Calretinin staining pattern aids in the differentiation of mesothelioma from adenocarcinoma in serous effusions

BACKGROUND, The differentiation between malignant mesothelioma and adenocar cinoma based on morphology alone can be a diagnostic challenge. The majorit y of the available antibodies recognize molecules expressed by adenocarcino ma whereas to the authors' knowledge specific markers for mesothelial cells are lacking. Calretinin, a calcium-binding protein, has been reported to b e a selective marker for mesothelioma and largely is absent from adenocarci noma on histologic material. The results with cytologic preparations have b een inconsistent. METHODS. To evaluate the specificity of calretinin in differentiating mesot helioma from adenocarcinoma in cytologic preparations, 21 paraffin embedded cells blocks of serous effusions from 15 patients with metastatic adenocar cinoma and 16 cell blocks from 9 patients with malignant mesothelioma were stained with a monoclonal antibody against calretinin. The immunoreactivity was evaluated blindly by two observers. Positive staining was defined as n uclear and cytoplasmic staining with or without intense membranous decorati on. The former resulted in a characteristic "fried egg" appearance. RESULTS. Calretinin staining was positive in all but 2 cases of mesotheliom a (14 of 16 cases; 87.5%). The latter contained predominantly spindle-shape d neoplastic mesothelial cells in the cell block preparations. All adenocar cinoma specimens were classified as negative for calretinin staining; 9 (42 .9%) lacked any immunoreactivity and 12 (57.1%) showed weak, sparse, coarse , granular cytoplasmic staining without nuclear or membranous staining. Ben ign reactive mesothelial cells, when observed in association with adenocarc inoma, also showed the characteristic "fried egg" appearance. The differenc e in the staining pattern of calretinin between cells of mesothelial origin and adenocarcinoma cells was statistically significant. CONCLUSIONS. Calretinin is a useful marker in differentiating mesothelioma of the epithelial type from adenocarcinoma in serous effusions. The "fried- egg" appearance or cytoplasmic and nuclear staining pattern is characterist ic of cells of mesothelial origin. (C) 2000 American Cancer Society.