MT2023-20: Hematopoietic Cell Transplant With Reduced Intensity Conditioning and Post-transplant Cyclophosphamide for Severe Aplastic Anemia and Other Forms of Acquired Bone Marrow Failure.

Inclusion Criteria: * Idiopathic Severe Aplastic Anemia (SAA), characterized by one of the following: 1. Refractory cytopenia(s), with 1+ of the following: 1. Platelets \<20,000/uL or transfusion dependent 2. Absolute neutrophil count \<500/uL without hematopoietic growth factor support 3. Absolute reticulocyte count \<60,000/uL AND bone marrow cellularity \<50% (with \< 30% residual hematopoietic cells) 2. Early myelodysplastic features (bone marrow (BM) blasts \<5%), without history of MDS/AML pre-treatment. 3. Idiopathic SAA with post-HCT graft failure (blood/marrow donor chimerism \<5%) requiring a 2nd allogeneic HCT * Paroxysmal Nocturnal Hemoglobinuria (PNH), including AA-PNH overlap syndrome, acquired pure red cell aplasia (aPRCA), or acquired amegakaryocytic thrombocytopenia (aAT), characterized by one of the following: 1. Refractory cytopenia(s), with 1+ of the following: 1. Platelets \<20,000/uL or transfusion dependent 2. Absolute neutrophil count \<500/uL without hematopoietic growth factor support 3. Absolute reticulocyte count \<60,000/uL or red cell transfusion dependent AND Bone marrow evidence of 1 to 3-lineage aplasia OR peripheral blood PNH clone \>/= 10% 2. Early myelodysplastic features (bone marrow (BM) blasts \<5%) without history of MDS/AML pre-treatment. 3. Idiopathic PNH, aPRCA, or aAT with post-HCT graft failure (blood/marrow donor chimerism \<5%) requiring a 2nd allogeneic HCT * Adequate organ function within 30 days of conditioning regimen Exclusion Criteria: * Pregnant, breastfeeding or intending to become pregnant during the study. Persons of childbearing potential must have a negative pregnancy test (serum or urine) within 7 days of the start of treatment * Uncontrolled infection * Evidence of moderate or severe portal fibrosis or cirrhosis on biopsy * Known allergy to any of the study components * Prior radiation therapy deemed excessive by radiation therapist for proposed low dose TBI exposure on this protocol * Diagnosis of an inherited bone marrow failure disorder such as Fanconi anemia, Telomere biology disorder, or Schwachman-Diamond syndrome, unless reviewed by the principal investigator and deemed appropriate for this approach (e.g. GATA2 deficiency) * Advanced myelodysplastic syndrome (MDS; BM blasts \>5%) or acute myeloid leukemia * Psychiatric illness/social situations that, in the judgement of the enrolling Investigator, would limit compliance with study requirements * Other illness or a medical issue that, in the judgement of the enrolling Investigator, would exclude the patient from participating in this study