Background The distinction between mesothelioma with epithelioid features and metastatic carcinoma may be challenging, particularly on cytology. A novel 2‐hit Claudin‐4 and BRCA‐associated protein 1 (BAP1) panel was investigated. Methods The objective of this study was to determine the sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of the panel on cytology from pleural effusions and matched biopsies, including 49 malignant mesotheliomas on cytology with 43 matched biopsies, 49 normal/reactive mesothelial proliferations, and 49 pleural metastatic carcinomas from different primaries with 21 matched pleural biopsies. The diagnostic role of the 4 categories obtained by crossing the immunostaining results was analyzed. Results Claudin‐4 strongly stained all metastatic carcinomas and tested completely negative in normal mesothelium, benign reactive mesothelial hyperplasia, and malignant mesothelioma. All normal and benign mesothelial proliferations and all carcinomas except 1 were immunoreactive for BAP1, whereas BAP1 loss was observed in 88% of malignant mesotheliomas. The expression of Claudin‐4 alone excluded all benign and malignant mesothelial growth, consistently characterizing all metastatic carcinomas. Double negativity was evident in all malignant mesotheliomas, and double positivity was observed in all metastatic carcinomas. BAP1‐positive/Claudin‐4–negative status was observed only in malignant mesotheliomas and benign mesothelial proliferations. A single metastatic anal squamous cell carcinoma had BAP1‐negative/Claudin‐4–positive staining. Conclusions Claudin‐4 expression was completely specific and sensitive for metastatic carcinoma, excluding mesothelial proliferations. BAP1 staining characterized 98% of metastatic carcinomas and 100% of benign mesothelial proliferations, whereas negativity was observed almost exclusively in mesotheliomas. This 2‐hit panel is probably the best compromise for differentiating malignant mesothelioma and metastatic carcinoma on either cytology or biopsy specimens.

A rare case of synchronous metastatic lung adenocarcinoma to the thyroid gland, diagnosed on routine fine‐needle aspiration cytology, is described. The combined use of morphological features associated with immunocytochemical analysis on smeared specimens as well as on cell‐block material, is a feasible tool to achieve a definitive diagnosis and it also permits determination of the required biomarkers.

In this case report, the authors describe the cytological features of a primary Merkel cell carcinoma of the parotid gland and discuss how the diagnosis was confirmed with immunocytochemistry.