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More recently, doctors have developed a newer type of allogeneic stem cell transplantation procedure called mini-transplants. These are also sometimes referred to as non-myeloablative transplants or reduced-intensity transplants. In contrast to a standard allogeneic stem cell transplant where the goal of treatment is to use high-dose chemotherapy to eradicate the remaining cancer cells and then restore the patient's stem cells that have been also destroyed by the chemotherapy, a mini-transplant uses significantly lower doses of chemotherapy (or radiation) and is, therefore, less toxic to the patient. This lower-dose or reduced-intensity approach kills only some of the remaining cancer cells but does not completely destroy the patient's diseased bone marrow blood-forming cells. The patient then receives a transplant of the donor's bone marrow or stem cells. The donor's transplanted immune cells serve as a "booster" to the recipient's own immune system by recognizing and destroying the remaining cancer cells that have not been killed by the low-dose chemotherapy or radiation therapy. This phenomenon is known as a "graft-versus-tumor" effect because the donor's transplanted immune cells are used as a means of targeting and destroying the patient's residual cancer cells.

Although mini-transplants are becoming more common and have been used for patients with a wide range of cancers, they appear to be most effective for patients with chronic myelogenous leukemia (CML). The outcome for patients with other types of leukemias, non-Hodgkin's lymphoma, Hodgkin's disease, or multiple myeloma have varied depending upon the type of mini-transplant procedure used. In general, mini-transplants are reserved for older patients (over age 60) or patients with serious underlying conditions who cannot tolerate a standard allogeneic stem cell transplant.

Graft-Versus-Host-Disease (GVHD)

This is perhaps the most serious potential complication that may develop in patients receiving an allogeneic stem cell transplant. As mentioned previously, in an allogeneic transplant, the source of the stem cells used for transplantation is another individual who serves as the donor. Graft-versus-host disease (GVHD) occurs when the donor's transplanted cells (the graft) begins to attack the recipient's (the host's) own tissues and organs. It should be noted that GVHD can occur with an allogeneic transplant even in cases where the donor and recipient's HLA markers are a "perfect match". This is because currently the degree of compatibility (match) between the donor and recipient is determined on the basis of evaluating similarities of tissue types for six major HLA markers. However, there are other antigens present on the donor's transplanted cells which may differ slightly from those of the recipient's own cells that can lead to the development of GVHD.

GVHD can develop within the first 3 months following allogeneic stem cell transplantation (acute GVHD) or it may develop after 3 months (chronic GVHD). Symptoms of acute GVHD include:

• Itchy, red rash on the hands and feet
• Nausea, diarrhea, and severe stomach cramps