Filigree-like Rete Ridges, Lobulated Nests, Rosette-like Structures, and Exaggerated Maturation Characterize Spitz Tumors With NTRK1 Fusion.

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.