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Background: Total lymphoid irradiation (TLI) plays a role in the conditioning of hematopoietic stem cell transplantation (HSCT) from non-sibling donors for severe aplastic anemia (SAA) patients. For the past decades, an anterior-posterior opposed technique has been used to deliver TLI. We present our singleinstitution experience of helical tomotherapy TLI.Methods: From October 2014 to February 2019, three consecutive patients of SAA who underwent tomotherapy TLI for pre-transplantation conditioning were enlisted. We retrospectively analyzed the survival, engraftment outcome, and toxicities of these patients and created a conventional TLI plan with anterior-posterior opposed fields for each patient to compare the dosimetry difference between helical tomotherapy and the conventional technique.Results: Among all three patients, the engraftment rate was 100%. The 18-month overall survival and event-free survival were both 66.7% among the 3 patients (2 underwent unrelated donor transplantation and 1 underwent haplo-identical related donor transplantation), and 100% for the patients who received transplantation from matched-or mismatched-unrelated donors. No patient experienced acute grade 3 or worse graft-versus-host disease (GVHD), and none of the three patients suffered from late GVHD. No significant acute or late adverse events of radiation were observed. Tomotherapy-based TLI provides better target volume coverage with an average 10% higher coverage along with greater sparing of normal tissues.The total body volume exposed to 5 Gy was an average 35% lower in the tomotherapy-based TLI.Conclusions: TLI with non-ablative conditioning regimen provided excellent engraftment and an acceptable survival in the SAA patients who underwent non-sibling donor HSCT.

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