ESTRO 2025 - Abstract Book

S2567

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

370

Digital Poster Evaluation of Cerenkov Light Calibration for Exradin W2 Scintillators in the CyberKnife System Bartosz Pawalowski 1 , Ewelina Nowak 1 , Zuzanna Wróblewicz 2 , Maksymilian Wosicki 2 , Felix Cabrera 2 , Sebastian Gajny 2 , Karolina Sanocka 2 1 Medical Physics, Greater Poland Cancer Centre, Poznan, Poland. 2 Medical Physics, Greater Poland Cancer Centre, Poznań, Poland Purpose/Objective: Dosimetry in small radiation fields, such as those used in stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT), presents significant challenges like perturbation caused by the detectors used. Unlike other types of detectors, the water-equivalent scintillator Exradin W2 (Standard Imaging, USA) does not perturb dose distributions when placed in the beam. However, another issue arises from the phenomenon of Cerenkov radiation. This study aims to investigate the effect of Cerenkov radiation on the accuracy of small field dosimetry using Exradin W2 scintillators. These scintillators offer high spatial resolution and are unaffected by variations in temperature and pressure, making them highly suitable for precise dosimetric measurements in small fields. Material/Methods: The presence of Cerenkov radiation, which generates electromagnetic radiation, impact the measurements. To address this issue, a correction factor called the Cerenkov Light Ratio (CLR) is introduced. CLR is calculated by comparing charge measurements in the blue and green channels of the detector under different configurations of fiber length. In this study, reference measurements were performed using the PTW microdiamond (PTW, Germany) detector for a range of collimator sizes, from 5 mm to 60 mm, on the CyberKnife system (Accuray, USA). This system generates high precision small field beams, and requires a dedicated methodology for commissioning and dose measurements. The calibration of the scintillator output was carried out using various collimator sizes, with particular focus on sizes below 12 mm, where conventional CLR calibration methods are known to be inadequate. The alternative calibration method developed in this study involved adjusting the length of the fiber exposed to the radiation field, thereby reducing discrepancies in dosimetric values. Results: The results demonstrated that the conventional CLR calibration methods, as recommended by the manufacturer, were inadequate for small field sizes, particularly those below 12 mm. The alternative calibration method, which adjusted the fiber length, significantly reduced discrepancies between the measured dose values and the reference dose values obtained using the Microdiamond detector. These findings suggest that the modified CLR calibration method provides a more accurate representation of the dose in small radiation fields.