Rosai-Dorfman Disease (RDD) is a rare, heterogeneous histiocytic disorder. Because of the rarity of RDD and a lack of prospective randomized trials, the treatment strategy for RDD is mostly based on retrospective study. Steroid was suggested as frontline treatment as RDD with only lymph nodes involvement. Studies showed thalidomide or lenalidomide showed some effect in recurrent/refractory skin RDD. The investigators want to analyze the efficacy and toxicity of lenalidomide combined with dexamethasone regimens in the treatment of RDD among adult patients at our hospital.

Inclusion Criteria:

• Histologically confirmed diagnosis of RDD
• Age ≥18 years
• Eastern Cooperative Oncology Group (ECOG) performance status 0-2.
• Patients must have adequate renal, liver, and bone marrow function as defined by the following criteria:

Absolute neutrophil count ≥1500 cells per mm3 Platelet count ≥100000 cells per mm3 Creatinine clearance [according to Cockcroft formula] ≥60 mL/min. Aspartate aminotransferase and alanine aminotransferase ≤2·5×upper limit of normal [ULN], and total bilirubin ≤2·5×ULN; or ≤10×ULN in the case of known liver involvement by RDD.

• No active or untreated infection.
• No cardiac abnormalities.
• Subject provide written informed consent.
• A female is eligible to enter and participate in this study if she is of:

Non-childbearing potential including ω Any female who has had a surgical procedure rendering her incapable of becoming pregnant.

ω Subjects have experienced total cessation of menses for more than 1 year and be greater than 45 years in age.

⎫ Childbearing potential, including any female who has had a negative serum pregnancy test within 2 weeks prior to the first dose of study treatment, and agrees to use adequate contraception.

• Male subjects must use an effective barrier method of contraception during the study and for 90 days following the last course of MA if sexually active with a childbearing potential

Exclusion Criteria:

• Patients had concurrent malignancies.
• History of myocardial infarction, or unstable angina, or New York Heart Association (NYHA) Grade III-IV within 6 months prior to Day 1.
• Women who were pregnant or of childbearing potential.
• Known HIV seropositive, active hepatitis C infection, and/or hepatitis B (defined as HCV RNA≥1000 copies or HBV DNA ≥1000 copies at screening).
• Major surgical procedure within 28 days prior to the first dose of study treatment.
• Presence of uncontrolled infection.
• Evidence of active bleeding or bleeding diathesis.
• Any serious and/or unstable pre-existing medical, psychiatric, or other condition that could interfere with subject's safety, provision of informed consent, or compliance to study procedures.