Mesothelial hyperplasia with reactive atypia: Diagnostic pitfalls and roleof immunohistochemical studies - A case report

The cytomorphologic features of highly reactive mesothelial cells can he di fficult to distinguish from malignant cells. We report on an unusual case o f mesothelial hyperplasia in a pericardial effusion. The specimen contained bizarre-shaped cells and large tissue fragments irt a patient with a histo ry of lung carcinoma. The atypical cells were negative for CEA and LeuM-1 a nd positive for cytokeratins (AE1/3) and HBME-1. Strong HBME-1 positivity s upported a mesothelial origin of the atypical cells and led to the diagnosi s of reactive mesothelium. While HBME-1 cannot be used us the sole marker t o establish an mesothelial origin; its use in a immunohistochemistry panel may be useful in individual cases to distinguish reactive mesothelial cells from carcinoma in effusion cytology. (C) 2000 Wiley-Liss, Inc.