ESTRO 2025 - Abstract Book

S2796

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

Purpose/Objective: The Linac-based single-isocenter technique (SIT) for stereotactic radiosurgery (SRS) of multiple brain metastases is a widely accepted treatment approach. Automated targeting and planning with specialized software enhances planning consistency and robustness. However, challenges can arise depending on the proximity of the metastases to the isocenter, the distance between metastases, or irregular contours. Therefore, this study aims to compare the dosimetric outcomes of the SIT and dual-isocenter technique (DIT) for treating multiple brain metastases in need of SRS. Material/Methods: Fifteen patients, each with 4-14 metastases (total of 97 metastases), who were treated with a single-isocenter technique (SIT) and prescribed a dose of 20 Gy, were retrospectively selected. For each patient, lesions were grouped into two clusters based on their locations and were replanned using two isocenters, one for each cluster. Elements Multiple Brain Mets v4.0 (Brainlab, Munich, Germany) protocols were pre-defined, including isocenter placement decisions and automatic geometric optimization, which involved 4-7 table rotations, 2-4 arcs per lesion, and gantry and collimator rotations. The following dosimetric parameters were evaluated: prescription dose covering 99% of the lesions, Paddick Conformity Index (PCI), mean (Dmean) and maximum (Dmax) doses to the lesions, and the volume of brain receiving 12Gy, 10Gy, 5Gy, and 3Gy (V12, V10, V5, V3, respectively). The dose to surrounding organs was limited to their maximum allowable doses. Wilcoxon signed-rank tests were used to assess statistically significant differences, with an alpha significance level set at 0.01 to correct for multiple comparisons. Median values were used to evaluate the differences. Results: When comparing the DIT to the SIT, we observed a statistically significant improvement in the PCI (0.73 vs. 0.69; p<0.001), along with a reduction in both maximum and mean doses to the lesions (26.4Gy vs. 26.8Gy for Dmax, and 23.7Gy vs. 24.1Gy for Dmean; p<0.001). The DIT also resulted in a decrease in the volume of brain tissue irradiated at V12 (-0.25cc), V10 (-0.82cc), V5 (-0.39cc), and V3 (-21.65cc) compared to SIT. Of these, only the difference in V3 was statistically significant (p=0.007). Conclusion: The dual-isocenter approach demonstrated significant improvements in both conformity and dosimetric outcomes, with enhanced normal tissue sparing in the treatment of multiple brain metastases. These advantages over the SIT can be attributed to the positioning of lesions farther from the isocenter, which are typically under thicker multi-leaf collimators in SIT. This setup compromises conformity, limiting the precision of dose delivery to the target.

Keywords: multiple brainmets; single/dual iso, dosimetry

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Digital Poster SRS for patients with benign disease: can extracranial dose be reduced whilst maintaining acceptable plan

quality with multi-arc non-coplanar VMAT? Esmé Votta 1,2,3 , David Eaton 3 , Christopher Dean 1

1 Radiotherapy Physics, Barts Health NHS Trust, London, United Kingdom. 2 Radiation Physics and Radiobiology, Imperial College Healthcare NHS Trust, London, United Kingdom. 3 Medical Engineering and Physics, King's College London, London, United Kingdom Purpose/Objective: At this institution, SRS patients are treated with HyperArc, Varian’s SRS solution which includes the use of non coplanar VMAT arcs with flattening-filter-free MV beams. This study investigates plan parameter selection for HyperArc planning with the objective of minimising extracranial absorbed dose whilst maintaining acceptable plan