3733 - Engraftment Rates of Image-Guided Volumetric Modulated Art Therapy for Total Body Irradiation Compared to Conventional

Purpose/Objective(s): Volumetric Modulated Arc Therapy (VMAT)-based Total Body Irradiation (TBI) is increasingly used in TBI with myeloablative intent. Clinical outcomes continue to emerge, but there remains no data on engraftment, an important early outcome after hematopoietic cell transplant, crucial for sustained long-term hematopoiesis. Furthermore, given the segmented nature of VMAT-TBI, there is potential for cancer cells to escape, which could impact relapse rates. The aim of this study is to analyze time to engraftment in patients receiving VMAT-TBI. We also calculate the TBV in the same cohort of patients to explore how that could potentially affect relapse rates.

Materials/Methods: In this IRB-approved retrospective single-institution study, all patients treated with VMAT-TBI Jan 2023-Sep 2024 were identified. Clinical data were abstracted from electronic records. Primary outcomes were time to neutrophil and platelet engraftment. To estimate the number of passes of the TBV through the field being radiated, we calculated the cardiac output (CO) and mean TBV for each patient and derived the time spent radiating the thorax from the radiation planning system.

Results: We identified 26 pts who received VMAT-TBI. Seventeen patients (65%) were male and 9 (35%) were female. The median age at TBI was 38 years (range: 12-61). ALL made up more than half of the population, with B-cell ALL being the most common diagnosis, accounting for 23.1%, followed by AML (15.4%) and CML (7.7%). Less frequent diagnoses included Anaplastic Large Cell Lymphoma, MDS, Ph+ B-cell ALL, T-cell lymphoma, and T-lymphoblastic CML, each constituting 3.8% of cases. Preparative regimen consisted of either Cyclophosphamide alone (11 pts), Fludarabine alone (11 pts), FC (3 pts), or Etoposide alone (1 pt). Most patients (92.3%) received 1200 cGy in 8 fractions BID, with 2 patients receiving 1320 cGy in 8 fractions BID. All but 1 patient had ANC engraftment and all but 2 patients had platelet engraftment. Each of those patients died prior to engraftment. Median time to ANC engraftment was 15 days (range: 11-44). Median time to platelet engraftment was 25 days (range: 12-248).

Conclusion: In the only series of its kind to date, we demonstrate no adverse impact on engraftment rate or timing after VMAT-TBI. Additionally, the high number of blood volume passes through the thorax during irradiation minimizes the chance of cancer cell escape, which should translate to no adverse effects on relapse rates. Further research comparing VMAT-TBI with conventional TBI is necessary to validate these findings.