ESTRO 2025 - Abstract Book

S2572

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

Conclusion: This investigation quantifies sensitivity of FOF as a function of field size, provides explanations and shed lights on the uncertainties of experimental measured in FOF especially for small fields (≤15x15 mm 2 ). The Monte Carlo data can significantly improve the accuracy in clinical practices, where measurements have large uncertainty.

Keywords: Dosimetry, small field, output factors

References: [1] Das IJ, Ding GX, Ahnesjo A. Small fields: Nonequilibrium radiation dosimetry. Medical physics. 2008;35(1):206 215. [2] Rogers DWO, Faddegon BA, Ding GX, Ma CM, We J, Mackie TR. BEAM: a Monte Carlo code to simulate radiotherapy treatment units. Medical physics. 1995;22(5):503-524. [3] Ding GX. Energy spectra, angular spread, fluence profiles and dose distributions of 6 and 18 MV photon beams: results of monte carlo simulations for a varian 2100EX accelerator. Physics in medicine and biology. 2002;47(7):1025-1046. [4] Constantin M, Perl J, LoSasso T, et al. Modeling the truebeam linac using a CAD to Geant4 geometry implementation: dose and IAEA-compliant phase space calculations. Medical physics. 2011;38(7):4018-4024.

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Digital Poster Experimental determination of beam quality correction factors for various ionization chambers in high energy photon beams using water calorimetry Lies Verpoest 1 , Séverine Rossomme 2 , Kevin Souris 2 , Norman Durny 3 , John Lee 1 1 MIRO, UCLouvain, Brussels, Belgium. 2 IBA Dosimetry, Ion Beams Applications S.A, Louvain-La-Neuve, Belgium. 3 IBA Dosimetry, Ion Beams Applications S.A, Schwarzenbruck, Germany Purpose/Objective: Based on international codes of practice such as TRS-398 [1], ionization chambers (ICs) are typically used to determine absorbed dose-to-water. When the user’s beam quality (Q) differs from the reference beam quality (Q 0 ), a beam quality correction factor (k Q ) is needed to correct for this difference in beam quality. Although k Q -factors are provided for several chambers in dosimetry protocols, data is limited for some other chambers. This study aims to experimentally determine k Q -factors for various ICs using water calorimetry in high energy photon beams. Material/Methods: Measurements were performed at IBA Dosimetry (Germany) using five high energy photon beams with beam qualities TPR 20,10 ranging between 0.680 and 0.764 at a water equivalent depth of 10 g/cm 2 . k Q -factors were derived by directly comparing calorimetry and ionometry results in terms of absorbed dose-to water. The calorimeter output was corrected for conductive heat transfer and perturbation of the beam through finite element and Monte Carlo simulations, respectively. Four types of cylindrical ICs were used: IBA RAZOR TM Nano (2), CC01 (2), CC04 (2) and CC08 (1). Two types of parallel plate ICs were used: IBA PPC40 (3) and PPC05 (1). The IC readings were corrected for temperature and pressure, polarity, recombination and volume averaging effects. Results: The obtained k Q -factors for the cylindrical ICs are presented in Figure 1, while those for the parallel plate ICs are shown in Figure 2. The results are compared to values found in literature obtained through Monte Carlo simulations or measurements and show overall good agreement. However, for the IBA CC01 chamber, differences of up to 3%