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ABSTRACT Background Method Results Conclusion Melanocytic tumors with perineuriomatous differentiation may pose diagnostic challenges. This study explores characteristics of perineuriomatous melanocytic nevi, an entity merging features of perineurioma and melanocytic nevi. The aim is to elucidate histopathological features of perineuriomatous nevi to allow dermatopathologists to recognize them and differentiate them from other spindle cell lesions.This study reviews four cases (2020–2023) of melanocytic nevi with perineuriomatous components.All cases comprised adults (median age: 64.50, mean age: 60.25), with a female predominance, and exhibited clinical features characterized by irregular brown macules or papules on the trunk and extremities. Histopathologically, all lesions were compound, and biphasic patterns were evident, encompassing superficial nevoid and deeper spindled populations arranged in whorled fascicles and embedded in a sclerotic or myxoid stroma. Immunohistochemistry revealed expression of at least one perineuriomatous marker in deeper cells. Cases were assessed and compared with previously published cases for comprehensive insights. Three of our four cases demonstrated the presence of a junctional component that was smaller than the dermal component. We suggest that this unusual feature, which we term the “reverse shoulder”, may allow dermatopathologists to help consider perineurial differentiation in the appropriate setting.Perineuriomatous nevi can pose diagnostic challenges. This study contributes to the growing body of literature on perineuriomatous nevi, emphasizing their unique features and the importance of accurate diagnosis to avoid unnecessary interventions. [ABSTRACT FROM AUTHOR]

Background: Exogenous materials may be encountered in skin biopsies as contamination. Contamination may occur during the biopsy procedure in the clinic or during tissue processing in the laboratory. Although the experienced pathologist may often instinctively ignore clear examples of contamination, such tissue may be a source of confusion for young pathologists and those in training. Foreign materials can typically be recognized as exogenous by morphology, polarizability, and the presence or absence of a tissue reaction, but we have rarely encountered situations in which exogenous materials could be misinterpreted as organisms, either by their morphologic appearance or staining properties., Methods: Exogenous materials used during skin biopsy and grossing were embedded in a nonhuman tissue scaffold and prepared into histologic slides. Hematoxylin and eosin (H&E), periodic acid-Schiff with diastase, and Grocott methenamine silver stains were performed, and each material was evaluated under polarized microscopy., Results: Exogenous materials were divided into the following 3 categories with shared morphologic appearances and staining properties: suture materials, plant-based materials, and synthetic materials., Conclusion: We present a comprehensive characterization of the morphologic and cytochemical staining properties of multiple exogenous materials that may contaminate a skin biopsy. This characterization will aid the pathologist by providing a mechanism to identify potential contaminants in skin biopsies., Competing Interests: Dr. Gardner has disclosed that he was a consultant for Bristol-Myers Squibb and Leica Biosystems. Lippincott CME Institute has identified and resolved all conflicts of interest relevant to this educational activity. The remaining authors, faculty, and staff in a position to control the content of this CME activity have disclosed that they have no financial relationships with, or financial interests in, any commercial organizations relevant to this educational activity., (Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.)