RT Journal Article SR Electronic T1 Erdheim-Chester disease with Rosai-Dorfman-like lesions: treatment with methotrexate, anakinra and upadacitinib JF RMD Open JO RMD Open FD EULAR SP e002852 DO 10.1136/rmdopen-2022-002852 VO 9 IS 1 A1 Jan Portegys A1 Anke Heidemeier A1 Andreas Rosenwald A1 Michael Gernert A1 Matthias Fröhlich A1 Sebastian Hueper A1 Patrick Pascal Strunz A1 Leo Rasche A1 Marc Schmalzing YR 2023 UL http://rmdopen.bmj.com/content/9/1/e002852.abstract AB Erdheim-Chester disease (ECD) is a non-Langerhans cell histiocytosis characterised by clonal expansion of histiocytes in various organs. These induce an inflammatory environment, which leads to damage of the affected areas. Recently, a new disease entity was proposed encompassing key features of ECD but also of Rosai-Dorfman-Destombes disease, another histiocytosis. Mitogen-activated protein kinase kinase 1 (MAP2K1) mutations seem to present a specific genetic lesion for this subtype.Here, we describe a case of this new disease entity with clinical, radiological and genetic findings compatible with ECD but histological findings compatible with Rosai-Dorfman-Destombes disease. In particular, there were intraabdominal and retroperitoneal lesions, which tested positive for a (c.167A>C; p.Q56P) mutation of the MAP2K1 gene. On histological examination, S100-positive, giant histiocytes with focal emperipolesis of haematological cells in addition to infiltration by lymphocytes and granulocytes were seen.As described for this rare variant of ECD, there was also bilateral testicular infiltration. We also describe a manifestation of oligoarthritis in this patient with ECD.The patient was treated with methotrexate and prednisolone. While radiological response to this regime was excellent, arthritis persisted. We added anakinra, which induced a response of the arthritis for more than a year. Due to treatment failure therapy was switched to upadacitinib, which induced a remission of the arthritis as well.This case adds a rare phenotype to an already rare presentation of ECD. The patient responded to immunosuppressive therapy.