### INTRODUCTION

### MATERIALS AND METHODS

### Coordinate system

*x*), the shot center (

_{f}, y_{f}, z_{f}*x*), and frame center coordinate systems. There is a coordinate system (

_{s}, y_{s}, z_{s}*x*) in which each gamma beam is a

_{b}, y_{b}, z_{b}*z*-axis, that is an axis of the coordinate system, and

_{b}*x*is a plane perpendicular to the gamma beam axis through the focus. The coordinate relations of the coordinate systems are given by

_{b}y_{b}*s*) is the position coordinate of the shot focus from the frame coordinate system. In Eq. (2), θ

_{x}, s_{y}, s_{z}_{1}and θ

_{2}represent the azimuth and latitude, respectively of the source from the shot center coordinate system in the GK Perfexion model. In particular, Eq. (2) is a coordinate relation related to the dose profile distribution on the xbyb plane that is introduced in the dose calculation.

### Model of the skull

##### (3)

*x*) is the coordinate from the shot center, α is the angle that the gamma angle (γ) is related to, α = 90 - γ and β is the angle of rotation of the image relative to the stereotactic frame in the MRI axial image. The point (l, m, n) is the coordinate of the papillae in the frame coordinate system.

_{s}, y_{s}, z_{s}### Dose calculation of the VEMT

##### (4)

^{60}C is positioned in the block position as shown in Fig. 1. For any position, owing to the gamma beam from each collimator, the radiation dose at the dose measurement position depends on the ring of the sector.

_{i}, θ) ≠ D(r

_{i}, θ)] depending on the distance and angle. In particular, the dose distribution value

^{i}from the foot perpendicular to the focus from the measurement position to the gamma beam axis. Table 1 presents the respective values [5].

### Dose profile measurement by EBT3 and EBT-XD films

*x-z*and

*x-y*with 4, 8, and 16 mm collimators by the solid phantom (Elekta). After selecting one of the

*x-z*and

*x-y*directions of the solid-water phantom, as shown in Fig. 2A, the eight films on the center face were irradiated with a 16 mm collimator at 0, 0.5, 1, 2, 3, 4, 6, and 8 Gy (EBT3) and 0, 1, 3, 5, 10, 20, 30, and 40 Gy (EBT-XD) for the measurement of the calibration curve.

### RESULTS

### Validation of LGP 10.1.1 system

### Validation of the modified VEMT by EBT3 and EBT-XD films

^{2}=9972, EBT-XD R

^{2}=0.9984). We determined optical density data from the film gray scale value corresponding to 5 Gy and 20 Gy, which is 50% the dose of 10 Gy and 40 Gy, respectively. Fig. 4 shows the dose calibration curve. The film gray scale corresponding to 5 Gy and 20 Gy of the dose calibration curve was 162 and 283, respectively, using the fitted curve. The FWHM of the x, y, and z axes was calculated using the value obtained from the dose calibration curve.