Online Articles: Case ReportsTTF-1-positive Metastatic Endometrioid Carcinoma: A Case Report and Review of Literature of a Potential Diagnostic PitfallCasteillo, François MD, MSc*; Fournel, Pierre MD†; Da Cruz, Vanessa MD*; Karpathiou, Georgia MD, PhD, BSc*; Boutet, Claire MD, PhD‡; Jacquin, Jean-Philippe MD†; Tissot, Claire MD§; Meyer-Bisch, Vincent MD∥; Péoc’h, Michel MD, PhD*; Forest, Fabien MD, PhD*Author Information Departments of *Pathology ‡Radiology §Pneumology ∥Neurosurgery, University Hospital of Saint Etienne, North Hospital, Cedex †Department of Oncology, Lucien Neuwirth Cancer Institute, Saint-Priest-en-Jarez, France The authors declare no conflict of interest. Reprints: Fabien Forest, MD, PhD, CHU de Saint Etienne, Hôpital Nord, Service d’Anatomie et Cytologie Pathologiques, Avenue Albert Raimond, 42055, Saint Etienne, Cedex 2, France (e-mail: [email protected]). Applied Immunohistochemistry & Molecular Morphology: January 2020 - Volume 28 - Issue 1 - p e6-e9 doi: 10.1097/PAI.0000000000000539 Buy Metrics Abstract A 75-year-old female patient, nonsmoker was addressed to our institution for a fracture of C5 vertebra with spinal cord compression by a tumor mass invading surrounding soft tissue. She had a previous history of breast ductal carcinoma and endometrioid carcinoma. Biopsy of the tumor mass showed a TTF-1-positive carcinoma. Molecular study showed a E545K mutation of PIK3CA. Lung imaging showed multiple nodules evocative of metastasis rather than a primitive tumor. Reviewing of slides of endometrioid carcinoma showed areas positive for TTF1, and the same E545K mutation was found in endometrial tumor. The final diagnosis was endometrioid metastatic carcinoma with aberrant TTF-1 expression. A subset of endometrial neoplasm expresses TTF-1, this situation might be confusing especially in case of metastatic disease. Copyright © 2017 Wolters Kluwer Health, Inc. All rights reserved.