Original ArticlesFiligree-like Rete Ridges, Lobulated Nests, Rosette-like Structures, and Exaggerated Maturation Characterize Spitz Tumors With NTRK1 FusionYeh, Iwei MD, PhD*; Busam, Klaus J. MD†; McCalmont, Timothy H. MD*; LeBoit, Philip E. MD*; Pissaloux, Daniel PhD‡; Alberti, Laurent PhD‡; de la Fouchardière, Arnaud MD, PhD‡,§; Bastian, Boris C. MD, PhD*Author Information *Departments of Dermatology and Pathology, UCSF, San Francisco, CA †Department of Pathology, Memorial Sloan Kettering Cancer Center, New York, NY ‡Department of Biopathology, Centre Léon Bérard §INSERM, Léon Bérard Center, Cancer Research Center of Lyon, University of Lyon, Claude Bernard University, Lyon, France A.D.L.F. and B.C.B. contributed equally. Conflicts of Interest and Source of Funding: Supported by the National Cancer Institute at the National Institutes of Health (1R35CA220481 to B.C.B.). B.C.B. is a consultant for Lilly Inc., outside the submitted work. The remaining authors have disclosed that they have no significant relationships with, or financial interest in, any commercial companies pertaining to this article. Correspondence: Iwei Yeh, MD, PhD, 1701 Divisadero St. Ste 280, San Francisco, CA 94115 (e-mail: email@example.com). The American Journal of Surgical Pathology: June 2019 - Volume 43 - Issue 6 - p 737-746 doi: 10.1097/PAS.0000000000001235 Buy SDC Metrics Abstract Activating NTRK1 fusions have been described as oncogenic events across the spectrum of Spitz tumors. Herein we report a series of 38 Spitz tumors with NTRK1 fusion. These Spitz tumors have distinctive histopathologic features characterized by filigree-like rete ridges which are elongated, thin and branched, dermal melanocytes arranged in a rosette-like configuration, and marked diminishment of melanocyte size with descent into the dermis. These features are distinct from those of other genetically defined subtypes of Spitz tumors and can aid in microscopic diagnosis and help prioritize in case selection for molecular testing in the rare patients that need targeted therapy. Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.