Impact of radiation dose in multi-isocentric volumetric arc-based total body irradiation on oncological outcomes in allogenic stem cell transplantation.

e18551 Background: Total body irradiation (TBI) remains a cornerstone in conditioning regimens for patients undergoing allogenic stem cell transplantation (allo-SCT). It plays a critical role in eradicating residual malignancies, immunosuppressing the host to prevent graft rejection, and creating space for donor cells to engraft. Despite decades of clinical application, there is a paucity of data directly comparing oncological outcomes across different TBI dose regimens. Modern techniques such as multi-isocentric volumetric arc-based TBI (VMAT-TBI) provide improved dose uniformity and reduced off-target toxicities, making higher dose regimens more feasible. This study aimed to evaluate the impact of two different VMAT-TBI regimens (8 Gy and 12 Gy) on overall survival (OS), relapse-free survival (RFS), engraftment success, and treatment-related toxicities. Methods: This retrospective analysis included patients treated with VMAT-TBI at doses >8 Gy between 2021 and 2023. The patients were divided into two groups: those receiving 8 Gy delivered in four bi-daily fractions (fx) and those receiving 12 Gy in six bi-daily fx. Patient selection accounted for age, Charlson Comorbidity Index (CCI), Disease-Related Index (DRI), and total conditioning score (TCI). Outcomes evaluated included OS, RFS, engraftment success, and toxicity profiles. Statistical adjustments were made to account for baseline differences between groups. Results: A total of 42 patients were included in the analysis, with 24 receiving 12 Gy and 18 receiving 8 Gy. The 12 Gy regimen was associated with a significant improvement in overall survival compared to the 8 Gy regimen. After adjusting for key covariates, the hazard ratio (HR) for OS significantly favored the 12 Gy group (HR: 0.02, 95% CI: 0.00–0.48; p = 0.01). The median RFS was not reached in the 12 Gy cohort, indicating durable disease control, while it was 11.8 months in the 8 Gy group. Toxicity profiles were similar between the groups, with no significant differences in acute or late complications. wo treatment-related deaths occurred in the 8 Gy arm. Engraftment success was notably higher in the 12 Gy group, with all patients achieving full donor engraftment. In contrast, one patient in the 8 Gy group experienced graft failure, emphasizing the superior efficacy of the higher dose regimen. Conclusions: This study demonstrates that VMAT-TBI with a 12 Gy dose offers superior oncological outcomes compared to 8 Gy, including improved OS, RFS, and engraftment time and rate, without a significant increase in toxicity. These findings highlight the potential benefits of higher-dose TBI regimens in modern radiotherapy techniques. However, randomized controlled trials are needed to validate these results and establish the optimal TBI dose for patients undergoing allo-SCT.