Phase III Study Comparing GVHD Prophylaxis With ATG-thymoglobulin to ATLG-grafalon in Elderly Patients With Acute Myeloid Leukemia or Myelodysplasic Syndrome and Receiving an Allogeneic Hematopoietic Stem Cell Transplantation With a 10/10 HLA Matched Unrelated Donor (OPTISAGE)

Inclusion Criteria: Age ≥ 50 and ≤ 70 years Patient between 50 and 55 years should be unfit for a myeloblative conditioning (SORROR score ≥2) AML requiring allogeneic stem cell transplantation (intermediate or high-risk AML) in complete cytologic response (CR1 or above) or MDS requiring allogeneic stem cell transplantation (IPSS≥ 1.5 or IPSS-R > 4.5 or IPSS-R > 3-4.5 with risk features [rapide blast increase, life-threatening neutropenia (<0.3 G/L) or thrombopenia (<30G/L) or high transfusion needs (>2/month for 6 months)] Without an HLA matched related donor Having an identified matched HLA 10/10 unrelated donor With usual criteria for HSCT: ECOG performans status ≤ 2 No severe and uncontrolled infection Cardiac left ventricular ejection fraction ≥50% Lung DLCO > 40% Adequate organ function: ASAT and ALAT ≤ 3N, total bilirubin ≤ 2N, creatinine clearance ≥ 50 mL/min (except if those abnormalities are linked to the hematological disease) With health insurance coverage Having signed a written informed consent Contraception methods must be prescribed during all the duration of the research NB: The authorized contraceptive methods are: For women of childbearing age and in absence of permanent sterilization: oral, intravaginal or transdermal combined hormonal contraception, oral, injectable or transdermal progestogen-only hormonal contraception, intrauterine hormonal releasing system (IUS), sexual abstinence (only if this the preferred and usual lifestyle of the participants). For man in absence of permanent sterilization: sexual abstinence, condoms Exclusion Criteria: Cancer in the last 5 years (except basal cell carcinoma of the skin or "in situ" carcinoma of the cervix) Uncontrolled infection Seropositivity for HIV or HTLV-1 or active hepatitis B or C Yellow fever vaccine and all others live virus vaccines within 2 months before transplantation Heart failure according to NYHA (II or more) or Left ventricular ejection fraction < 50%. Lung DLCO ≤ 40% Preexisting acute hemorrhagic cystitis Renal failure with creatinine clearance < 50ml / min Pregnancy (β-HCG positive) or breast-feeding Patients with any debilitating medical or psychiatric illness, which would preclude the realization of the SCT or the understanding of the protocol Patient under state medical aid Patient under legal protection (protection of the court, or in curatorship or guardianship). For Grafalon: Hypersensitivity to the active substance or to any of the excipients For Thymoglobulin: Hypersensitivity to rabbit proteins or to any of the excipients Participation in other interventional clinical trials Any contraindication mentioned in the SmPC of all auxiliary medicinal products planned to be used in the trial: cyclosporine, mycophenolate mofetil, fludarabine, treosulfan