Session Item

The position of X-ray beam focal spot can affect the position of the radiation isocentre, symmetry of the beam, and the width of penumbra, which is especially important for small fields used in stereotactic radiotherapy. The purpose of the work was to determine position of the X-ray beam focal spot relative to the rotation axis of the collimator for two TrueBeam Varian accelerators

Based on the Chojnowski’s paper (J Appl Clin Med Phys. 2017;18(5):175-183), a method for determination of the location of the focal spot X-ray beam relative to the collimator rotation axis was developed and implemented. Portal images generated with high-energy X-ray beams with a flattening filter were registered regularly over a period of four months for two Varian TrueBeam linear accelerators. Each time a set of images was obtained, with separate fields shaped by the jaws of the main collimator and by the multi-leaf collimator (MLC). The images were analyzed using own plugin for ImageJ software to find positions of beam centers. If the beam focal spot was not on the collimator''s rotation axis, the location of the center of the field shaped by the jaws of the main collimator differed from the location of the center of the field shaped by the MLC. Based on this difference, the distance between the focal spot and the axis of collimator rotation was calculated.

For both accelerators, the distance between the beam focal spot and the collimator rotation axis measured in the cross-plane direction did not exceed 0.2 mm, and in more than 90% of cases, it was within 0.1 mm. The distances measured in the in-plane direction were larger and for one accelerator reached even 0.76 mm, whereas for the other one the range of results was smaller and did not exceed 0.4 mm.

Differences between two linear accelerators of the same type were observed. Accelerator with a smaller variation of distance between the beam focal spot and the collimator rotation axis should be probably chosen as a primary unit for stereotactic radiotherapy