Extraordinary Case ReportPerineural Invasion as a Clue to Malignant Behavior in a DermatofibromaSantos-Briz, Angel MD, PhD*,†,‡; García-Gavín, Juan MD§; Pastushenko, Ievgenia MD, PhD¶; Sayagués, José María PhD*,†,║; Rodríguez-Peralto, José Luis MD, PhD‡,**; Requena, Luis MD, PhD‡,††Author Information *Department of Pathology, Complejo Asistencial Universitario de Salamanca, Salamanca, Spain; †Institute for Biomedical Research (IBSAL), Salamanca, Spain; ‡Dermatopatología Diagnóstica, Salamanca, Spain; §Pérez & Gavín dermatólogos Clinic, Vigo, Spain; ¶Laboratory of Stem Cells and Cancer, Université Libre de Bruxelles, Brussels, Belgium; ║Service of Hematology, Complejo Asistencial Universitario de Salamanca, Salamanca, Spain; **Department of Pathology, Hospital 12 de Octubre, Madrid, Spain; and ††Department of Dermatology, Fundación Jiménez Díaz, Universidad Autónoma, Madrid, Spain. Correspondence: Angel Santos-Briz, MD, PhD, Departamento de Anatomía Patológica, Hospital Universitario de Salamanca, Paseo de San Vicente 58-182, 37007 Salamanca, Spain (e-mail: firstname.lastname@example.org). The authors declare no conflicts of interest. The American Journal of Dermatopathology: July 2020 - Volume 42 - Issue 7 - p 533-538 doi: 10.1097/DAD.0000000000001570 Buy Metrics Abstract Dermatofibroma (DF) represents one of the most common mesenchymal proliferations of the skin. Their recurrence rate, even when incompletely excised, is very low, whereas the atypical, aneurysmal, and cellular variants have recurrence rates of up to 20% each. Extraordinary rare malignant lesions with metastases to lymph nodes and/or lung have been described. We report a 64-year-old woman with a long history (years) of a skin lesion on her right arm that became painful during the last months. Histologically, it consisted of a conventional cellular DF in which perineural invasion was present. Subsequently, the lesion showed a clinically aggressive course with recurrences, sarcomatous transformation, and pulmonary metastases. Given that no predictive morphological features have been identified to separate classical benign DF from rare metastasizing forms, perineural invasion in an otherwise conventional DF could be a histopathologic clue for an adverse prognosis and should provoke a closer clinical follow-up. Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.