This study is looking to see how well new bone marrow cells grow after bone marrow transplantation (also called **HSCT**, which means hematopoietic stem cell transplantation). Researchers will use a special imaging test called **FLT** (fluorothymidine) alongside blood tests to check if the cells are growing well. The study aims to help patients who are at high risk of their new cells not growing properly. About 56 patients will join, and the study will last up to 7 years across 4 centers. Participants will undergo several FLT imaging tests and give blood samples to help find out how well the new cells are doing. This study is important to improve outcomes in people receiving bone marrow transplants.

• **Study Duration**: Each participant will be in the study for up to one year.
• **Visits**: Participants will have several imaging tests and blood draws during the study.
• **Eligibility**: Patients aged 4 to 80 years undergoing specific types of HSCT.