S1838
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
calculated from the jaw setting. Conversely, on Halcyon units (jawless), it is determined from an effective field size. A matrix f(x,y) is generated to represent the field opening: pixels with a value of 1 are in the open area, while 0 are in the MLC-closed region. The effective field size is derived, similar to optical physics, as the size of a light field with intensity f(x,y). It represents the beam’s effective dimension within the MLC opening and serves as the input for the CBSF calculation. To test the implementation of the effective field size for CBSF in MU calculation, a small 1x1 cm2 field centred on the beam axis was created, being the smallest field size in the CBSF pre-configured table. The field was set in 4 different conditions, as shown in the Figure:
calculation. Beam penumbra was underestimated by v.16.1, whereas differences were minimal in v.18.x. None of the versions distinguished penumbrae from distal or proximal MLC layers. Field sizes calculated in the Y direction (perpendicular to leaf motion) generally matched measured values (except for 0.5x0.5 cm2 in v.16.1). However, in the X direction, v.16.1 overestimated field size for fields larger than 2x2 cm2.Clinical cases analysis indicated that approximately 50% of VMAT control points involved apertures smaller than 1x1 cm2, suggesting a potential systematic dose error with v.16.1. Conclusion: Small fields Acuros calculations for Halcyon are accurate in version 18.x. Caution is advised when using version 16.1, especially for small lesions, due to its propensity to overestimate dose in very small fields. References: [1] TRS-483. Dosimetry of small static fields used in external beam radiotherapy. IAEA, 2017[2] Fogliata A et al. Small elongated MLC fields: novel equivalent square field formula and output factors. Med Phys 2025;52:5032-5038 Keywords: Acuros, small field, Halcyon Collimator backscatter factor for jawless linac Antonella Fogliata 1 , Antonella Stravato 2,3 , Francesca Dusi 4 , Sebastiano Menna 5 , Elisa Pilloni 5 , Flavio V Quaranta 5 , Luca Vellini 5 , Luca Cozzi 1 , Davide Cusumano 5 , Marco Fusella 6 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 University, Rome, Italy. 4 Med, Abano Terme Hospital, Abano Terme, Italy. 5 Medical Physics, Mater Olbia Hospital, Olbia, Italy. 6 Medical Physics, Abano Terme Hospital, Abano Terme, Italy Purpose/Objective: Along a primary photon beam of a linac, the collimating device produces backscatter radiation (CBS). The monitor chamber, positioned upstream and close to the collimation system, measures this backscatter radiation, including this secondary component in the MU, which varies with the field size. The Acuros algorithm (Varian Eclipse) estimates the CBS Factor (CBSF) based on measured output factors. The aim of this work is to evaluate the impact of the CBSF from Varian Halcyon unit heads (jawless), using the pre-configured Acuros algorithm. Digital Poster 1491
Case a): the standard fieldCases b) to d): small leaf openings at the beam edges, involving the last one or two extreme leaves, with an expected negligible dosimetric impact on the central axis. The effective field sizes for CBSF ranged from 0.9x1 cm2 (case a)) to 4x4 cm2 (case d)). Dose at 10 cm depth at the isocentre was measured for all cases. The same fields were calculated with Acuros versions 16.1, 18.0, and 18.1, and the results were compared with measurements.
Material/Methods: In the Eclipse system, for C-arm linacs, the CBSF is
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