® for Histiocytosis. Histiocytoses are rare disorders characterized by a buildup of white blood cells called histiocytes, which leads to tissue damage. This group of disorders is diagnosed in a few thousand people every year across the United States. As a result of this low incidence rate, people are often diagnosed late and receive treatment from doctors who have limited or no prior experience with the disease. These new clinical practice guidelines from NCCN provide the latest evidence and expert-consensus for diagnosing and treating the three most-common forms of histiocytosis in adults: Langerhans cell histiocytosis (LCH), Erdheim-Chester disease (ECD), and Rosai-Dorfman disease.