Diagnostic Utility of PAX8 and PAX2 Immunohistochemistry in the Identification of Metastatic Mullerian Carcinoma in Effusions

Morphologic distinction of Mullerian carcinomas from non-Mullerian carcinomas in effusion specimens by cytomorphology alone can be diagnostically challenging. Therefore, immunohistochemical adjuncts can be useful in differentiating Mullerian from non-Mullerian metastases. In this study, we evaluated the expression of PAX8 and PAX2 in malignant effusions collected from patients with known Mullerian and non-Mullerian carcinomas. Sections from cell blocks prepared from 152 effusion specimens (54 and 98 cases representing metastases from Mullerian and non-Mullerian primaries, respectively) were immunostained with rabbit polyclonal antibodies against PAX8 and PAX2. Immunopositivity was defined as the presence of strong nuclear staining in at least 25% of the tumor cells. Fifty-two (96%) and 13 (24%) of the 54 Mullerian carcinomas were positive for PAX8 and PAX2, respectively. PAX8 positivity was seen in only four (4%) of 98 non-Mullerian carcinomas; these represented metastasis from a large cell neuroendocrine lung carcinoma, papillary thyroid carcinoma, renal cell carcinoma, and acinic cell carcinoma of the parotid gland. PAX2 positivity was not seen in any of the non-Mullerian carcinomas. The results demonstrate that both PAX8 and PAX2 are highly specific markers for metastatic Mullerian carcinomas in cell block preparations from effusion specimens (96% and 100%, respectively). PAX8, however, is more sensitive than PAX2 in identifying Mullerian carcinomas in fluids (96% versus 24%). Overall, immunohistochemistry for PAX8 and PAX2 represent diagnostically useful adjuncts in identifying a Mullerian carcinoma as a source of a malignant effusion. Diagn. Cytopathol. 2011; 39: 651-656. (C) 2010 Wiley-Liss, Inc.