ALK-positive histiocytosis: a new clinicopathologic spectrum highlighting neurologic involvement and responses to ALK inhibition
Authors: Paul G. Kemps, Jennifer Picarsic, Benjamin H. Durham, Zofia Hélias-Rodzewicz, Laura Hiemcke-Jiwa, Cor van den Bos, Marianne D. van de Wetering, Carel J. M. van Noesel, Jan A. M. van Laar, Robert M. Verdijk, Uta E. Flucke, Pancras C. W. Hogendoorn, F. J. Sherida H. Woei-A-Jin, Raf Sciot, Andreas Beilken, Friedrich Feuerhake, Martin Ebinger, Robert Möhle, Falko Fend, Antje Bornemann, Verena Wiegering, Karen Ernestus, Tina Méry, Olga Gryniewicz-Kwiatkowska, Bozenna Dembowska-Baginska, Dmitry A. Evseev, Vsevolod Potapenko, Vadim V. Baykov, Stefania Gaspari, Sabrina Rossi, Marco Gessi, Gianpiero Tamburrini, Sébastien Héritier, Jean Donadieu, Jacinthe Bonneau-Lagacherie, Claire Lamaison, Laure Farnault, Sylvie Fraitag, Marie-Laure Jullié, Julien Haroche, Matthew Collin, Jackie Allotey, Majid Madni, Kerry Turner, Susan Picton, Pasquale M. Barbaro, Alysa Poulin, Ingrid S. Tam, Dina El Demellawy, Brianna Empringham, James A. Whitlock, Aditya Raghunathan, Amy A. Swanson, Mariko Suchi, Jon M. Brandt, Nabeel R. Yaseen, Joanna L. Weinstein, Irem Eldem, Bryan A. Sisk, Vaishnavi Sridhar, Mandy Atkinson, Lucas R. Massoth, Jason L. Hornick, Sanda Alexandrescu, Kee Kiat Yeo, Kseniya Petrova-Drus, Stephen Z. Peeke, Laura S. Muñoz-Arcos, Daniel G. Leino, David D. Grier, Robert Lorsbach, Somak Roy, Ashish R. Kumar, Shipra Garg, Nishant Tiwari, Kristian T. Schafernak, Michael M. Henry, Astrid G. S. van Halteren, Oussama Abla, Eli L. Diamond, Jean-François Emile
Journal: Blood (2021)

Abstract ALK-positive histiocytosis is a rare subtype of histiocytic neoplasm first described in 2008 in 3 infants with multisystemic disease involving the liver and hematopoietic system. This entity has subsequently been documented in case reports and series to occupy a wider clinicopathologic spectrum with recurrent KIF5B-ALK fusions. The full clinicopathologic and molecular spectra of ALK-positive histiocytosis remain, however, poorly characterized. Here, we describe the largest study of ALK-positive histiocytosis to date, with detailed clinicopathologic data of 39 cases, including 37 cases with confirmed ALK rearrangements. The clinical spectrum comprised distinct clinical phenotypic groups: infants with multisystemic disease with liver and hematopoietic involvement, as originally described (Group 1A: 6/39), other patients with multisystemic disease (Group 1B: 10/39), and patients with single-system disease (Group 2: 23/39). Nineteen patients of the entire cohort (49%) had neurologic involvement (7 and 12 from Groups 1B and 2, respectively). Histology included classic xanthogranuloma features in almost one-third of cases, whereas the majority displayed a more densely cellular, monomorphic appearance without lipidized histiocytes but sometimes more spindled or epithelioid morphology. Neoplastic histiocytes were positive for macrophage markers and often conferred strong expression of phosphorylated extracellular signal-regulated kinase, confirming MAPK pathway activation. KIF5B-ALK fusions were detected in 27 patients, whereas CLTC-ALK, TPM3-ALK, TFG-ALK, EML4-ALK, and DCTN1-ALK fusions were identified in single cases. Robust and durable responses were observed in 11/11 patients treated with ALK inhibition, 10 with neurologic involvement. This study presents the existing clinicopathologic and molecular landscape of ALK-positive histiocytosis and provides guidance for the clinical management of this emerging histiocytic entity.