Malignant effusion, consistent with Adenocarcinoma.
This is a very interesting case. The clinical history provided suggested a parapneumonic effusion, clinically suspicious of benign.
The cells in the cell block are single and looks mesothelial like. However, they are very atypical with prominent nucleoli and frequently bi or even multinucleated.
I requested Calretinin that was totally negative. The epithelial markers BerEp4 and Moc31 shows strong membranous positivity. p53 shows strong nuclear staining.
Additionally I requested TTF1 and Napsin that are strongly positive.
So, the final diagnosis is Adenocarcinoma, lung origin. Interesting malingnant case with cells that shows a morphology similar to atypical or malignant mesothelial cells.