29 cases), ACA (39 cases), and SBT (10 cases) were used for CR immunohistochemistry (IHC).

Moderate to strong staining in > 50% of tumor cells was noted in 26 of 29 (90%) MMs but in only 3 of 39 (8%) ACAs and 1 of 10 (10%) SBTs. Admixed reactive mesothelial cells (when present) were strongly positive in all fluid specimens, but the staining pattern of benign non-reactive mesothelial cells in washing specimens was less reliable.

CR is both a sensitive and specific marker of reactive and neoplastic mesothelial cells in cytologic cell block preparations and is thus useful in the differential diagnosis of MM and metastatic ACA in malignant effusions. However, CR IHC does not appear to allow definitive identification of SBT in peritoneal washings.