Millennium 120 multileaf collimator

The tumor was irradiated by seven coplanar beams collimated to the tumor with a Millennium 120 multileaf collimator (Varian Medical Systems). The leaves were positioned so as to create a 5-mm gap around the tumor. The isocenter for all beams was at the tumor center, and the source-to-axis distance was 1 m. For the FF beams, the beam energy was set at 6 MV because this is the most common energy used for treating lung tumors. For FFF beams, we also chose 6 MV because two recent studies have shown better sparing of organs at risk with 6 MV FFF beams than with 10 MV FFF beams (Lu et al. 2015 (link); Tambe et al. 2016 ). All dose distributions were normalized to deliver 50 Gy (prescription dose, Dx) to the isocenter. We used this point normalization, rather than a planning target volume (PTV) coverage-based normalization, to remove the impact of dose calculation uncertainties in the transition region between the tumor and the lung, as that region was the focus of our study. The impact of electronic disequilibrium at the edge of the tumor was evaluated by comparing the dose calculated in the phantom as described above (for each lung density and tumor volume) versus the dose calculated in the phantom where the lung tissue was replaced with water.