Rosai-Dorfman Disease of the Pericardium Accompanied by Monoclonal Gammopathy: A Case Report with CT and PET Imaging Findings.

Rosai-Dorfman disease (RDD) is an uncommon proliferative histiocytic disorder of unknown etiology typically presenting as massive lymphadenopathy, and in some cases, with extranodal involvement. However, serous membranes are rarely involved in extranodal RDDs, and reports regarding pericardial involvement are scarce. Herein, we report a case of extranodal RDD manifesting as diffuse pericardial thickening and effusion in a 79-year-old man with monoclonal gammopathy. The patient complained of dyspnea, hence chest CT and PET scans were performed. They showed irregular thickening of the pericardium with a marked increase in metabolic activity. Pericardial biopsy showed the characteristic S100-positive and CD68-positive histiocytes exhibiting emperipolesis. The disease progressively evolved to bilateral pleural thickening with effusion of the pericardium, and finally led to death even with corticosteroid treatment. Although exceedingly rare, this case demonstrates the importance of RDD in the differential diagnosis of effusion in serous cavities based on imaging findings.