Targeted Approach to Langerhans Cell Histiocytosis (LCH) Using MEK Inhibitor, Trametinib

Inclusion Criteria: * Diagnosis/disease status: * Patients with newly diagnosed Langerhans cell histiocytosis (LCH) OR * Patients with relapsed or refractory disease OR * Patients with newly diagnosed or relapsed/refractory disease who are receiving the liquid formula of trametinib OR * Patients who have been receiving trametinib as a treatment for LCH since January 1, 2020 may be included in the observational chart review to track long-term follow-up. Eligibility for chart review cohort will include receiving trametinib as treatment. * Diagnosis confirmed with biopsy prior to start of treatment * Patient must have adequate cardiac function evident through Echocardiogram (ECHO) and Electrocardiogram (EKG) within 30 days of starting treatment. * Shortening fraction of ≥ 27% by echocardiogram or * Ejection fraction of ≥ 50% by gated radionuclide study * QTC \< 480 msec * Performance status: Patients must have a performance status corresponding to ECOG scores of 0, 1, or 2. Use Karnofsky ≥ 50% for patients \> 16 years of age and Lansky ≥50% for patients ≤16 years of age. * Adequate organ and marrow function as defined below: * Absolute Neutrophil count ≥ 1,500/μL * Platelets ≥ 100x103/μL * Total bilirubin ≤ 1.5X ULN for age * AST/ALT ≤ 2.5 X ULN for age * Serum creatinine based on age/gender * Hemoglobin ≥ 8 g/dL * Patients with bone marrow disease must have hemoglobin ≥ 8 g/dL with transfusion support allowed * Women of childbearing potential …