VMAT-based total body irradiation on a conventional LINAC: workflow, procedure and preliminary results

The aim of this study was to evaluate the feasibility of implementing volumetric modulated arc therapy (VMAT)-based techniques and extended CBCT image guidance for total body irradiation (TBI) treatment using a conventional linear accelerator.

Patients eligible for TBI between November 2016 and December 2024 were included in the analysis. Patients received a total dose of 4-12 Gy, given in six fractions within 3 days, two fractions/day with 6 h minimum interval between fractions, or 2 Gy in one fraction, depending on the clinical indication. During the initial phase of the protocol, PET-CT imaging was used to obtain full-body CT datasets. Subsequently, CT simulation was performed using a multislice Siemens CT scanner available in the Radiation Oncology Department. In this setting, two CT studies were acquired per patient, one extending from the pelvis to the head (upper scan) and a second from the pelvis to the feet (lower scan), which were merged into a single dataset for treatment planning. Dosimetric planning was performed using a multi-isocenter approach with the Eclipse™ treatment planning system, employing volumetric modulated arc therapy (VMAT) to achieve the prescribed dose distribution. In the initial stage of the treatment program, treatments were delivered on a Varian CLINAC DHX linear accelerator. Following its decommissioning, treatment delivery was transitioned to Varian TrueBeam linear accelerators (models SN3790 and SN2137). Treatment delivery, including verification and patient positioning, was performed sequentially, beginning with the upper body followed by the lower body. Image guidance was initially based on kV-MV imaging and was later replaced by extended-field cone-beam computed tomography (CBCT), which was registered to the simulation CT to enable automated setup corrections. Dosimetric parameters and setup verification metrics were subsequently analyzed.

Between November 2016 to December 2024, 27 patients fulfilled the inclusion criteria. All scheduled sessions were completed, amounting to a total of 148 treatment fractions. The average number of isocenters used to generate the treatment plans was 7,11 (6-12). The mean lung dose was 10.12 Gy (range 8.97-11.07 Gy). Dose homogeneity achieved across all sessions was 1.24 (1.11-1.41). After image acquisition, mean setup corrections were 0.06 cm lateral (range 0.00-2.00 cm), 0.26 cm vertical (0.00-2.00 cm), and 0.03 cm longitudinal (0.00-1.30 cm) in head-first plan. Laterally, 0.26 cm (range: 0.00-2.00 cm) vertically, and 0.03 cm (range: 0.00-1.30 cm) longitudinally in feet first plan. The average duration of each session, from the first image acquisition to the completion of the final field, was 87 min (range 60-159).

Our study demonstrates that VMAT-based TBI is a feasible and promising alternative to conventional 2D-TBI, providing improved dose homogeneity, enhanced organ sparing and with reproducibility comparable to previously reported HT systems. These findings support the integration of VMAT techniques on conventional LINACs for TBI treatments, although further prospective studies are needed to confirm long-term clinical benefits.