ESTRO 2025 - Abstract Book

S2606

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

Purpose/Objective: The TRS-483 Code of Practice (CoP) recommends the procedure to measure small field output factors (FOF) as the ratio of the test and reference field size readings, multiplied by the Output Correction Factor (OCF). OCF is tabulated as a function of the detector and the equivalent square field size, S. S, in the CoP, is defined as the equivalent field area. Field dimensions are dosimetric and determined as FWHM of the profile. From the CoP, S as defined is valid only for the X/Y ratio in the interval (0.7,1.4), hence unsuitable for an elongated field. This work aims to define a new equivalent square field formula, validated for small elongated fields shaped by Varian MLC mounted on C-arm linacs. Material/Methods: FOFs were obtained using two Varian TrueBeam, equipped with Millennium MLC and an HD-MLC. Symmetric square and rectangular fields, ranging from 0.5 cm to 4 cm were MLC-shaped, with the jaws set to 4.4x4.4 cm². The measuring point was located at the isocenter, 10 cm depth. Three detectors were used: microDiamond, DiodeE, and PinPoint3D, on 6 and 10 MV beams, both with and without flattening filter. Consistently with the CoP, the measured output ratios were corrected using the OCF from CoP Tables, using two equivalent square field approaches: - equivalent area from CoP, S=√(X·Y) - new proposal: EqSqFS=[2·min(X,Y) a ·max(X,Y) 2-a ]/(X+Y) , with a empirically set to 1.12. OCFs were obtained using the two approaches respectively. The dosimetric field size for each setting was determined as FWHM from profiles acquired with the microDiamond. FOF were displayed in a single plot including all the measured elongated fields, as a function of the specific equivalent square field size. Results: The measured dosimetric field size was systematically smaller than the geometrical setting in the direction perpendicular to the leaf motion, while it was systematically larger in the direction parallel to the leaf motion. The following plots show the results of the FOF by using the two methods, for the 6 MV with flattening filter beam quality, HD-MLC.

The proposed EqSqFS effectively addresses the inadequacy of the equivalent area method of determining equivalent square field size, resulting in a continuous and fluent OF pattern.

Conclusion: A new approach for equivalent square field size estimation is introduced, which is valid for elongated fields as well. Results are confirmed for MLC-shaped fields from Varian TrueBeam linacs.

Keywords: Small elongated fields, equivalent square fields

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Digital Poster Measurement of key temporal parameters in Hitachi’s dose-driven continuous scanning proton therapy system Clifford Ghee Ann Chua 1,2 , Kang Hao Lee 1 , Kah Seng Lew 1 , Calvin Wei Yang Koh 1 , Andrew Wibawa 1 , Zubin Master 1 , Wen Siang Lew 2 , James Cheow Lei Lee 1,2 , Sung Yong Park 1,3 , Hong Qi Tan 1,2