TU-F-BRE-02: A Practical QA Method for Respiration-Gated SBRT Verification Using 2D Detector Array.
PURPOSE
Pre-treatment plan verification for respiration-gated SBRT is challenging but desired since this type of treatment involve more delivery uncertainty, e.g., tumor motion effect, tissue heterogeneity and gating system QA. This study is to design and verify a QA method for respirationgated SBRT pre-treatment verification using a 2D detector array.


METHODS
A lung phantom was designed based on the MatriXX QA system, which includes a detector board and a MultiCube phantom. The major modification was that an air chamber (∼5cm thick) was inserted between the top portion of MultiCube phantom and detectors board. A half plastic ball (4cm diameter) was put in the air chamber and attached to the detector board surface to mimic a tumor. The whole lung phantom was placed on a moving stage with a superior-inferior sinusoidal motion. The whole phantom was aligned to the treatment machine so that the S-I and L-R axes of the MatriXX were parallel to the corresponding machine axes during movement. In this way, the moving phantom is equivalent to a stationary phantom with only tumor motion. Following a typical planning procedure for respiration-gated SBRT treatment, SBRT plans were created using the gated-MIP as PTV. Treatment delivery was performed on Varian iX machine with respiration gating.


RESULTS
The measured 2D plane dose showed blurring effect induced by motion and this blurring effect was more noticeable at the peripheral region of the PTV. Nevertheless, the tumor dose was able to be identified by analyzing the detectors attached to the tumor and was well above the prescribed dose.


CONCLUSION
The designed lung phantom for respiration-gated SBRT pre-treatment verification was tested and was able to identify the dose received by the tumor. Further tests based on large number of patients would be needed to for future routine application.