SU-E-T-325: Study of the Accuracy of the Dynamic Tracking System Using Radiographic Films

Purpose: To investigate the accuracy of the dynamic tracking system on a linear accelerator using radiographic films. Methods: The Vero 4DRT system (Mitsubishi Heavy Industries, BrainLAB) installed in Joetsu General Hospital in Japan was used in the present study. It can perform dynamic tracking irradiation using gimbal mechanism by swinging an accelerator tube directly. A respiratory motion platform was used to move a water equivalent phantom, in which EDR2 radiographic films were placed. The amplitude of the motion was 10 mm and the periods of the motion were 3, 4, and 5 seconds. The moving phantom was irradiated with dynamic tracking and without dynamic tracking. The static irradiation, in which the phantom did not move, was also performed. The field size was 3 × 3 cm2. After the irradiation the films were developed and scanned. The area as a function of irradiated dose was obtained from the scanned films. The variation of the areas was used to evaluate the tracking accuracy. Results: The profiles for the moving phantom without dynamic tracking were blurred if compared to the static irradiation. The irradiated areas above the 50 % isodose level became smaller but those under the 50 % isodose level became larger. In the dynamic tracking irradiation, the changes of the areas became smaller because the irradiating fields approach to static fields from the film point of view. The maximum variation of the areas corresponded to 1 mm tracking error, which was deduced from the comparison of the areas with those for the irradiation to the static phantom. Conclusion: The tracking accuracy of the Vero 4DRT system was investigated using the radiographic films. It was evaluated less than 1 mm under the irradiation condition in this study. This work was supported by JSPS KAKENHI Grant Number 23791449.