CME ArticleMorphologic Diversity of Merkel Cell CarcinomaOgawa, Toru MD, PhD*; Donizy, Piotr MD, PhD†; Wu, Cheng-Lin MD‡; Cornejo, Kristine M. MD§; Ryś, Janusz MD, PhD¶; Hoang, Mai P. MD‖Author Information *Visiting Physician, Department of Pathology, Massachusetts General Hospital, Boston, MA; †Assistant Professor of Pathology, Department of Pathomorphology and Oncological Cytology, Wroclaw Medical University, Wroclaw, Poland; ‡Assistant Professor of Pathology, Department of Pathology, National Cheng Kung University Hospital, College of Medicine, National Cheng Kung University, Tainan, Taiwan; §Assistant Professor of Pathology, Department of Pathology, Harvard Medical School, Massachusetts General Hospital, Boston, MA; ¶Professor of Pathology, Department of Pathology, Center of Oncology, M. Sklodowska-Curie Memorial Institute, Krakow, Poland; and ‖Professor of Pathology, Department of Pathology, Harvard Medical School, Massachusetts General Hospital, Boston, MA. Correspondence: Mai P. Hoang, MD, Department of Pathology, Massachusetts General Hospital, 55 Fruit Street, Warren 828, Boston, MA 02114 (e-mail: [email protected]). All authors, faculty, and staff in a position to control the content of this CME activity and their spouses/life partners (if any) have disclosed that they have no financial relationships with, or financial interests in, any commercial organizations relevant to this educational activity. The American Journal of Dermatopathology: September 2020 - Volume 42 - Issue 9 - p 629-640 doi: 10.1097/DAD.0000000000001548 Buy CME Test Metrics Abstract Merkel cell carcinoma (MCC) is a rare and highly aggressive neuroendocrine carcinoma of unknown origin. We performed a retrospective histologic review of primary cutaneous MCCs diagnosed from 1997 to 2018 in several clinical institutions and literature review to determine the frequency of various unusual morphologic appearances of MCC. Of the 136 primary MCCs identified, intraepidermal carcinoma or epidermotropism was noted in 11/136 (8%) cases. An association with pilar cyst in 1/136 (0.7%) case, with actinic keratosis in 2/136 (1.5%) cases, with either invasive or in situ squamous cell carcinoma (SCC) in 14/136 (10%) cases, with poroma in 1/136 (0.7%), and with basal cell carcinoma in 1/136 (0.7%) case was noted. Trabecular pattern and rosettes were noted in 7/136 (5%) and 3/136 (2%) cases, respectively. There was one case of metastatic MCC in a lymph node with chronic lymphocytic leukemia and one rare case of metastatic MCC and SCC in a lymph node. Although uncommon, differentiation toward other cell lineage can be observed in both primary and metastatic MCCs. The tumor can assume a variety of histologic appearances including association with SCC, basal cell carcinoma, melanocytic neoplasm, and follicular cyst; as well as exhibit glandular, sarcomatous, and mesenchymal differentiation. This diversity of morphologic appearance of MCC reflects the complexity of its underlying pathogenesis. Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.