Original ArticlesBizarre Chorionic-type Trophoblast in Second-trimester and Third-trimester Placentas Clinicopathologic Characterization of a Placental Pseudoneoplastic LesionMurdock, Tricia A. MD*; Varghese, Aaron MD†; Xing, Deyin MD*; Schoolmeester, J. Kenneth MD‡; Alexander, Caitlin MD, MPH*; Baergen, Rebecca N. MD§; Dahoud, Wissam MD*; Hopkins, Mark R. MD*; Askin, Frederic MD∥; Vang, Russell MD* Author Information Departments of *Pathology †Gynecology and Obstetrics, The Johns Hopkins Medical Institutions, Baltimore, MD ‡Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN §Department of Pathology and Laboratory Medicine, Weill Cornell Medicine, New York, NY ∥Department of Pathology and Laboratory Medicine, University of North Carolina, Chapel Hill, NC Conflicts of Interest and Source of Funding: The authors have disclosed that they have no significant relationships with, or financial interest in, any commercial companies pertaining to this article. Correspondence: Tricia A. Murdock, MD, Department of Pathology, The Johns Hopkins Hospital, 401 North Broadway, Weinberg 2242, Baltimore, MD 21231 (e-mail: [email protected]). The American Journal of Surgical Pathology: February 2022 - Volume 46 - Issue 2 - p 258-267 doi: 10.1097/PAS.0000000000001838 Buy Metrics Abstract Bizarre (atypical/symplastic) cells have been described in various gynecologic normal tissues and benign neoplasms. This type of bizarre cytologic change is usually an incidental finding and is regarded as a benign process. We describe 17 cases of bizarre chorionic-type trophoblast in second-trimester and third-trimester placentas that created concern for an underlying/undersampled or incipient intraplacental trophoblastic neoplasm, predominantly found in intervillous trophoblastic islands (11/17), placental septae (6/17), chorionic plate (1/17), and/or the chorion layer of fetal membranes (2/17). The bizarre trophoblastic cells exhibited sheet-like or nested architecture, had a multifocal/patchy distribution, and/or were present as individual cells within hyaline stroma; they were characterized by large nuclei with smudgy chromatin and occasional intranuclear pseudoinclusions. The degree of atypia was classified as mild (0/17), moderate (3/17), or severe (14/17). Mitotic figures and necrosis were not identified. A dual immunohistochemical stain for trophoblast (hydroxyl-delta-5-steroid dehydrogenase) and a proliferation marker (Ki-67), performed in 15 cases, demonstrated 0% to very low proliferative activity within the bizarre trophoblast (0% to 2% [10/15], 3% to 8% [5/15]). Immunohistochemical stains for fumarate hydratase showed intact/retained expression in the bizarre cells in 7 of 7 cases. Clinical follow-up ranged from 1 to 45 months, and all patients were alive and well without subsequent evidence of a gestational trophoblastic or other neoplasms. We conclude that bizarre chorionic-type trophoblast in second-trimester or third-trimester placentas have the potential to mimic an intraplacental trophoblastic neoplasm but are likely a benign degenerative change. This study expands the spectrum of bizarre cells that occur in the gynecologic tract. Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.