Original StudyPD-L1 Expression in Extramammary Paget Disease: A Case SeriesFowler, Mark R. MD*; Flanigan, Kendall L. BS, BA†; Googe, Paul B. MD*,‡Author Information *Departments of Pathology and Laboratory Medicine, University of North Carolina School of Medicine, Chapel Hill, NC; †AU/UGA Medical Partnership, Medical College of Georgia, Athens, GA; and ‡Department of Dermatology, University of North Carolina School of Medicine, Chapel Hill, NC. The authors declare no conflicts of interest. The American Journal of Dermatopathology: January 2021 - Volume 43 - Issue 1 - p 21-26 doi: 10.1097/DAD.0000000000001622 Buy Metrics Abstract The PD-1/PD-L1 pathway plays a critical role in the physiologic inhibition and modulation of the immune response in normal tissue. Many tumors evade immune detection and response by upregulating PD-L1 expression. Humanized monoclonal PD-1 and PD-L1 antibodies have proven as both tolerable and effective treatment in many neoplasms. Extramammary Paget disease (EMPD) is a deformative and debilitating cutaneous malignancy in which definitive treatment options are limited with high recurrence rates after surgical excision. To the best of our knowledge, there is little published information regarding EMPD and PD-L1 expression. We evaluated 18 EMPD surgical pathology cases for tumor cell and tumor-associated inflammatory (TAI) cell PD-L1 expression. We identified PD-L1 tumor cell expression in 3 (17%) of the cases: 2 of 4 invasive cases (50%) and 1 of 14 (7%) noninvasive cases. One invasive case had lymph nodal metastasis with PD-L1 tumor cell expression. The host inflammatory response intensity and PD-L1 expression were variable in cases negative for tumor cell PD-L1 expression; however, a marked inflammatory response and TAI PD-L1 expression were present in all cases positive for tumor cell PD-L1 expression. In conclusion, 1 in 14 (7%) in situ EMPD cases showed tumor cell PD-L1 expression and 2 of 4 invasive cases (50%) showed tumor cell PD-L1 expression. TAI cells were more often positive (83%) than tumor cells (17%) for PD-L1 expression. Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.