Cutaneous Rosai-Dorfman disease.

We report a patient with purely cutaneous Rosai-Dorfman disease (RDD) who presented with a solitary, asymptomatic plaque on the back of her left thigh, with characteristic, large histiocytoid cells exhibiting emperipolesis histologically. Cutaneous lesions occur in 27% of patients with lymph node involvement in RDD however purely cutaneous disease has only been reported on 18 previous occasions. The aetiology is unknown, although it is though to be a reactive disorder rather than neoplastic, possibly an immunological response to an infectious agent.