Comparable Results Between 8 and 12 Gray TBI in Combination With Fludarabine and Post‐Transplant Cyclophosphamide in MRD‐Negative but Not in MRD‐Positive Acute Lymphoblastic Leukemia Patients Transplanted in First Complete Remission

Background The optimal TBI dose for ALL patients undergoing allogeneic SCT is still not clearly defined. Methods Single‐center retrospective analysis of high‐risk ALL patients in CR1 treated with 8 Gy ( n = 22) or 12 Gy ( n = 50) TBI in combination with fludarabine and PTCy. Median patient age in the 8 Gy TBI cohort was 63 (37–79) and 37 (18–56) in the 12 Gy TBI cohort and median follow‐up time was 21 months (range 1–92). Results OS and LFS at 2 years after 8 Gy were 65% and 55% versus 74% and 74% after 12 Gy ( p = 0.3 and p = 0.2, respectively). CIR and NRM at 2 years were 27% and 14% after 8 Gy versus 4% and 20% after 12 Gy ( p = 0.004 and p = 0.4, respectively). MRD‐positive (+) patients ( n = 26) receiving 12 Gy ( n = 19) showed better OS ( p = 0.01), LFS ( p = 0.009), GRFS, lower CIR ( p = 0.02), and similar NRM than did MRD+ patients receiving 8 Gy ( n = 7). MRD‐negative (−) patients ( n = 38) receiving 12 Gy ( n = 27) had similar OS, LFS, GRFS, lower CIR, and higher NRM ( p = 0.04) than did MRD− patients receiving 8 Gy ( n = 11). Conclusion Our study demonstrates that 8 Gy TBI in comparison to 12 Gy TBI results in low NRM but a high relapse rate with similar OS, LFS, and GRFS. In MRD+ high‐risk ALL patients, allogeneic SCT with 12 Gy TBI leads to improved OS, LFS, GRFS, and a low relapse rate. Prospective studies comparing the different treatment regimens with larger MRD patient cohorts are needed to confirm this data.