 The ultimate objective will be to use the TK substrate 18FHBG to locate the donor T cells within the recipient as they exert anti-leukemic effects, and the T cells can then be eliminated in response to in vivo administration of ganciclovir, before morbidity and mortality from GvHD occurs. We will use the imaging strategy to define patterns of T cell trafficking in humans pre and post-DLI infusion, and to determine where the cells reside while they mediate GVL in contrast to GvHD. We expect to obtain in vivo PET imaging markers predictive of GvHD before clinical symptoms occur. 