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Conditioning

The conditioning given to a patient depends on the type of disease and the age of the patient. Usually, a combination of cyclophosphamide (Cy) with total body irradiation (TBI) is given to create space for the transplanted HSC and to suppress the recipient's immune system to prevent the rejection of the donor’s HSC. TBI in fractionated doses is better tolerated than in a single dose (for instance 6 x 200 cGy instead of 1 x 1200). Conditioning regimens without TBI often consist of the alkylating reagent busulfan alone or in combination with Cy.

Conditioning may have severe side effects. Because the conditioning is mainly given to create space and for immune suppression while the anti-leukemic effect (GVL) of an HSCT comes from the allogeneic donor cells, reducing the intensity of the conditioning might be an option in particular in elderly patients who had been excluded from allogeneic transplantation programs previously. Such non-myeloablative conditioning regimens do not destroy hematopoiesis and its goal is to establish a stable engraftment of the donor HSC and to count on the allogeneic GVL-effect of the graft to eradicate residual tumor cells. During the first months after transplantation, the hematopoietic lineages of the patient are of donor as well as of patient origin (mixed chimerism). With time, the allogeneic cells of the donor will start to eradicate the residual tumor cells as well as the patients HSC so that all the hematopoietic lineages become of donor origin. Non-myeloablative regiments usually consist of Fludarabine in combination with Busulfan or with low doses (200 cGy) of TBI.