25 Dosimetric impacts of FFF large-field beams for lung cancer VMAT treatment

Abstract

Introduction Volumetric modulated arc therapy (VMAT) using 6 MV (X6) flattening filter free (FFF) beams is generally used for stereotactic body radiation therapy (SBRT) in the treatment of lesions less than 4 cm diameter. FFF beams provide a higher dose rate and a reduced treatment delivery time compared to flattened beams (FF) [1] . This study offers an analysis of the dosimetric impacts caused by large field X6 FFF beams as part of the VMAT treatment for lung cancer. Methods A retrospective cohort of 30 patients with lung cancer who received VMAT X6 FF radiotherapy with wild-field going from 10 × 10 cm to 30 × 30 cm for a prescribed dose ranging from 50 to 66 Gy between 2016 and 2017 was included in this study. Planned target volume (PTV) ranges from 50.0 to 1850.0 cm3 with a margin of 5–7 mm around the clinical target volume (CTV). Determination of the X6 FFF dose distribution was made possible by the anisotropic analytical algorithm (AAA version 13.6) implemented in the Eclipse® treatment planning system (Varian Medical Systems) with a new objective function (Varian PRO/PO). Standardization of the treatment plans was implemented (95% of the dose covers 98% of the PTV) without exceeding 110% of the prescribed dose (1% of the PTV). The number of monitor unit (MU) and the treatment time of each plan were recorded as well as the 2 cm distance dose of PTV and organ-at-risk (OAR). Results A treatment delivery time reduction was found but limited by the linac gantry rotation speed and multi-leaf collimator (MLC) speed. A relative deviation of less than 1% was observed for the mean PTV dose between the X 6 FFF and X 6 FF beams. A 4,0% ± 0,17% reduction of the dose at 2 cm near from the PTV and 15,2% ± 0,25% to OAR was obtained using X 6 FFF . A relative dosimetric discrepancy of 7,1% ± 0,13% between X 6 FFF and X 6 FF beams for the average lung dose was found. However, the MU increased to 10.0% ± 0,21%. for the use of X 6 FFF . Conclusions Large field VMAT treatment with FFF mode provides a highly homogenous dose distribution to target volumes while generating high dose gradients that allows to spare surrounding healthy tissues.