Our study further confirms that C-RDD is a distinct entity with different age and possibly race distributions from RDD. Compared with its nodal counterpart, C-RDD demonstrates a wider histopathological spectrum with different clinicopathological phases depending on duration of the lesions. Awareness of these features is helpful in making a correct diagnosis. The associations of C-RDD with other histiocytoses may have important implications for the pathogenesis of this rare histiocytosis.

Rosai-Dorfman disease (RDD; sinus histiocytosis with massive lymphadenopathy) and Langerhans cell histiocytosis (LCH) are two different yet pathogenetically related histiocytic disorders. While systemic and localized forms have been identified in both diseases, each has its own characteristic histological, immunohistochemical and ultrastructural profile. Rarely, either RDD or LCH can also occur in the context of certain malignant neoplasms. However, the coexistence of RDD and LCH has never been described. We report a case of cutaneous RDD in which a focus of LCH was found. Clinical and laboratory examinations revealed no evidence of extracutaneous involvement of RDD or LCH. We believe that this is the first report of such a coexistence, and the possible pathogenesis is discussed.