Isolated Multinodular Soft-Tissue Rosai-Dorfman Disease on FDG PET/CT.

Rosai-Dorfman disease (sinus histiocytosis with massive lymphadenopathy) is a rare systemic histiocytic disorder of unknown etiology characterized by the accumulation of enlarged non-Langerhans histiocytes within lymph nodes and extranodal sites. The histiocytes display characteristic emperipolesis (nondestructive engulfment of inflammatory cells) and are CD68 and S100 positive and CD1a negative. Although extranodal disease frequently occurs with nodal involvement, isolated extranodal disease is uncommon. We report a case of isolated localized subcutaneous multinodular disease on FDG PET/CT. We also include a companion classic Rosai-Dorfman case with extensive nodal involvement and a characteristic benign clinical course with spontaneous improvement.