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.