ESTRO 2024 - Abstract Book

S4738

Physics - Quality assurance and auditing

ESTRO 2024

Radboud university medical center, radiation oncology, Nijmegen, Netherlands

Purpose/Objective:

In our clinic we treat stereotactic radiosurgery (SRS) patients with different couch rotations during treatment to optimize the dose delivery to the patient. It is therefore important to understand impact of these couch rotations on the target position due to possible deviation of the couch as a result of rotating the couch. The purpose of this study is to check the accuracy of the SGRT system (advanced Horizon AlignRT, Vision RT, UK) compared to the MV EPID based positions for a clinical range of couch rotations from 270 to 90 degrees.

Material/Methods:

We made a home-made phantom with ball bearing inside to mimic the clinical patient treatment at the linac. Using this phantom we can monitor the phantom postition using both the WL test with EPID, and the SGRT system. We can thus estimate the robustness of the SGRT system for couch rotations. The home-made phantom is fixed with an open face mask (Orfit Industries, Belgium) at the head adjuster (VisionRT, UK). Figure 1 shows the phantom in treatment position with couch rotation of 270 degrees. Winston-Lutz (WL) measurements using EPID for different couch rotations with the head phantom are related to the optical measurements for a variation of phantom shifts in different directions.

Figure 1 shows a screenshot of the home-made phantom at couch with a rotation of 270 degrees.

Results:

Figure 2 shows a typical measurement of ball bearing position at gantry 0 degrees with EPID (gray) and position of the selected region of interest (ROI) of the surface of the phantom’s face with SGRT (red). The plotted lines of the SGRT and EPID measurements follow the same shape. Small deviations between the SGRT system and EPID measurements were found for different couch angle positions in a range of 0 to 0.8 mm. In this example the phantom with ball bearing was shifted from isocenter position to a couch position of -1.4 mm lateral and -1.1 mm longitudinal. The difference in target position for EPID and SGRT showed under different couch angle positions are within 0.5 mm except for couch rotation 90 degrees.

Figure 2 shows the measured shifted ball bearing position with EPID versus measured shifts of ROI surface by the SGRT system for different couch rotations.