A Study of Memory, Thinking, and Brain Imaging in Adults With Histiocytosis

Inclusion Criteria: * Age greater than or equal to 18 years. * Fluency in English * Diagnosis of a histiocytic disorder as determined, in the opinion of the study PI, by a corroborating constellation of histopathology, clinical, and/or radiologic findings. * Will undergo Standard of Care MRI. 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