Aberrant Cytokeratin 20 Reactivity in Epithelioid Malignant Mesothelioma: A Case Report

Malignant mesothelioma is an uncommon neoplasm that should be distinguished from the more common pulmonary adenocarcinomas and other metastatic lesions. Although diagnosis is based on morphologic features, immunohistochemical stains such as Calretinin, WT-1, CK-5/6, D2-40, Ber-Ep4, and MOC-31 are routinely used. Other organ-specific immunohistochemical markers are used when metastases from unknown primary lesion is suspected clinically. Here, we report a case of pleural epithelioid malignant mesothelioma expressing CK20. A 68-year-old male presented to the Emergency Department with nonproductive cough and progressive shortness of breath. Chest x-ray showed a large left-sided pleural effusion. Metastasis from a gastrointestinal primary was clinically suspected. Cytopathologic examination of the pleural fluid demonstrated atypical cells singly and in clusters with round nuclei, prominent nucleoli, and dense cytoplasm. The cell block demonstrated single and clusters of atypical cells positive for calretinin, D2-40, WT-1, CK-5/6, and CK7. Ber-EP4, MOC-31, TTF-1, Napsin-A, and CDX-2 were negative. CK20 was diffusely positive. A diagnosis of atypical mesothelial proliferation with aberrant CK20 expression was made. A subsequent pleural biopsy demonstrated sheets of highly atypical cells that were diffusely and strongly positive for the mesothelial markers and CK20. Multiple studies have shown malignant mesotheliomas to lack CK20 reactivity. To our knowledge, this is the first case report of a diffuse and strong CK20-positive mesothelioma. Such aberrant expressions should be kept in mind when cases are histologically atypical or lack reactivity for multiple mesothelial markers, especially when a gastrointestinal primary malignancy is suspected.