Differentiating reactive mesothelial cells from metastatic carcinoma and malignant mesothelioma is critical in effusion cytology. Numerous immunohistochemical/cytochemical reports use various antibodies in effusion samples, and most antibodies differentiate metastatic adenocarcinoma from malignant mesothelioma, but no antibodies help distinguish malignant mesothelioma from reactive mesothelial cells. A mouse monoclonal antibody (IMP3/L523S) against KOC is a 580-amino acid oncofetal RNA-binding protein containing 4 K homology domains. IMP3/L523S has been identified in several human malignant tumors. The immunocytochemical staining profile of IMP3 was determined in 95% alcohol-fixed cytologic effusion specimens. A total of 229 cases of pleural and peritoneal effusion cytospecimens were evaluated for the study, including 39 benign effusions with reactive mesothelial cells and 190 metastatic malignant effusions. IMP3 immunoreactivity was observed in 2 (5.1%) of 39 cases of reactive mesothelial cells, 138 (72.6%) of 190 cases of malignant effusion, 4 (36.4%) of 11 cases of malignant mesothelioma, 106 (75.7%) of 140 cases of metastatic adenocarcinoma, and 8 (100%) of 8 cases of squamous cell carcinoma. The overall specificity for the diagnosis of malignancy was 94.9%, whereas the sensitivity was 72.6%. In the peritoneal effusions, the sensitivity for the diagnosis of metastatic adenocarcinoma to distinguish reactive mesothelial cells was 92.3%. In conclusion, IMP3 staining is present in many carcinomas and is not a useful marker for distinguishing between carcinomas arising in different organs. However, the IMP3 antibody is a highly specific marker for malignant lesions, and thus, IMP3 staining is useful for distinguishing neoplastic cells from reactive mesothelial cells in effusion samples.
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