Dosimetric evaluation and feasibility of total body irradiation with multiple isocenter VMAT technique on the halcyon...

The current planning study sought to assess the potential clinical feasibility and dosimetrics of employing volumetric modulated arc therapy (VMAT) with the HalcyonTM linear accelerator (Linac) using photon optimizer algorithm for total body irradiation (TBI).

The upper body and lower body PTVs were optimized independently using a 6 MV FFF beam energy with ten isocenters in the Eclipse treatment planning system (TPS) and photon optimizer algorithm. The treatment plans were delivered using a HalcyonTM E linac (Varian Medical Systems Inc., Palo Alto, CA). A dose of 300 cGy in single fractions was prescribed for the whole-body PTV. Pretreatment quality assurance included portal dosimetry comparisons and ArcCheckTM (Sun Nuclear), assessing the delivery-to-calculation agreement with the gamma agreement index.

The PTV D98%, D95%, D50%, D5%, D2%, and Dmean were 292.4 cGy (97.5%), 301.6 cGy (100.5%), 324.9 cGy (108.3%), 346 cGy (115.3%), 351 cGy (117%), and 325 cGy (108.3%), respectively. The dose at the Dmax within the PTV body was 377.0 cGy (125.7% of the prescribed dose). Portal dosimetry and arc check patient-specific quality assurance results demonstrated mean and standard deviation values within acceptable tolerances: 3%/3 mm and 2%/2 mm with individual measurements of 100 ± 0.05, 99.9 ± 0.15, 99.9 ± 0.41, and 98.2 ± 1.28, respectively. Overall portal dose gamma pass rates were greater than 96%. Target delineation took 6 hours. Optimization and dose calculation were performed 13 to 14 hours for one time. To create clinically acceptable plans, multiple repeated reoptimization and calculation are required. The overall time required to create clinical acceptable plans is 38 to 40 hours. Quality assurance took 6 to 8 hours.

HalcyonTM VMAT TBI achieves a satisfactory dose distribution within the PTV. Nevertheless, the planning and optimization time is considerably extensive. It is recommended to have more than two linear accelerators and high-end hardware for the TPS to ensure optimal performance.