ESTRO 2022 - Abstract Book

S783

Abstract book

ESTRO 2022

Results The crossing point across and along the bore ( U, V ) on the EPID represents the vertical fan-line of the radiation beam at the actual gantry angle, Fig. 2. As expected, the crossing point in the direction across the bore ( U ) had a linear variation with gantry rotations U fit = af+b , with a =114.8/º, b =510.5, and f being the gantry angle in degrees. Thus, the U cross point is sensitive to the definition of the zero gantry angle. The standard deviation of the crossing point of the four lines was approximately 1.3 ( U ) and 1.4 ( V ) pixels. The vendor calibration (Cal) is also shown in Fig. 2. A slight discrepancy between Cal and string phantom of 1.2 pixels (0.3 mm in iso-plane) and 3.4 pixels (0.7 mm in iso-plane) was observed in the U and V direction, respectively. The effect of CW and CCW rotations seems negligible.

Conclusion A simple phantom able to detect the centre EPID position was developed. Only small differences between vendor calibration and phantom measurements were observed. The measured U direction depends on a correct definition of the zero position of the gantry. That uncertainty can be removed by using a single CS on the centre string in a Winston-Lutz test that can define the U position. However, only the string phantom can detect the EPID centre position in the longitudinal direction ( V ). Furthermore, the string phantom does not need precise alignment to correct measurements, making it easy and fast to use.