In the case of an autotransplant (ASCT) the value is that the very high dose of melphalan (or similar drugs) can wipe out more myeloma cells than lower, more conventional doses. Unfortunately, except in rare instances, the high dose chemotherapy does not wipe out all the myeloma. Nonetheless the myeloma tumor burden can be substantially reduced, which in most cases is a major benefit for the patient.
For a twin donor transplant, the advantages are very similar to those with an ASCT. The added benefit is that there is no risk of contamination of the twin’s blood stem cells with myeloma cells, which is a concern with ASCT.
For an allogeneic transplant, the situation is more complicated. Because toxicities are much higher with allotransplant, lower doses of chemotherapy are now used: mini-allotransplant. The potential benefit is from a transplant or graft versus myeloma effect in which the transplanted cells attack the myeloma. This is good. But, they can also attack the patient’s normal cells in different parts of the body (liver, lungs, intestines, etc.). This is called graft versus host disease (GVHD) which can be very dangerous, even life-threatening. Research is ongoing to balance and assess the benefits and risks.