ESTRO 2025 - Abstract Book

S2697

Physics - Dose calculation algorithms

ESTRO 2025

Purpose/Objective: The output of very small fields generated by static MLC is challenging for both measurement and calculation. Regarding measurements, the TRS-483 Code of Practice (CoP) recommends small field output factors (FOF) measurement as the ratio of the test and reference field size readings, multiplied by the Output Correction Factor (OCF) tabulated as a function of the detector and the equivalent square field size (EQSQF). Critical is the situation of rectangular fields, where the equivalent area proposed for EQSQF does not hold. Concerning calculation, in particular with Acuros (Eclipse, Varian), the dosimetric leaf gap DLG and effective spot size ESS parameters have to be properly estimated and tuned to improve MU calculation accuracy. From version 18 of Acuros, the DLG is modelled according to the Enhanced Leaf Modeling ELM. This work aims to evaluate the accuracy of the Acuros MU calculation in the condition of very small MLC-shaped fields. Material/Methods: FOF were obtained using a Varian TrueBeam equipped with an HD-MLC, for 6 and 10 MV beams both with (X) and without flattening filter (FFF). Symmetric square and rectangular fields, from 0.5 cm to 4 cm were MLC-shaped, with the jaws set to 4.4x4.4 cm². Geometric conditions: 10 cm depth, 90 cm SSD. Three detectors were used: microDiamond, DiodeE, and PinPoint3D. The measured output ratios were corrected using the OCF from CoP Tables. However, instead of the equivalent area, the EQSQF was estimated with a modified Sterling formula, which gave consistent FOF also for elongated fields. Acuros (v.18) was configured with the ELM following the vendor procedure. ESS were varied to find the best calculations/measurements agreement, in the range of 0.5-0.8 mm for X and Y. Results: DLG computed in the ELM process were: -0.011, -0.019, -0.009, and -0.007 mm for 6X, 6FFF, 10X, and 10FFF, respectively. The ESS parameters value giving the best calculations/measurements agreement, for all the beam qualities, was 0.7 mm in both X and Y directions (collimations). Here is the table with the percentage difference between calculated and measured FOF for the 6X beam quality.

FOF differences of less than 1% were found also for the other beam qualities.

Conclusion: Acuros with ELM modelling for DLG estimation, and an ESS setting of 0.7 mm can result in MU accuracy within 1% for MLC-shaped fields as small as 0.5x0.5 cm 2 .

Keywords: Acuros, Elongated small fields, DLG and ESS