Dosimetric and Delivery Assessment of Stereotactic Body Radiotherapy Using Flattened and Unflattened Beams for the Single-Isocenter Treatment of Multiple Liver Targets

BACKGROUND: Stereotactic Body Radiotherapy (SBRT) is increasingly applied in the management of liver cancers. Flattening filter-free (FFF) beams, which offer higher dose rates, enable faster delivery and improved patient comfort. This retrospective study compares the dosimetric and delivery characteristics of SBRT using FFF and conventional flattened beams for the treatment of multiple liver targets. METHODS: Twenty-six patients with 2-11 hepatic targets were treated using a single-isocenter volumetric modulated arc therapy (VMAT) technique. Treatment plans were created for 6X, 6FFF, and 10FFF photon beams, with prescription doses ranging from 27.5 Gy to 50 Gy in 5 fractions. Plan evaluation metrics included Planning Target Volume (PTV) coverage (V95%, V98%, V100%), mean liver dose (MLD), normal liver volume receiving <15 Gy, and maximum doses to Organs at Risk (OARs). Delivery parameters were assessed by gamma passing rate, monitor units (MU), and beam-on time (BOT). Repeated-measures ANOVA, with post-hoc least significant difference (LSD) testing, was used for statistical analysis. RESULTS: Comparable Planning Target Volume (PTV) coverage was achieved with all beam modalities. No statistically significant differences were noted in mean liver dose (MLD), normal liver volume receiving <15 Gy, or other Organ at Risk (OAR) doses, except for lower spinal cord doses with FFF beams (p = 0.003). The 2%/2 mm gamma passing rate for 6FFF was 1% lower than for 6X and 10FFF. Monitor unit (MU) values were higher for 6FFF (4.3%) and 10FFF (1.5%) compared with 6X (p = 0.01). Beam-on time (BOT) was significantly shorter with 6FFF (3.8 ± 0.28 min) and 10FFF (3.8 ± 0.30 min) relative to 6X (5.1 ± 0.22 min) (p = 0.001). CONCLUSION: Flattening filter-free (FFF) beams reduce beam-on time (BOT), providing an advantage for breath-hold techniques. However, despite the higher dose rate of 10FFF, no additional reduction in overall treatment time was observed.