Rosai-Dorfman disease (RDD) is a rare non-malignant proliferation of histiocytes of unknown aetiology that mainly affects lymph nodes. Here we report a case of RDD that presented a diagnostic dilemma due to its atypical presentation and the overlap with IgG4 disease. Our case presented with interstitial lung involvement without lymphadenopathy. Open lung biopsy suggested the diagnosis of RDD. However, the predominant IgG4 positive plasma cells together with the absence of lymphadenopathy were not typical of RDD. Within 1 year, the patient developed diffuse lymphadenopathy and immunohistochemical staining of lymph node aspirates confirmed the diagnosis. Despite trials of corticosteroid therapy, the disease progressed.
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