Sustained remission after discontinuation of ALK inhibition in ALK-positive histiocytosis

ALK-positive histiocytosis is a rare histiocytic neoplasm first described in 2008 and recently classified as a distinct entity [ 1 , 2 ]. The disease is typically driven by KIF5B::ALK fusions and often involves the neurologic system [ 3 , 4 ]. We previously conducted the largest study of ALK-positive histiocytosis to date, investigating 31 children and 8 adults with the disease [ 4 ]. Our study demonstrated robust responses in all 11 patients treated with ALK inhibition, including 10 with neurologic involvement. However, the durability of response and potential for therapy cessation remained uncertain. Here, we present long-term outcomes of our original 11 cases, along with 16 additional patients, demonstrating universal responses to ALK inhibition and sustained remissions after therapy discontinuation.