This is a research study among patients with Rosai-Dorfman disease.

Rosai-Dorfman disease is a rare disorder characterized by overproduction (proliferation) and accumulation of a specific type of white blood cell (histiocyte) in the lymph nodes of the body (lymphadenopathy), most often those of the neck (cervical lymphadenopathy). In some cases, abnormal accumulation of histiocytes may occur in other areas of the body besides the lymph nodes (extranodal). These areas include the skin, central nervous system, kidney, and digestive tract. The symptoms and physical findings associated with Rosai-Dorfman disease vary depending upon the specific areas of the body that are affected. The disorder predominantly affects mainly adolescents or young adults. The exact cause of Rosa Dorfman disease is unknown. The purpose of this research study is to learn the prevalence , therapies and long term prognosis of Rosa Dorfman disease.

Inclusion Criteria:

Diagnosis of a histiocytic disorder as determined by a corroborating constellation of histopathology, clinical, and/or radiologic findings. All participants will undergo Standard of Care MRA, CTA or imaging examination.

Exclusion Criteria:

Known intracranial involvement of histiocytosis (including dura, leptomeninges and brain parenchyma) Prior stroke or intracranial hemorrhage Other (non-histiocytic) intracranial neoplasm or neurological disorder deemed by the PI or Co-PI to confound neuroimaging studies (e.g., demyelinating disease) Existing diagnosis of a psychiatric disorder or untreated mood disturbance Existing diagnosis of a neurodegenerative disease, such as Alzheimer's disease Chronic or daily excessive alcohol consumption as determined by the PI. History of chronic use of corticosteroids, defined as continuous treatment for six months or longer at any time in the past History of severe claustrophobia or other contraindications to patient SOC brain MRI Prior intravenous cytarabine or cladribine Other current or prior treatments (e.g., high-dose chemotherapy for a different cancer) deemed by the PI or Co-PI to confound imaging studies or cognitive performance