[Development of Isocenter Measurement Method Considering the Displacement of the Focal-spot Position of a Medical...

Measurement of beam alignment, including an isocenter, is a fundamental part of commissioning and quality assurance for a medical linear accelerator (linac). An alignment shift is caused by the focal spot displacement from the beam axis. In this study, we present a procedure for the analysis of the focal spot positions in the radiation alignment using a newly developed quality control tool (QCT) device and analysis software.

The device has the function to perform the light/radiation field coincidence test, and the isocenter test at the same time. First, the light/radiation field was evaluated by using cross-slits on the surface of the QCT device. Second, the focal spot positions of beam alignment were measured using QCT, and the acquired images were collectively analyzed using the developed analysis software. Finally, measurements of focal spot positions with different energies were made using the accelerators at the 2 facilities.

The focal spot position was changed with a gantry angle. The focal spot displacement was 0.15-1.37 mm. With the validation of 2 accelerators at 2 facilities, the displacement of the focal spot position could be measured using the QCT phantom.

The focal spot position measurement using the QCT test captured the change in the beam alignment between the beam axes without being affected by the jaw collimator. This study suggested that focal spot position measurement using a QCT device is useful for evaluating beam alignment at linac.