S1839
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Results: Six-field plans were generated with different rod/spot sizes, slit aperture and distances. A 21 Gy single fraction dose was prescribed (in absence of consensus) as the median dose to a circular volume matching the slit aperture diameter. Among other parameters, target volumes receiving >90% dose levels V(>90%) and receiving <30% (V(<30%)) were evaluated. The 30% dose level aimed for a peak-to-valley dose ratio (PVDR) near 3 or less. These metrics describe volumes of immune-suppressing high dose and the immune-stimulating low dose. Figure shows dose distribution for 5 mm slits, 2 cm spacing (6MV flat beam, HD-MLC).
Results: Measurements of the four 1x1 cm2 field settings were within ±0.1%, as expected. Acuros calculations, influenced by the different effective field size and CBSFs, resulted in dose overestimations up to 8% in the worst case with version 16.1. The Table summarises the discrepancies between calculated and measured doses.
Conclusion: The CBSF implementation in Eclipse for jawless linacs might lead to dose overestimations, especially in cases involving unintended MLC apertures far from the central axis. Keywords: Halcyon, Acuros, collimator backscatter factor
Digital Poster 1494 Static MLC-based patterns for SFRT-GRID
Luca Cozzi 1 , Antonella Stravato 2,3 , Antonella Fogliata 1 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 Purpose/Objective: Spatially Fractionated Radiotherapy (SFRT) has gained interest for its immune-modulation effects, which enhance radiotherapy efficacy with low toxicity. Among SFRT methods, GRID therapy uses a fluence grid pattern (lines or spots) created with cerrobend masks or standard MLC. Typical GRID apertures are 1- 2 cm, while apertures smaller than 1 mm belong to mini-beam therapy, where the standard dosimetry fails. This work proposes different GRID patterns for implementation on standard clinical linacs and MLCs and evaluates their dosimetric parameters. Material/Methods: GRID patterns were generated in the Varian Eclipse treatment planning system for a Varian TrueBeam with Millennium or HD-MLC to cover a simple spherical target shape. Static slits with defined apertures and spacing (centre-to-centre distance) were delivered, varying aperture size and distance. Up to six equispaced fields arranged hexagonally generated the plans. Plans were assessed either as high-dose rods (along the slit length) or high-dose spots by combining two fields at collimator angles of 0 and 90 degrees. Calculations used Acuros v.18.1 on a virtual phantom with 1.25 mm grid resolution.
Target dose inhomogeneity is shown in the DVHs. V(>90%) and V(<30%) were about 20% and 0%, 10% and 5%, 8% and 15% for slit/distance pairs of 5/10mm, 10/20mm, 15/30mm, respectively. With larger distances, V(>90%) and V(<30%) were about 5% and 8%, 3% and 40%, 8% and 20% for slit/distance combinations of 5/20mm, 10/30mm, 15/40mm, respectively. Those values were not substantially different between spots and rods setting. These results highlight the need to understand biologically optimal high- versus low-dose volume ratios when selecting aperture and distance.
Perliminar EPID-based plan deliveries showed good
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