Abstract

Purpose: Total Body Irradiation (TBI) is a critical component of conditioning regimens prior to hematopoietic stem cell transplantation (HSCT). This report presents the first clinical implementation of VMAT-based TBI for a 19-year-old male patient using a Halcyon™ Elite linear accelerator, with treatment planning performed in Eclipse™ Treatment Planning System (TPS) version 17.0. The prescribed dose was 12 Gy delivered in 10 fractions, consistent with a reducedintensity conditioning protocol. Methods: The patient was simulated in head-first supine (HFS) positions using vaclock immobilization and full-body support. A planning target volume (PTV) was defined by cropping 3 mm from the external contour to avoid build-up issues. MAT plans were created using 12 isocenters with 6 MV flattening-filter-free (FFF) beams. The treatment was planned using Eclipse v17.0, with emphasis on achieving uniform PTV coverage while minimizing dose to critical organs-atrisk (OARs) including lungs, kidneys, lenses, and testes. Pre-treatment quality assurance (QA) was performed with portal dosimetry. Results: The treatment plan achieved adequate PTV coverage, with D95% of 10.76 Gy, ensuring acceptable dose homogeneity (HI = 0.26). The mean doses to the left and right lungs were 11.45 Gy and 11.48 Gy, respectively. The mean kidney doses were 11.09 Gy for the left kidney and 10.98 Gy for the right kidney. The doses to the lenses (5.38 Gy) and testes (14.27 Gy) remained within institutional tolerance limits. The total monitor units (MU) delivered were 2704.9.The total beam-on time per fraction was approximately 15 minutes, with overall treatment time including setup and imaging averaging 1 hour 30 minutes. Gamma analysis using 3%/3 mm criteria demonstrated 100% passing rates (γ < 1.0) for both head-first supine (HFS) and feet-first supine (FFS) setups, exceeding the 97% institutional tolerance. Conclusion: Halcyon VMAT-based planning provided satisfactory target coverage and organ sparing within an efficient and reproducible workflow. Halcyon may be considered a suitable platform for TBI delivery in adolescent and young adult (AYA) patients undergoing transplantation.

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