Diagnostic utility of PAX8 and PAX2 immunohistochemistry in the identification of metastatic Müllerian carcinoma in effusions

Morphologic distinction of Müllerian carcinomas from non‐Müllerian carcinomas in effusion specimens by cytomorphology alone can be diagnostically challenging. Therefore, immunohistochemical adjuncts can be useful in differentiating Müllerian from non‐Müllerian metastases. In this study, we evaluated the expression of PAX8 and PAX2 in malignant effusions collected from patients with known Müllerian and non‐Müllerian carcinomas. Sections from cell blocks prepared from 152 effusion specimens (54 and 98 cases representing metastases from Müllerian and non‐Müllerian 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 Müllerian carcinomas were positive for PAX8 and PAX2, respectively. PAX8 positivity was seen in only four (4%) of 98 non‐Müllerian 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‐Müllerian carcinomas. The results demonstrate that both PAX8 and PAX2 are highly specific markers for metastatic Müllerian carcinomas in cell block preparations from effusion specimens (96% and 100%, respectively). PAX8, however, is more sensitive than PAX2 in identifying Müllerian carcinomas in fluids (96% versus 24%). Overall, immunohistochemistry for PAX8 and PAX2 represent diagnostically useful adjuncts in identifying a Müllerian carcinoma as a source of a malignant effusion. Diagn. Cytopathol. 2010. © 2010 Wiley‐Liss, Inc.