ALK-positive adult histiocytosis with a TFG-ALKfusion gene.

ALK-positive histiocytosis is a rare condition that can affect multiple systems in infants and adults. We identified a rare case of ALK-positive histiocytosis with fusion of the ALK gene with TFG. A 35-year-old previously healthy male has been complaining of back and hip discomfort for seven months. A radiologic examination of the spine and pelvis revealed several hypermetabolic osteolytic lesions. Immunostaining for LCA, CD68, CD117and ALK were positive, whereas immunostaining for CD1a, Langerin, and S100 were negative. Analysis with fluorescence in situ hybridization (FISH) confirmed the ALK rearrangements, and next-generation sequencing (NGS) revealed the fusion of the TFG and ALK genes. After receiving alectinib, an ALK inhibitor of the second generation, the patient showed a durable remission. ALK-positive histiocytosis is a distinct form of histiocytosis that has the potential to be treated with an ALK inhibitor.