NTRK1 ‐Rearranged Cutaneous Non‐Langerhans Cell Histiocytosis With Durable Response to TRK Inhibitor in a Patient With Primary Myelofibrosis

Non-Langerhans cell histiocytoses (non-LCH) are heterogeneous histiocytic neoplasms driven by recurrent kinase pathway activation. NTRK1 fusions have recently been identified as recurrent, actionable alterations as drivers of xanthogranulomatous proliferations. We report a 62-year-old man with primary myelofibrosis (PMF) who developed multifocal, symptomatic cutaneous non-LCH harboring an IRF2BP2::NTRK1 fusion. The cutaneous eruption was refractory to ruxolitinib but showed complete response to larotrectinib, with successful rechallenge after treatment discontinuation. This case illustrates targeted therapy in concurrent myeloid and histiocytic neoplasms and expands the clinical spectrum of NTRK1-rearranged histiocytosis.