- Some of the patients are anticipated to be evaluated using FLT (to include only patients needed for the immunological primary endpoint, not increasing total patient numbers). 23 adult NCI patients in total will undergo FLT PET/CT imaging on day -1, at day +5 or day +9, at 4 weeks, and at a future point to include evidence of GVHD relapse, or immune recovery. An estimated 50 patients (including subset of the 23 patients undergoing serial scanning) will be imaged approximately at 1 year for evaluation of thymus reconstitution. The total possible numbers will include no more than 118 patients to achieve the 68 evaluable adults for the immunological primary endpoint. However, all NCI patients will undergo a single 1 year FLT for evaluation of thymus reconstitution. 10 pediatric patients at CNMC will undergo FLT PET/CT imaging on Day -1, day+9, and day +28 (if possible). Initial images will be correlated with