ESTRO 2025 - Abstract Book

S2592

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

Conclusion: We showed the need for 2D real-time dosimetry to determine beam characteristics in non-homogeneous UHDR beams using a preclinical FLASH radiotherapy setting; one solution is scintillating based dosimetry.

Keywords: 2D real-time dosimetry, UHDR-FLASH dosimetry

2063

Digital Poster Farmer chamber electron reference dosimetry in the range of 6 to 15 MeV reduces measurement uncertainty H.R. van de Glind, A. Broekhuis, K.H. van der Boom, G.Y. Vonk, A.M. Kamerbeek Radiotherapy, Isala, Zwolle, Netherlands Purpose/Objective: In current practice, electron reference dosimetry is performed using plane-parallel chambers. However, this approach has several disadvantages, including a complicated and error-prone cross-calibration. Historically, Farmer chambers, typically used for photon beam dosimetry, were not employed for electron dosimetry due to concerns about their stability at low electron energies [1]. In a recent publication by Muir et al., the k Q factor has been established for beam qualities as low as 4 MeV [2], [3], allowing the direct use of Farmer chambers for electron reference dosimetry [4]. To investigate the applicability in clinical practice, we examined the associated uncertainty and dose calibration differences using both plane-parallel and Farmer chambers for electron reference dosimetry. Material/Methods: We compared the existing dosimetry procedure, which involves cross-calibration between Farmer and plane parallel chambers, with the new method using only the Farmer chamber. Reference dose measurements and uncertainty analyses were conducted for both methods for the clinically available electron energies (6 MeV, 9 MeV, 12 MeV, and 15 MeV). For the reference dose measurements, a PTW MP1 water phantom was used with PTW ROOS (plane-parallel) and PTW 30013 (Farmer) ionization chambers. Results: The deviation between dose measurements using the plane-parallel chamber and the Farmer chamber is well within the associated uncertainties (Figure 1). We quantified most contributors to uncertainty through measurements. The primary source of increased overall uncertainty in dose measurements with the plane-parallel chamber was the cross-calibration process with the Farmer chamber. This is because cross-calibration introduces k Q uncertainties multiple times, whereas the procedure using a Farmer chamber directly does not. Moreover, the calibration coefficient determined through cross-calibration, depends on measurements that introduce additional errors. This leads to a combined standard uncertainty in the calibration coefficient together with k Q of 0.8% for the Farmer and 1.4% for the plane-parallel chamber. When all uncertainties are considered, the total combined standard uncertainty in the dose measurement was 1.1% and 1.5% for the Farmer and plane-parallel chamber respectively.