Beam focal spot position: The forgotten linac QA parameter. An EPID‐based phantomless method for routine Stereotactic linac QA

Abstract

Modern day Stereotactic treatments require high geometric accuracy of the delivered treatment. To achieve the required accuracy the IGRT imaging isocenter needs to closely coincide with the treatment beam isocenter. An influence on this isocenter coincidence and on the spatial positioning of the beam itself is the alignment of the treatment beam focal spot with collimator rotation axis. The positioning of the focal spot is dependent on the linac beam steering and on the stability of the monitor chamber and beam steering servo system. As such, there is the potential for focal spot misalignment and this should be checked on a regular basis. Traditional methods for measuring focal spot position are either indirect, inaccurate, or time consuming and hence impractical for routine use. In this study a novel, phantomless method has been developed using the EPID (Electronic Portal Imaging Device) that utilizes the different heights of the MLC and jaws. The method has been performed on four linear accelerators and benchmarked against an alternate ion chamber‐based method. The method has been found to be reproducible to within ±0.012 mm (1 SD) and in agreement with the ion chamber‐based method to within 0.001 ± 0.015 mm (1 SD). The method could easily be incorporated into a departmental routine linac QA (Quality Assurance) program.

Beam focal spot position determination for an Elekta linac with the Agility® head; practical guide with a ready‐to‐go procedure

“…It was noticed that the mean discrepancy in validating beam focal spot offsets against the ionization chamber method was four times higher on Elekta linacs with the Agility ® head (0.004 ± 0.052 mm) compared to published results for Varian linacs (0.001 ± 0.015 mm). No specific reason for this was found other than the fact that the reproducibility of beam focal spot position is substantially higher on the Elekta machines.…”

“…An innovative phantom‐less method of measuring beam focal spot offset using the EPID has been presented for Elekta linacs with the Agility ® head. It is a modification of the method described previously for Varian linacs, which have two sets of jaws as opposed to one for the Agility ® head. It has the same advantages of being an accurate, practical and fast technique.…”

“…A significant improvement of the beam focal spot offset measurement methodology proposed by Chojnowski et al using an EPID‐based and phantom‐less technique, has since been shown to produce quick and accurate measurements. The idea was developed from the fact that if the radiation source is aligned with the collimator axis then the radiation isocenter position determined by the collimator rotation is independent of the type of field collimation used: jaws (diaphragms) or MLC.…”

“…The procedure of measuring the beam focal spot offset as described by Chojnowski et al is specific to Varian machines. It uses four open fields; two of which use two sets of jaws (X and Y) to form the collimation aperture and the other two using the MLC.…”

An EPID‐based method to determine mechanical deformations in a linear accelerator

“…The source displacement was relative to the source position in the first test execution. It was introduced by modifying the source position on the target using the linac beam steering and adjusting the beam steering servos, and the amplitude of the source displacement was measured with the independent verification method presented in Chojnowski et al, but with a manually positionned ionisation chamber. Table presents the averages of errors detected at each gantry angle.…”