Development of a generic focal spot measurement method suitable for bore-type linacs

In linear accelerators, deviations in the x-ray focal spot position significantly affect the accuracy of radiation therapy. However, as the focal spot position in bore-type linac systems such as the Radixact system, cannot be assessed using conventional methods, a new evaluation method is required. This study aimed to develop a novel method to measure the focal spot position of Radixact and evaluate any deviations from the ideal x-ray focal spot position.

A structurally simplified measurement system was developed to evaluate the focal spot position of the Radixact system. This system consisted of a vertically aligned metal bar and an ionization chamber, which was moved stepwise to acquire the beam profiles. The focal spot position deviation was calculated based on the center differences of the profiles obtained from two different upstream and downstream locations of the metal bar.

The measurement results indicated that the focal spot position shift was 0.42 mm and -0.36 mm at the target height in the IEC-X and -Y directions, respectively. The measurement uncertainty was 0.187 mm, confirming a slight deviation from the ideal focal position.

This study developed a novel method to accurately evaluate the x-ray focal spot position of the Radixact system, which can potentially be applied to other conventional linear accelerators and bore-type systems, such as Halcyon, to improve the accuracy of radiotherapy. However, its generalizability and applicability to different radiotherapy machines must be explored further.