Flattening Filter Free Volumetric Modulated Arc Therapy (FFF-VMAT) Approach for SBRT in Patients With NSCLC: Dosimetric Potential Compared With Flattening Filtered Volumetric Modulated Arc Therapy (FF-VMAT) and Helical Tomotherapy (HT)

Abstract

SBRT is an emerging trend in the treatment of NSCLC as studies provided encouraging results comparable to conventional regimes. HT and VMAT provide exceptional dose conformity to targets while also effectively sparing organs at risk (OARs). The purpose of this study is to explore the potential of FFF-VMAT, FF-VMAT and HT in a SBRT regime for patients with NSCLC who were treated by conventional fractionations. Twenty NSCLC patients with a single target (size 1.9-7.6cm; median 5.3cm) were selected retrospectively in which thirteen of the targets were centrally and seven were peripherally located within the lung. Retrospective dual-half-arc FFF-VMAT, FF-VMAT and HT (2.5cm fan-beam, Pitch 0.287) re-plans were developed under a SBRT regime of 48Gy in 4 fractions. The doses to OARs including lungs, bronchi, trachea, brachial plexus, spinal cord and esophagus were compared. Dosimetric analysis was performed using repeated measures ANOVA followed by a post hoc multiple comparison test. Patients with centrally located tumors have higher doses to OARs than those peripherally located such as ipsilateral lung (Dmean: 7.4±2.63Gy vs 4.9±2.25Gy), ipsilateral bronchus (Dmax: 32.31±15.14Gy vs 9.89±7.31Gy) and proximal trachea (Dmax: 15.35±10.32Gy vs 7.09±6.09Gy). In the peripheral group, FFF- and FF-VMAT plans were comparable in all dosimetric aspects. FFF-and FF-VMAT achieved a superior conformity in CN (p=0.0006) and CI50% (p=0.0012) than HT. Both VMAT plans generated a lower mean dose in total (FFF: 3.44±1.52Gy, FF: 3.50±1.51Gy, HT: 4.35±1.83Gy), ipsilateral (FFF: 4.43±2.09Gy, FF: 4.50±2.06Gy, HT: 5.79±2.66Gy) and contralateral lung (FFF: 1.18±0.55Gy, FF: 1.22±0.57Gy, HT: 1.51±0.74Gy) while also achieving a lower V5, V10 and V11.6 of total and ipsilateral lung. HT excelled in comparison with FFF-VMAT by providing a lower Dmax to ipsilateral bronchus (p=0.0216) and FF-VMAT with a lower Dmax to the spinal cord (p=0.0181), esophagus (p=0.0098) as well as Dmax (p=0.0322) and Dmean (p=0.0389) of the contralateral bronchus. The option of SBRT is more suitable for peripherally located tumors in all the techniques due to lower toxicities to OARs. Both VMAT plans surpass in conformity and reducing dose to lung tissues which is essential to NSCLC patients with impaired lung function. The higher dose rate in FFF-VMAT can reduce delivery times and improve tumor motion ambiguities. FFF-VMAT is an appealing option for SBRT treatments to peripherally located tumors for NCSLC patients and can be further enriched by integrating respiratory gating to allow further decrease in target margins for lung sparing.