No More Feet-first Supine: A Simplified Total Body Irradiation Planning and Delivery Using Single Head-first Supine CT Simulation and SGRT for Intra-fraction Motion Monitoring on Radixact Tomotherapy

<i>Introduction</i>: Total Body Irradiation (TBI) remains a crucial preparatory regimen for hematopoietic stem cell transplantation. However, traditional techniques involve complex workflows requiring dual CT simulations-Head-First Supine (HFS) and Feet-First Supine (FFS)-which pose logistical, dosimetric, and patient comfort challenges. This study introduces a simplified TBI approach using a single HFS CT scan combined with Surface Guided Radiation Therapy (SGRT) for setup verification and intra-fraction motion monitoring on the Radixact® TomoTherapy system. <i>Materials and Methods</i>: From August 2024 to April 2025, five adult patients with hematologic malignancies scheduled for TBI were retrospectively analyzed. Each patient underwent a single HFS simulation scan on PET-CT with full-body coverage. Immobilization was achieved using a vacuum cushion and thermoplastic mask. Treatment planning was performed using Accuray Precision® version 3.5.0.2 on a single CT dataset. Two helical plans, one for the upper body and one for the lower body, were generated with a 14 cm overlap at the mid-thigh. Seven feathered Planning Target Volumes (PTVs) were created at the junction to ensure dose homogeneity. Surface Guided Radiation Therapy (SGRT) was used for initial setup and intra-fraction motion monitoring, while ClearRT-based kVCT imaging served as a secondary internal verification. Patient-specific quality assurance (PSQA) was conducted using Delta4 with 3%/3 mm gamma criteria, and junction dose validation was performed using ion chamber measurements. <i>Results</i>: SGRT enabled accurate patient setup for TBI, with mean translational errors ≤2 mm and rotational errors ≤0.6° when compared to kVCT verification. Real-time motion monitoring during treatment enabled automatic beam-holds with an average latency of 0.0038 seconds, ensuring precise delivery. Dosimetric results showed excellent target coverage (≥95% of the PTV receiving ≥95% of the prescribed dose) and homogeneous dose distribution. Smooth dose profiles were observed at plan junctions, and lung doses remained within safe limits (~1.7 Gy). Pre-treatment QA demonstrated high gamma pass rates across all body regions (mean ≥96.6%), confirming treatment accuracy. <i>Discussion</i>: This simple modified workflow utilizing a single HFS CT scan for full-body TBI on Radixact eliminates the need for patient repositioning and FFS imaging, thereby simplifying the planning and delivery process. Integration of SGRT provides non-ionizing, real-time monitoring, enhancing setup precision and intra-fraction control. The feathered PTV junction design effectively mitigated dose inhomogeneities. This method demonstrates not only dosimetric robustness but also improved workflow efficiency and patient comfort.