Pediatric Cutaneous Anaplastic Lymphoma Kinase-Positive Histiocytosis with DCTN1::ALK Fusion: A Case Report and Literature Search

<b>Background and Clinical Significance</b>: 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 <i>ALK</i> gene and exhibiting excellent response to systemic tyrosine kinase inhibitors. <b>Case presentation</b>: 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 <i>ALK</i> rearrangement, whereas, upon next-generation sequencing, a <i>DCTN1::ALK</i> fusion was identified. <b>Conclusion</b>: 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 <i>DCTN1::ALK</i> 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.