Pretransplant Conditioning for Hematopoietic Cell Transplants: The Past Is Prologue

We examine reasons underlying slow progress in identifying the best conditioning regimens for allogeneic hematopoietic cell transplantation in specific situations and suggest solutions. The current categorization of pretransplant conditioning regimens based on intensity of bone marrow injury is flawed. It groups regimens of different intensities into the same category and, sometimes, the wrong category. Using this flawed categorization, retrospective and prospective studies often combine regimens of different intensities or unintentionally compare regimens of different intensities. These and other heterogeneities limit critical analyses of these studies, resulting in a lack of consensus on the best regimen(s). More accurate assessment of regimen intensity, collaborative harmonization of conditioning regimens, and randomized clinical trials designed to compare specific regimens are needed.