Hematopoietic stem celltransplantation (HSCT) has the potential to cure a variety of benign andhematologic diseases that may be incurable with conventional therapy. In itsbroadest form, HSCT consists of three parts: a conditioning phase, stem cellinfusion, and for allogeneic procedures, a method for prophylaxis ofgraft-versus-host disease (GVHD). There are, however, many variations ofthis framework. Conditioning regimens include various combinations ofchemotherapy, radiotherapy, and immunotherapeutic agents. All conditioningregimens must produce at least enough immunosuppression to prevent graftrejection; beyond this, they can vary considerably in intensity, rangingfrom high-dose regimens that result in complete ablation of the patient’sbone marrow to reduced-intensity regimens that cause only mildmyelosuppression. Stem cells can be obtained from bone marrow (BM),peripheral blood (PB), or umbilical cord blood (UCB). Finally, GVHDprophylaxis can be achieved through immunosuppressive medications or graftmanipulation (in particular T-cell depletion). The choice of conditioningregimen, stem cell source, and GVHD prophylaxis regimens varies on the basisof patient and disease characteristics, as well as donor availability. Inthe case of allogeneic HSCT for hematologic malignancies, one of theprincipal goals is to allow engraftment and development of a donor-derivedimmune system that can effect an immunologic attack against the recipientlymphohematopoietic system, and in particular against the tumor cells. Thisgraft-versus-tumor (GVT) effect is a fundamental and unique aspect ofallogeneic HSCT. In autologous transplantation, the main goal is to providean opportunity for hematologic recovery after the administration ofhigh-dose therapy.

Not surprisingly, the outcomeof HSCT depends on many patient factors, such as age and comorbidities;disease factors, such as diagnosis, disease stage, and prior therapy; donorfactors, including human leukocyte antigen (HLA) and gender match; andtransplantation factors, including conditioning regimen, stem cell source,and GVHD prophylaxis. Table 1.1 lists estimates of long-term survival forsome of the hematologic malignancies and marrow disorders for whichtransplantation is commonly performed.