Stem cell boost for severe CAR T-related hematotoxicity is safe and clinically feasible. (A) Median cumulative duration of severe thrombocytopenia (platelet count < 50 G/L) and severe neutropenia (ANC/µ) in days between CAR infusion and stem cell rescue. Box and whiskers indicated the median and interquartile range. (B) Overview of management strategies of hematological toxicity prior to stem cell rescue. (C) Cumulative incidence rate of neutrophil (left) and platelet (right) engraftment in the first 30 days after CAR infusion. *An additional patient (#7) engrafted on day 124. **Two additional patients engrafted after day 30 (#8, 11). (D) Spider plot depicting neutrophil (left) and platelet (right) recovery for the first 45 days following stem cell boost. Each patient is color coded. Patient #7 (light blue) received a second autologous stem cell boost due to persistent thrombocytopenia. (E) Kaplan-Meier estimates of OS (gray) and PFS (dark gray) for the entire study cohort (n = 12). Median survival and 1-year survival rates are depicted. (F) Summary of best overall response at day 90. CR, complete response; PD, progressive disease; PR, partial response.