Evaluation of dosimetric impact of the interplay effect between multi-leaf collimator (MLC) movement and tumor respiratory motion during volumetric modulated arc therapy (VMAT) delivery using polymer gel dosimeter was taken as an example in this article. An excellent gas barrier PAN (polyacrylonitrile) bottle filled with polyacrylamide-based gel dosimeter contained magnesium chloride as a sensitizer (iPAGAT dosimeter) was set to the QUASAR™ respiratory motion phantom (Modus), and was moved with motion amplitudes (peak-to-peak amplitude) of 1 and 2 cm with a 4 second period during VMAT delivery by the Novalis Tx linear accelerator (Varian/BrainLAB). Two spherical GTVs with 2 cm diameter and two PTVs were defined considering the respiratory motion and setup uncertainties. Three-dimensional (3D) dose distribution in iPAGAT dosimeter was read out by the 3T MRI system, and was evaluated by the dose profiles, gamma analysis and the dose-volume histogram (DVH) using in-house developed software. As a result, interplay effect was negligible since dose coverage of GTV was sufficient during VMAT delivery with simulated respiratory motion.