Impact of lung density on isolated lung tumor dose in VMAT using inline MR-Linac

Purpose: This study investigated the impact of lung density on the isolated lung tumor dose for volumetric modulated arc therapy (VMAT) in an inline magnetic resonance linear accelerator (MR-Linac) using the Monte Carlo (MC) simulation. Methods: CT images of the thorax phantoms with lung tumors of 1, 2, and 3 cm diameters were converted into voxel-base phantoms with lung densities of 0.1, 0.2, and 0.3 g/cm(3), respectively. The dose distributions were calculated for partial-arc VMAT. The dose distributions were compared using dose differences, dose volume histograms, and dose volume indices. Results: In all cases, the inline magnetic field significantly enhanced the lung tumor dose compared to that at 0 T. For the 1 cm lung tumor, the inline magnetic field of 1 T increased the minimum dose of 95% of the Planning target volume (PTV D-95) by 14.0% in 0.1 g/cm(3) ling density as compared to that in 0.3 g/cm(3) at 0 T. In contrast, at 0 and 0.5 T, the PTV D-95 in 0.3 g/cm(3) lung density was larger than that in lung density of 0.1 g/cm(3). For the 2 cm lung tumor, a similar tendency to 1 cm was observed, whereas the dose impact of lung density was smaller than that for 1 cm. For the 3 cm lung tumor, the lung tumor dose was independent of lung density at 0.5 T and 1.0 T. Conclusion: The inline MR-Linac with the magnetic field over 1 T can enhance the PTV D-95 for VMAT regardless of the lung density.