In adult patients with relapsed/refractory Langerhans cell histiocytosis (LCH), a treatment regimen of XTD regimen (Selinexor, Thalidomide and Dexamethasone) is planned to be used.

Inclusion Criteria:

• Organ pathology confirmed diagnosis of LCH;
• Age 18 years or older;
• Multi-system involvement, or single system with multiple lesions;
• Disease not relieved after receiving at least one systemic treatment, or disease relapsed after improvement;
• ECOG performance status score ≤2;
• Clinical physician determines suitability for this treatment protocol;
• Subjects can understand the study protocol and are willing to participate in this study, providing written informed consent.

Exclusion Criteria:

• Single system single lesion LCH
• Underwent major surgery within 4 weeks prior to the first administration of the study drug;
• Underwent radiotherapy within 4 weeks prior to the first administration of the study drug;
• History of myocardial infarction within the past year; suffers from New York Heart Association (NYHA) class 3 or 4 congestive heart failure, or has a history of NYHA class 3 or 4 congestive heart failure, unless left ventricular ejection fraction (LVEF) ≥ 50% in the echocardiogram (ECHO) screening performed within 1 month before entering the study;
• Pregnant or breastfeeding women (women of childbearing age with positive pregnancy test at baseline or who have not undergone pregnancy testing. Postmenopausal women must have been menopausal for at least 12 months);
• Abnormal liver and kidney function: creatinine level ≥176.8μmol/l (2mg/dl), transaminase and bilirubin levels more than 2 times the upper limit of normal (for LCH patients with liver involvement, transaminase levels more than 10 times and bilirubin levels more than 3 times the upper limit of normal);
• Severe hematological abnormalities: absolute neutrophil count less than 1 × 10^9/L, platelet less than 50×10^9/L;
• Presence of uncontrolled infections;
• Any other circumstances that the investigator believes to be inappropriate for the patient to participate in this trial;