Pediatric Cutaneous Anaplastic Lymphoma Kinase-Positive Histiocytosis with Fusion: A Case Report and Literature Search.

: Anaplastic lymphoma kinase (ALK)-positive histiocytosis is a relatively novel entity, affecting single or multiple organ systems; it is characterized by aggregates of neoplastic cells of the histiocytic lineage, harboring molecular alterations in the gene and exhibiting excellent response to systemic tyrosine kinase inhibitors. : Herein, we present a pediatric case with cutaneous-only involvement: the 6-month-old male patient presented with an elevated, tan-colored lesion on his left forearm. Following surgical excision, histopathological evaluation reported spindle cells with wide eosinophilic cytoplasm and Touton-type giant cells. The tumor cells were positive for CD163, ALK, phosphorylated ERK, and cyclin D1. Fluorescent in situ hybridization revealed rearrangement, whereas, upon next-generation sequencing, a fusion was identified. : Our case serves as a great addition to the limited number of cases reported in the literature, and it represents the first published pediatric case with the rare fusion. The novelty of this genetic alteration and the lack of knowledge about its potential effects on the clinical aspects of ALK-positive histiocytosis highlight the importance of ancillary molecular testing, when available.