ESTRO 2025 - Abstract Book

S3646

Physics - Quality assurance and auditing

ESTRO 2025

Dosimetric indicators were measured using the Nomex Multimeter T11049 (PTW, Freiburg, Germany), attached to the EPID panel in front of the X-ray tube, across all acquisition protocols available on the Elekta CBCT XVI (Elekta, Crawley, UK). These indicators included kerma area product (KAP), incident air kerma at the detector (Ka), and field-of-view average dose (D FOV ) [1,2], which were originally introduced for diagnostic CBCT and are less commonly used for radiotherapy CBCT. Measurement repeatability was evaluated, and protocol variability was assessed by testing four different VersaHD linacs. Organ dose estimates were obtained for all protocols in various anatomical regions and for different patient sizes (6 adult and 7 pediatric sizes) using the simulation software PCXMC 2.0 (STUK, Helsinki, Finland), with adapted input parameters to model exposure geometry and dose distribution [3].

Results: A substantial variability in dosimetric indices was observed across different protocols (Figure 1).

Measurement repeatability was excellent, with a dose deviation below 1%. The maximum Ka deviation detected across different linacs was 2.2 mGy, with an average deviation of (0.0±1.0) mGy and an average absolute deviation of (0.8±0.6) mGy. These findings suggest that our dataset could be valuable for other Elekta users. This considerable variability in dosimetric indices corresponded to a large variability in organ doses, as demonstrated in Figure 2 for an adult patient (175cm height, 70kg weight) and a pediatric patient (126 cm height, 14 kg weight).