ESTRO 2024 - Abstract Book

S3266

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Characterization of EBT3, EBT-XD and OC-1 Films for Absolute Dose measurement in Electron Beams

Cathyanne Schott 1 , Imane Saïd Mansour 1 , Nathalie Fournier-Bidoz 1 , Pauline Maury 1 , Charlotte Robert 1 , Julie Colnot 1,2

1 Gustave Roussy, Radiation Oncology, Villejuif, France. 2 THERYQ, PMB-Alcen, Peynier, France

Purpose/Objective:

FLASH radiotherapy, characterized by ultra-high dose rate (UHDR, >100 Gy/s), shows potential for effective tumor treatment while sparing healthy tissue. Nevertheless, precise dose measurements in such high-dose-rate beams is challenging as standard dosimeters from conventional radiotherapy saturate under UHDR conditions (1), requiring alternative dosimetry solutions. EBT3 films have shown promising characteristics for UHDR beam dosimetry, thanks to their dose-rate-independent response (2). In light of this, we developed a robust dosimetry protocol that relies on film-based measurements. To identify the most suitable candidate for absolute dose measurements in UHDR electron beams, we compared EBT3 films, EBT-XD (Ashland) and OC-1 (OrthoChrome Inc.) films. This study was first conducted in conventional electron beams in order to assess the properties of the films, with the aim of using them in the very near future to commission the FLASHKNiFE system (THERYQ) newly installed at Gustave Roussy.

Material/Methods:

Films were irradiated and calibrated against ion chamber measurements using a Clinac 2300 C/D (Varian) accelerator at 12 MeV. Optical density uncertainty was evaluated at two post-irradiation times and various doses (0 – 30 Gy), using four films per dose. Within- and between-film response uniformity, scanner repeatability, and film positioning on the scanner glass were considered in this aim. Absolute dose measurement accuracy was evaluated 24 hours (day 1) and 4 days (day 4) post-irradiation, utilizing films irradiated at several “unknown” doses (2.5 – 27 Gy). The energy dependence of the films was assessed by irradiating them at “unknown” doses at different beam energies, and calculating the difference between the delivered dose (measured with ion chamber) and the measured dose with the calibration curve performed at 12 MeV. The study also investigated the influence of a background correction method, which involves using a pre-irradiation scan to compute the optical density for each irradiated film.

Results:

The optical density uncertainty of EBT3 and EBT-XD remained stable between day 1 and day 4, with average values of 0.24% and 0.33% respectively for unirradiated films, and of 0.50% and 0.57% respectively for irradiated films. OC-1 exhibited higher uncertainties, increasing from 0.59% on day 1 to 0.78% on day 4. EBT3 showed the highest dose accuracy in comparison with the ion chamber on days 1 and 4 (0.78% and 0.74%). EBT-XD maintained comparable accuracy on both days (1.02% and 0,96%), while OC-1 accuracy decreased on day 4 (1.65% to 1.83%). A slight dependence on energy is observed for high irradiation doses (>10 Gy) for the three types of film. The accuracy of absolute dose measurements was only enhanced by background correction in the case of OC-1 films, resulting in a reduction of the absolute variation from the delivered dose, decreasing from 1.65% to 1.05%.

Conclusion: