ESTRO 2024 - Abstract Book

S4913

Physics - Quality assurance and auditing

ESTRO 2024

The machine quality assurance (QA) of stereotactic radiosurgery (SRS) systems like the CyberKnife by Accuray [1] requires high precision in the measurement workflow [2]. Radiochromic film [3] is the state of the art method for many QA measurements due to its high resolution [4]. However, the workflow of film measurements contains multiple steps making the handling difficult and time consuming. Therefore, a new completely digital QA workflow was implemented, that does not require film anymore.

Material/Methods:

Measurements were performed with the myQA SRS by IBA, a semiconductor detector array with a 0.4 mm resolution. The characteristics of this detector with respect to its application for the CyberKnife QA have been investigated previously [5]. Here, new workflows were developed for the implementation in the CyberKnife QA. This includes the modification of QA tests specified by the manufacturer as well as the implementation of self-written python scripts for the analysis of the measured data. The implemented tests included field size tests for the Iris collimator, positioning accuracy of the MLC and a test for the X-ray based automatic targeting system of the CyberKnife (automatic quality assurance, AQA [2]). The field sizes of the Iris collimator were measured in two dimensions. Flatness and symmetry of the beam profiles were used as consistency check over time. For the MLC QA, a Bayouth test [6] with five stripes of 10 mm width and gaps of 15 mm width at 800 mm SAD was performed and evaluated. The AQA test was implemented for all three collimator systems (fixed cones, Iris collimator and MLC) with the detector positioned at the isocenter tilted to 45° as shown in Figure 1 instead of using the AQA ball-cube phantom. The target is tracked via X-ray markers (fiducials) included in a plate, that can be attached to the detector hull. After irradiating circular isocentrical beams from 0° and 90°, the middle points of the resulting measured ellipses were evaluated.

A series of measurements were taken for all three QA procedures to establish the sensitivity and stability of the new workflows and evaluate their consistency with the current practice.

Figure 1: Measurement setup for the AQA test. The myQA SRS is placed on a fixture to tilt it to 45°. With the help of a plate with metal fiducials attached to the detector hull the detector is positioned isocentrical.