Crystal deposition in a case of cutaneous Rosai-Dorfman disease.

Rosai-Dorfman Disease (RDD) is an idiopathic reactive proliferation of distinctive histiocytes that have abundant cytoplasm and commonly exhibit intracytoplasmic ingestion of inflammatory cells (emperipolesis). The histiocytes are immunopositive for S100 protein and are typically associated with an infiltrate of lymphocytes, plasma cells, and neutrophils. The classic clinical presentation is massive enlargement of (usually) cervical lymph nodes with a histologic appearance that mimics exaggerated sinus histiocytosis. RDD can also involve extra-nodal sites and skin involvement is common either as part of disseminated disease or as a result of primary disease. We report an exceptional case of cutaneous RDD with crystal deposition in a young male presenting with skin nodules. Skin biopsy showed classic features of cutaneous RDD with the additional feature of conspicuous rhomboidal and needle-shaped crystals within the cytoplasm of many lesional plasma cells, histiocytes, and also in an extra-cellular location. The plasma cells were polyclonal by light chain immunostaining. Crystal deposition has not been reported to date in RDD and is likely a result of the reactive plasma cell proliferation.