Purpose: VMAT plans with high amount of complex apertures could lead to less confidence in dose calculation accuracy due to machine modelling and dose algorithm limitations. It was hypothesized that VMAT plans generated with FFF beams would result in more complex aperture shapes due to the non-flattened beam profile. The purpose of this study was to test this hypothesis by assessing various plan complexity metrics. Methods: Target and organ-at-risk structures were created on an anthropomorphic phantom in various sites: lung, prostate, spine, pancreas, breast, rectum and HN. Hypo-fractioned VMAT plans were generated using Elekta Agility MLC in Pinnacle TPS. Both FF and FFF plans were optimized using the same objectives and constraints. The following plan complexity metrics were assessed: total plan MU, mean aperture area (MAA), mean leaf gap (MLG), mean perimeter area ratio (MPAR), and modulation complexity score (MCS). Paired 2-sided Wilcoxon signed-rank test was used to test for differences. Results: A total of 20 pairs of plans were generated. Similar plan qualities were achieved between FF and FFF plans. FFF plans resulted in higher aperture complexities, as indicated by larger MU/MPAR and smaller MAA/MLG/MCS values. The differences were: 278 ± 178 for total plan MU (p < 0.01), -2.0 ± 2.6 cm² for MAA (p < 0.01), -0.2 ± 0.2 cm for MLG (p < 0.001), 0.18 ± 0.17 cm⁻¹ for MPAR (p < 0.001) and -0.02 ± 0.02 for MCS (p < 0.001). Similar results were observed when divided all plans into two subgroups based on PTV volumes (Vptv < 100 cm³ and > 200 cm³, 10 cases each). Conclusion: There were small but statically significant differences in all complexity metrics suggesting slightly more complex aperture shapes in FFF plans. However, the differences were quite small and not likely to cause any difference in dose calculation uncertainties.
Liu Q, Liang J. Comparison of aperture complexity between VMAT plans using conventionally flattened (FF) and flattening-filter-free (FFF) beams. Med Phys. 2022 June;49(6):E422-E423.