This is a special kind of allogeneic transplant that can only be used when the patient has an identical sibling (twin or triplet) – someone who has the exact same tissue type. An advantage of syngeneic stem cell transplant is that graft-versus-host disease will not be a problem. Also, there are no cancer cells in the transplanted stem cells, as there might be in an autologous transplant.
A disadvantage is that because the new immune system is so much like the recipient’s immune system, there’s no graft-versus-cancer effect. Every effort must be made to destroy all the cancer cells before the transplant is done to help keep the cancer from coming back.