Laryngeal ALK Positive Histiocytosis: Potential Mimic of Inflammatory Myofibroblastic Tumor-Case Report and Literature Review.

INTRODUCTION: ALK positive histiocytosis (APH) is a rare histiocytic neoplasm defined by an ALK fusion and generally characterized by a favorable prognosis. Often occurring as either a solitary lesion or a systemic disease, APH can present with a wide range of phenotypic expression. Although the outcome is generally favorable, a correct diagnosis is often crucial especially in a multifocal setting. ALK inhibitors serves as a highly effective treatment regimen associated with excellent clinical response in cases of systemic disease.

CASE PRESENTATION: Here we present a case of laryngeal APH occurring in a 34-year-old male patient that histologically mimics an inflammatory myofibroblastic tumor. The lesion was excised and no evidence of systemic disease was found.

RESULTS: Immunohistochemical analysis revealed the tumor positive for CD68, CD163, and Factor XIIIa. ALK1 immunohistochemistry showed diffuse cytoplasmic expression. Fluorescence in-situ hybridization was performed to confirm ALK rearrangement which revealed positive. Combining the morphologic features, immunohistochemical evidence of histiocytic differentiation, and ALK rearrangement, the diagnosis of ALK positive histiocytosis was rendered.

DISCUSSION: While the classic hepatosplenic and hematopoietic manifestation have been well described, APH manifesting in the head and neck is a distinct phenomenon that has only been reported on rare occasions. We provide a review of the literature of all cases of ALK positive histiocytosis affecting the head and neck to provide an overview of the clinicopathologic features as well as the molecular findings of this rare entity.