S1644
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
Digital Poster 1935
The clock is ticking: in vivo dose reading stability analysis of the Gafchromic™ in vivo dosimetry system. Agnieszka Walewska, Paulina Wesołowska, Paweł F Kukołowicz Medical Physics Department, Maria Sklodowska-Curie National Research Institute of Oncology, Warsaw, Poland Purpose/Objective: The Gafchromic™ in vivo dosimetry system (Ashland), utilising the pnt-dos™ radiochromic film dosimeter, is designed as a quality control system for the verification of the delivered radiation dose in radiotherapy. The pnt-dos™ device is designed to verify doses across both low (< 80 cGy) and high (80– 400 cGy) ranges. The purpose of this work was to investigate the dependence of the dose readout value on the time elapsed following detector irradiation. Material/Methods: To account for the characteristic post-irradiation optical density growth inherent to radiochromic films, a time-resolved calibration workflow was implemented. The calibration detectors were automatically scanned at specified intervals to build a set of time-resolved, triple-channel calibration curves. Calibration curves were estimated for the low- dose range using a 6 MV Flattening Filter Free (FFF) photon beam generated in Ethos (Varian) according to the manufacturer’s instructions. To quantify the effect of time-resolved calibration approach, six detectors were irradiated at a 10 cm depth in a solid water- equivalent phantom, with a source-to-surface distance (SSD) of 90 cm. The following doses were delivered: 69.57, 49.69, 39.75, 29.82, 19.88, and 9.94 cGy. The low-dose range was selected to evaluate the detectors’ performance for potential use in in vivo dosimetry on the patient’s body surface, where the delivered doses are typically low. Each detector was scanned (Epson Perfection V600) five times at specific intervals following irradiation: 24 minutes, 55 minutes, 1 hour 24 minutes, 19 hours 17 minutes, and 23 hours 21
Figure 2. Dosimetry and beam diagnostics results. (a) Three repeated measurements at 6 MeV electron energy with average dose rates. (b) Five repeated measurements at 2 MeV electron energy with average dose rates. (c) Stability test over 8 hours at 0.1 mA CW average current and the corresponding dose rate. (d) Micro pulse measured by FCT. Conclusion: We developed a superconducting accelerator platform with broad energy and highly tunable dose rates, providing a stable, versatile tool for FLASH-RT mechanistic studies, protocol optimization, and potential pre-clinical translation. References: [1] Huang S, Liu K, Zhao K, Chen J. DC-SRF photocathode gun. Chin Sci Bull 2022;68:1036–46. [2] Sun J, Lv J, Tian S, Liu J, Zhang Z, Xu H, et al. Dosimetry study of high repetition rate 2 MeV electron beam from a continuous-wave photocathode gun. Phys Scr 2025;100:065010. [3] Jia H, Li T, Wang T, Zhao Y, Zhang X, Xu H, et al. High-brightness megahertz-rate beam from a direct-current and superconducting radio- frequency combined photocathode gun. Phys Rev Res 2024;6:043165. [4] Lv J, Sun J, Luo Y, Liu J, Wu D, Fang Y, et al. Ultrahigh Dose Rate Irradiation Regulates Mitochondrial DNA-induced Interferon-β Secretion via Cytochrome c Leakage. MedComm 2025;6:e70457. Keywords: FLASH radiotherapy, Superconducting accelerator
minutes. Results:
Data analysis involved calculating the standard deviation (SD) across the five readings of each dose. Furthermore, absolute deviations of individual time- point measurements from the reference dose were determined (Tab.1). The maximum SD across the five time-resolved readings obtained within the 24 hours post-irradiation was 0.6 cGy. The maximum absolute deviation of the measured dose relative to the
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