S1639
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
Material/Methods: FOF measurements were measured with
microDiamond and DiodeE detectors (PTW) at the isocentre, 10 cm depth, for 6 and 10 MV beams, flat (X) and FFF, from two Varian TrueBeams. The “SmallField” dataset included 64 square and rectangular fields shaped by the jaws (MLC retracted), with sides from 0.6 to 4.0 cm. The “ElongatedField” dataset contained fields with one dimension from 0.8 to 3.0 cm and the other from 10 to 40 cm. The Output Correction Factors followed TRS-483 [2], estimating the ESF with the formula from [1] (F1) in place of the equivalent area method (F2). Data were normalised to 4x4 cm2 as an intermediate field. Results: For the “SmallField” dataset, the ESF with F1 proved valid for jaw-defined small fields up to 4x4 cm2, as shown in the Figure, where the 6X FOF is plotted against ESF for all field sizes. The plot on the left uses ESF from F1, producing a smooth, unique FOF curve; the plot on the right, using the F2 equivalent area method, shows its limitations for non-square fields. In all the plots, the square fields, from 0.5x0.5 to 4x4 cm2, are reported in red symbols.
Conclusion: The ESF formula F1 is valid for fields up to 4x4 cm2, including jaw-collimated geometries. However, for larger fields (>4x4 cm2), it requires correction. References: [1] Fogliata A et al. Small elongated MLC fields: novel equivalent square field formula and output factors. Med Phys 2025;52:5032-5038[2] TRS-483. Dosimetry of small static fields used in external beam radiotherapy. IAEA, 2017 Keywords: small fields, output factor, jaw-collimation Mini-Oral 1586 Experimental comparison of magnetic field correction factors for Advanced Markus chamber and microDiamond detector in MR-integrated proton therapy Krishna Godino Padre 1,2 , Aswin Hoffmann 1,3 , Hermann Fuchs 4 , Felix Horst 1,5 1 OncoRay National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany. 2 National Center for Tumor Diseases (NCT), NCT/UCC Dresden, a partnership between DKFZ, Faculty of Medicine and University Hospital Carl Gustav Carus, TUD Dresden University of Technology, and Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Dresden, Germany. 3 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany. 4 Division of Medical Physics, Department of Radiation Oncology, Medical University of Vienna, Vienna, Austria. 5 Institute of Radiooncology-OncoRay, Helmholtz-Zentrum Dresden- Rossendorf, Dresden, Germany Purpose/Objective: Accurate dosimetry in magnetic fields (MFs) requires correction factors to account for MF-induced dose perturbations. Although detector responses in proton therapy have been widely studied [1–4],
In the “ElongatedField” dataset, FOFs do not follow a smooth curve, indicating that the ESF formula F1 requires modification for fields larger than 4x4 cm2.
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