LAUSR

DLBCL: diffuse large B-cell lymphoma ECD: Erdheim-Chester disease HCT: hematopoietic cell transplantation LCH: Langerhans cell histiocytosis NGS: next-generation sequencing PNH: progressive nodular histiocytosis STAMP: Solid Tumor Actionable Mutation Panel XGs: xanthogranulomas INTRODUCTION The histiocytoses are rare myeloid disorders characterized by the accumulation of dendritic cell, macrophage, or monocyte-derived cells in tissues and organs. Based on the revised 2016 classification of histiocytoses, non-Langerhans cell histiocytosis of skin and mucosa (C group) is a subclass that includes various forms of xanthogranulomas (XGs) and several non-XG family disorders. Because these histiocytoses have overlapping clinical and histologic presentations, a definitive diagnosis and appropriate therapy regimen is often difficult to ascertain. We present a case report describing the use of targeted next-generation sequencing (NGS) with the Stanford Solid Tumor Actionable Mutation Panel (STAMP) to identify a rare case of IRF2BP2NTRK1eassociated progressive nodular histiocytosis (PNH)/xanthogranulomatosis after autologous hematopoietic cell transplantation (HCT) for diffuse large B-cell lymphoma (DLBCL). STAMP targets 198 genes based on their known impact as actionable targets of existing and emerging anticancer treatments. This case of IRF2BP2-NTRK1 fusion-related histiocytosis demonstrates the applicability of NGS to the diagnosis and treatment of clinically ambiguous pathologic conditions.