Feasibility of high-energy flattening filter-free beams in stereotactic radiotherapy of centrally located non-small cell lung cancer

Background Stereotactic body radiation therapy (SBRT) is an established treatment option for early-stage non-small cell lung cancer (NSCLC) and can be delivered using intensity-modulated radiotherapy (IMRT) or volumetric-modulated arc therapy (VMAT). Flattening filter–free (FFF) beams enable ultra-high dose rates and improved delivery efficiency. However, the higher effective energy of 10 MV FFF beams compared with conventional 6 MV flattened beams raises concerns regarding lateral electron disequilibrium and penumbra widening, particularly in centrally located lung tumors. This study aimed to evaluate the dosimetric feasibility of 10 MV FFF (10FFF) and 6 MV flattened (6FF) photon beams using IMRT and VMAT techniques for SBRT of centrally located NSCLC. Materials and methods Eighteen patients with early-stage central NSCLC were retrospectively selected. Target volumes and organs at risk (OARs) were contoured according to RTOG 0813 guidelines. Four treatment plans were generated for each patient by a single planner: IMRT_6FF, IMRT_10FFF, VMAT_6FF, and VMAT_10FFF. Target coverage, dose gradient indices, OAR dose metrics, and treatment delivery parameters were evaluated. Statistical comparisons were performed using RM-ANOVA with a Bonferroni post-hoc test. Results All techniques achieved acceptable target coverage and conformity. VMAT_6FF plans demonstrated significantly lower R 50% and D 2cm compared with IMRT_10FFF. OAR doses were comparable among techniques, except for skin dose, which was significantly reduced in FFF-based plans. FFF beams significantly reduced beam-on time and overall treatment time, and reduced the calculated number of deep-inspiration breath-holds, albeit with an increase in monitor units. Conclusion High-energy FFF beams are dosimetrically feasible for SBRT of centrally located NSCLC, offering substantial improvements in treatment efficiency. However, VMAT_6FF plans exhibited superior dose gradient characteristics, which remains highly relevant for sparing critical central structures. The choice between 10FFF and 6FF should be individualized based on the clinical prioritization of dose gradient versus delivery speed.