Description:

The differential diagnosis between reactive mesothelial cells, malignant mesothelioma, and adenocarcinoma is often difficult in cytologic specimens obtained from fluid material. Various immunohistochemical markers have been proven useful. Recent immunohistologic studies have suggested that E-cadherin and calretinin may be useful markers for epithelial and mesothelial differentiations, respectively. The differential diagnosis in our case includes reactive mesothelial cells which can present as single cells or groups of round cells with centrally or eccentric nuclei with nucleoli versus poorly differentiated adenocarcinoma, signet ring cell type. Macrophages can occasionally mimic signet ring cells.  In this case presence of large single cells with eccentric nuclei and vacuolated PAS positivity, that stain positive with cytokeratins (AE1:3 and Cam 5.2) and negative with calretinin, can confirm the diagnosis of signet ring carcinoma rather than reactive mesothelial cells.  Other helpful immuno stains are CEA (positive in this case), TTF-1 to rule-out lung or thyroid primaries (negative in this case), CA125 (negative in this case), ER/PR and GCDFP15 (negative in this case).   Common malignancies that can present as single cells in fluid cytology: Melanoma, Breast carcinoma, Gastric carcinoma, Lymphoma.

Diagnosis

Adenocarcinoma, signet ring type, of probable GI origin. Favor Gastric Adenocarcinoma.

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