Abstract
ABSTRACT
Some of the rarest diseases with cell accumulation in different organs are histiocytoses. Since these diagnoses are uncommon, other differential diagnoses must be excluded. We report a case of a 42‐year‐old man with episodes of knee pain and fever. Multiple sclerotic bone lesions were discovered by
18
F‐fluorodeoxyglucose positron emission tomography‐computed tomography (FDG PET/CT). A bone biopsy was taken, and it showed a mixed infiltrate consisting of mature lymphocytes, plasma cells, and many histiocytes (CD68
+
/CD163
+
/S‐100
−/+
/CD1a
−
/langerin
−
), including some foamy macrophages and cells with emperipolesis. Histology showed characteristics for both Erdheim–Chester disease (ECD) and Rosai–Dorfman disease (RDD). Molecular analysis showed MAP2K1 mutation that can be seen with both diseases. Considering the clinical manifestations that resembled ECD and histologic RDD‐like features, a diagnosis of overlap between ECD and RDD with isolated multifocal bone involvement was made. Treatment with pegylated interferon alfa (Peg‐IFN‐α) was started, and after three months clear clinical improvement was reported by the patient, although PET/CT demonstrated metabolically stable disease.