Unexpected PAX8 Immunoreactivity in Metastatic High-grade Breast CancerKilgore, Mark R. MD; Bosch, Dustin E. MD, PhD; Adamson, Kathi H. MD; Swanson, Paul E. MD; Dintzis, Suzanne M. MD, PhD; Rendi, Mara H. MD PhDApplied Immunohistochemistry & Molecular Morphology: October 2019 - Volume 27 - Issue 9 - p 637–643 doi: 10.1097/PAI.0000000000000707 Research Articles Buy Abstract Author InformationAuthors Article MetricsMetrics Immunohistochemistry (IHC) is often critical for distinction between metastatic carcinomas of Mullerian organ and breast origin. Paired box family protein 8 (PAX8) has been described as a transcription factor highly specific to neoplasms derived from Mullerian organs, thyroid, and kidney. PAX8 IHC with polyclonal and monoclonal antibody reagents was performed on 27 primary and 22 metastatic breast carcinomas. Eight of 27 primary breast carcinomas (30%) were positive for PAX8 with the monoclonal antibody reagent only; 0 of 22 were polyclonal anti-PAX8 immunoreactive. Substantial numbers of metastases had positive immunoreactivity for polyclonal anti-PAX8 (23%). Each of these metastases and additional cases (45% total) also had positive immunoreactivity for monoclonal anti-PAX8, including 5 of 7 brain metastases. IHC with monoclonal anti-PAX8 was positive on 6 of 7 primary breast carcinomas corresponding to PAX8-positive metastases. Together, these results indicate a significant fraction of breast carcinoma metastases and corresponding primary neoplasms have immunoreactivity for PAX8, and positivity rates depend on the antibody used. Diagnoses of metastatic breast carcinoma were achieved with the aid of clinical history and additional IHC in cases of PAX8 immunoreactivity. Contextual interpretation is imperative for PAX8 IHC, particularly when the differential diagnosis includes metastatic breast carcinoma with limited diagnostic material available. Department of Pathology, University of Washington, Seattle, WA The authors declare no conflict of interest. Reprints: Mark R. Kilgore, MD, Department of Pathology, University of Washington, P.O Box 356100 1959 NE Pacific St. Seattle, WA 98195 (e-mail: email@example.com). Received February 16, 2018 Accepted August 15, 2018 Copyright 2019 Wolters Kluwer Health, Inc. All rights reserved.