Impact of flattening-filter-free techniques on delivery time for lung stereotactic volumetric modulated arc therapy and image quality of concurrent kilovoltage cone-beam computed tomography: a preliminary phantom study

To the Editor, Volumetric modulated arc therapy (VMAT) serves as a means for stereotactic hypofractionated treatment of lung tumors [1]. The present authors proposed an efficient VMAT sequence by restricting leaf speeds of a multi-leaf collimator per gantry rotation angle to below 1 mm/degree, thereby reducing the dose delivery time down to 210 s for a D95 prescription dose of 50 Gy in four fractions [2, 3]. The authors also suggested further reduction of the dose delivery time to below 100 s using flattening-filter-free (FFF) techniques [3]. Recently we evaluated a non-clinical FFF research configuration by adding an FFF filter to our linac system, a Synergy with an MLCi multi-leaf collimator (Elekta AB, Stockholm, Sweden). The FFF filter is made of stainless steel and has a constant thickness of 2 mm for 6- and 10-MV beams. Furthermore, we also tested a non-clinical on-board cone-beam CT (CBCT) system, XVI 5.0 research version, that allows concurrent 4D CBCT imaging during VMAT delivery. It is anticipated that the reduced VMAT delivery time may degrade CBCT image quality due to a much lower number of projection images. The purposes of this study were: (i) to quantify the stereotactic lung VMAT delivery time with FFF, and (ii) to visually inspect CBCT image quality with FFF.