The main purpose of this study is to discover how 18F-cholineFCH distributes in the body, and see if it can distinguish active histiocytes (tumor) from inflammatory (non- tumor) cells. This tracer has the potential to give the investigators' team more information when trying to identify the tumor cells that are most important to collect with biopsy. In addition, the study will measure levels of 18F-cholineFCH in the biopsy tissue.

Inclusion Criteria:

• Age ≥ 18 years old
• Suspected but pathologically unconfirmed diagnosis of Erdheim-Chester disease or Langerhans cell histiocytosis or other histiocytic disorder on the basis of clinical diagnosis, radiologic features, or findings from prior biopsies

Or

• Confirmed diagnosis of Erdheim-Chester disease or Langerhans cell histiocytosis or other histiocytic disorder and requiring additional diagnostic imaging and biopsy to determine mutational status in order to determine therapeutic options.

Exclusion Criteria:

• Inability to undergo or cooperate with PET/CT scan (e.g., claustrophobia)
• Pregnant or nursing female

Patients for this study will be recruited by physicians from the Departments of Neurology.