What is the recommended allogeneic hematopoietic stem cell transplant procedure (including conditioning regimen, donor stem cell collection, graft‑versus‑host disease prophylaxis, and supportive care) for a patient under 70 years old with adequate cardiac, pulmonary, hepatic, and renal function and an HLA‑matched donor?

Allogeneic Hematopoietic Stem Cell Transplantation Procedure

For a patient under 70 years old with adequate organ function and an HLA-matched donor, proceed with myeloablative conditioning (MAC) using busulfan/fludarabine or busulfan/cyclophosphamide, peripheral blood stem cells as the graft source, and calcineurin inhibitor-based GVHD prophylaxis with ATG for matched unrelated donors.

Donor Selection and Graft Source

HLA-matched sibling donors (MSD) are the first priority 1, 2. When an MSD is unavailable, select an 8/8 or 10/10 HLA-matched unrelated donor (MUD) if the search can be completed within 3 months 2. If urgent transplantation is needed or no matched donor is available, consider haploidentical donors or cord blood based on center experience 1, 2.

Graft Source Hierarchy:

• Peripheral blood stem cells (PBSCs) are preferred for faster engraftment and lower graft failure rates 3
• Bone marrow grafts may be considered for specific conditions like severe aplastic anemia 3
• PBSCs carry higher acute GVHD risk but this is manageable with appropriate prophylaxis 3

Conditioning Regimen Selection

For patients ≤55 years with HCT-CI <2, use myeloablative conditioning (MAC) 2. The evidence strongly supports MAC in younger, fit patients to maximize disease eradication while accepting higher toxicity that this population can tolerate.

MAC Regimen Options (in order of preference):

1. Fludarabine/Busulfan (Flu/Bu4): 150 mg/m² fludarabine days -7 to -3, plus 9.6 mg/kg IV busulfan days -7 to -4 1, 2

   • Associated with less toxicity than Bu/Cy in younger fit patients 2

2. Busulfan/Cyclophosphamide (BuCy): Equivalent efficacy to Flu/Bu4 2

3. Cyclophosphamide/TBI: Reserve for patients with CNS disease or myeloid sarcoma 2

Important Conditioning Considerations:

• ATG (anti-thymocyte globulin) is essential for MUD transplants at doses of 6-10 mg/kg (Thymoglobuline) or 20-40 mg/kg (ATG-F) 1
• ATG reduces GVHD risk without significantly compromising graft-versus-leukemia effects 1
• For MSD transplants, ATG may be added to prevent GVHD but is not mandatory 1

GVHD Prophylaxis

Standard GVHD prophylaxis consists of a calcineurin inhibitor (cyclosporine or tacrolimus) combined with methotrexate 2. Follow EBMT-ELN working group standardized recommendations 2.

Prophylaxis Protocol:

• Calcineurin inhibitor (tacrolimus preferred): Start day -1, continue 6-12 months post-transplant
• Methotrexate: Days +1, +3, +6, +11
• Add ATG to conditioning for MUD transplants (as noted above) 1

Common pitfall: Inadequate ATG dosing in MUD transplants leads to excessive acute GVHD. Ensure proper weight-based dosing and timing.

Stem Cell Collection and Infusion

Mobilize donor PBSCs with G-CSF (granulocyte-colony stimulating factor) at 10 mcg/kg/day for 4-5 days 3. Target collection of ≥2 × 10⁶ CD34+ cells/kg recipient weight for adequate engraftment.

Collection Process:

• Begin apheresis on day 4-5 of G-CSF
• Process 2-3 blood volumes per collection
• Cryopreserve if needed, though fresh infusion is preferred
• Infuse stem cells on day 0 after conditioning completion

Supportive Care Framework

Pre-Transplant Evaluation:

• HCT-CI score: Predicts non-relapse mortality; scores <2 support MAC 2
• EBMT score: Combines age, disease stage, donor type for overall survival prediction 4
• Cardiac function: LVEF >50%
• Pulmonary function: DLCO >50% predicted
• Hepatic function: Bilirubin <2× ULN
• Renal function: CrCl >60 mL/min

Infection Prophylaxis:

• Antibacterial: Fluoroquinolone from neutropenia until engraftment
• Antifungal: Fluconazole or posaconazole from conditioning through day +75
• Antiviral: Acyclovir/valacyclovir for HSV prophylaxis; monitor CMV weekly with pre-emptive therapy
• PCP prophylaxis: Trimethoprim-sulfamethoxazole starting at engraftment

Transfusion Support:

• Maintain platelets >10,000/μL (>50,000/μL if bleeding)
• Maintain hemoglobin >7-8 g/dL
• Use only irradiated, leukoreduced blood products to prevent transfusion-associated GVHD

Post-Transplant Monitoring

Engraftment Assessment:

• Monitor CBC daily until neutrophil engraftment (ANC >500/μL × 3 days)
• Expected neutrophil engraftment: Days 14-21 for PBSCs
• Chimerism studies at days +30, +60, +100, then every 3 months

Disease Monitoring:

• Bone marrow evaluation at days +30, +100, then every 3-6 months
• MRD monitoring is critical: Use flow cytometry or molecular methods based on disease 4
• Rising MRD warrants intervention before hematologic relapse

Critical distinction: While RIC regimens (fludarabine 150 mg/m² + busulfan 6.4 mg/kg IV) are appropriate for patients >55 years or those with HCT-CI ≥3 1, 2, your patient under 70 with adequate organ function should receive MAC for optimal disease control and survival outcomes. The evidence from multiple guidelines consistently demonstrates superior relapse-free survival with MAC in fit, younger patients 2, 4, 2.