A Stranger in the Slide: A Rare Collision of a Spitz Melanocytoma With a Novel MYH9::LTK Fusion and a Common BRAF Mutated Nevus Mimicking a Melanoma With a Preexistent Nevus.

Abstract: In this case report, we present a rare collision tumor consisting of a Spitz melanocytoma with a novel MYH9::LTK fusion, and a BRAFV600E mutated common dermal nevus in a 25-year-old man. Based on morphology alone, the lesion could easily have been misdiagnosed as a melanoma with a preexisting nevus. However, only the common dermal nevus showed BRAFV600E expression on immunohistochemical analysis, pointing toward the possibility of a collision tumor and guiding further molecular analyses. Although MYH9::LTK fusions have not been previously described in Spitz tumors, we classified this lesion as Spitz based on the epithelioid morphology and kinase fusion driver. Because there was dermal mitotic activity and heterozygous CDKN2A loss, we signed this lesion out as Spitz melanocytoma, in association with an unrelated BRAFV600E mutated dermal nevus. This case underscores the relevance of performing immunohistochemistry and if needed additional molecular analyses to confirm the lineage and dignity of an atypical melanocytic lesion, in which a Spitz tumor is a diagnostic consideration, to prevent misdiagnosis.