The goal of this clinical trial is to learn if drugs (fludarabine and melphalan) combinated with a targeted irradiation (in bone marrow and in lymphoid tissue) works to treat blood cancers (high risk myelodysplastic syndrome or acute myeloid leukemia) in adults. It will also learn about the safety of this combination of drugs and irradiation. The main questions it aims to answer are:
* Does this combined therapy result in an improved survival and decreased relapse/progression rate after a bone marrow transplant?
* What toxicities or complications do participants have when taking this combined therapy?
Researchers will use this combined therapy to see if it works to treat high risk myelodysplastic syndrome or acute myeloid leukemia through the evaluation of the length of time (during the treatment and 2 years after the treatment) that a patient lives with the disease but it does not get worse (progression-free survival).
Participants will:
* Receive the combined therapy (study treatment) one week before the bone marrow transplant.
* Receive too post-transplant medication as per usual clinical practice.
Patients will have to attend medical visits (checkups and tests) for 2 years.