S1837
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Digital Poster 1490 Dosimetric accuracy of Acuros in Halcyon small field calculations Antonella Fogliata 1 , Antonella Stravato 2,3 , Francesca Dusi 4 , Sebastiano Menna 5 , Elisa Pilloni 5 , Flavio V Quaranta 5 , Luca Vellini 5 , Davide Cusumano 5 , Marco Fusella 4 1 Radiotherapy and Radiosurgery, Humanitas Research Hospital IRCCS, Milan-Rozzano, Italy. 2 Medical Physics, A.O. San Giovanni-Addolorata, Rome, Italy. 3 Medical University, UniCamillus, Rome, Italy. 4 Radiation Oncology, Abano Terme Hospital, Abano Terme, Italy. 5 Medical Physics, Mater Olbia Hospital, Olbia, Italy Purpose/Objective: The accuracy of dose calculation in small fields is challenging and requires detailed evaluation. For many linacs and dose calculation algorithms, fine- tuning is possible to optimise agreement between dose measurements and calculations, especially for small fields and stereotactic treatments. However, Varian Halcyon linacs utilise pre-configured, non- tunable Acuros algorithms. This study aims to assess the accuracy of Acuros small-field dose calculations on a Halcyon unit across three different algorithm versions. Material/Methods: A set of 49 square and rectangular fields (ranging from 0.5x0.5 to 4x4 cm2) was measured with microDiamond and microSilicon detectors (PTW, Germany) at 10 cm depth in water, at the isocentre, including field output factors (FOF) and profiles. Measurements followed TRS-483 Code of Practice [1] and the equivalent square field formula was adopted for small rectangular fields [2]. The same fields were calculated in a virtual water phantom with Acuros (Eclipse, Varian, USA) at a 1 mm grid resolution. Three Acuros versions were tested: 16.1 (Dosimetric Leaf Gap model, DLG), 18.0, and 18.1 (both with Enhanced Leaf Modelling, ELM). Other configuration parameters, such as effective spot size and collimator backscatter, differed across algorithm versions. FOFs and profiles were compared between measurements and calculations. Additionally, five clinical brain SRT cases were analysed across these versions. Results: Version 18.0 and 18.1 showed good agreement with FOF measurements, with mean±standard deviation (range) were 0.2±0.3% (-1.1%, +2.2%) and - 0.2±0.3% (-2.7%, +2.5%), respectively. A dose overestimation was observed with version 16.1, averaging +3.2±3.4% (-2.0%,+27.2%). The largest discrepancy (27%) occurred for the 0.5x0.5 cm2 field. The main difference between v.16.1 and v.18.x is the leaf modelling; the latter employs ELM, which significantly improves the Monitor Units
Conclusion: The clinical goal achievement rates of RAD were comparable to those of VMAT; moreover, RAD maintained target coverage with a shorter optimization time while significantly reducing doses to the larynx and oral cavity compared with VMAT. References: [1]Clark R, Magliari A, Rosa L, Li T, Beriwal S, Cozzi L. Comparison of Advanced Dynamic Arc Therapy With Collimator Rotation and Fixed Integrated Gantry
Positions to the Standard of Care Across Five Treatment Sites. Cureus 2025;17:e86280.
https://doi.org/10.7759/cureus.86280.[2]Chung CV, Nair SS, Khan MS, Nguyen CI, Martin-Paulpeter RM, Ludmir EB, et al. Novel volumetric modulated arc therapy approach for lattice radiation therapy for bulky liver tumors. Frontiers in Oncology 2025;15. https://doi.org/10.3389/fonc.2025.1680342. Keywords: RapidArc Dynamic, Nasopharyngeal cancer
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