Reduced Intensity Hematopoietic Cell Transplantation for Patients With Resistant Langerhans Cell Histiocytosis

Inclusion Criteria: * Histologically confirmed Langerhans cell histiocytosis (LCH) by demonstration of CD1a positivity or Birbeck granules in lesions * Considered poor-risk, defined as multisystem disease with involvement of one or more risk organs (i.e., liver, spleen, lungs, and/or hematopoietic system) * No isolated "lung only" LCH * Progressive disease after one of the following treatments: * LCH-III protocol or other standard LCH-directed therapies * At least 1 course of the current salvage protocol (i.e., LCH-2 2005) or similar therapy (e.g., cytosine arabinoside or cladribine-based regimens) * HLA-matched related or unrelated donor OR unrelated umbilical cord blood (UCB) available * 1 locus mismatch for donor allowed * Up to 2 loci mismatch for unrelated UCB allowed * Any hematologic status (transfusion support allowed) * Adequate hepatic, renal, cardiac, and pulmonary function to undergo reduced-intensity hematopoietic cell transplantation (RI-HCT) including the following: * Transaminases \< 5 times upper limit of normal (ULN) * Bilirubin \< 3 times ULN (unless secondary to hepatic LCH) * Creatinine ≤ 2 mg/dL (adults) (if creatinine \> 1.2 OR history of renal dysfunction, must have estimated creatinine clearance \> 40 mL/min) * Creatinine clearance \> 40 mL/min (pediatrics) * Glomerular filtration rate ≥ 50mL/min * Negative pregnancy test Exclusion Criteria: * Decompensated congestive heart failure, uncontrolled arrhythmia, or left ventricul…