They destroyed her bone marrow twice. Leukemias are cancers of bone marrow. Usual tactic is chemo + bone marrow replacement and hope new tcells mop up cancer

They destroyed her bone marrow twice. Leukemias are cancers of bone marrow. Usual tactic is chemo + bone marrow replacement and hope new tcells mop up cancer cells. Didn't quite work so they started with new marrow and made some changes to a supply of tcells they flooded her with. Once they couldn't find cancer they put the marrow on.