S1810
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
expanding schwannoma after radiotherapy. Discov Onc 15, 79 (2024). https://doi.org/10.1007/s12672-024- 00936-y Keywords: Acoustic neuroma, Radiosurgery, Cyberknife
optimized using 5 concentric rings (3-8-15-20-28 mm) around the GTV as shown in Fig1.
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The treatment plan optimization was particularly focused on minimizing cochlear dose according to the ALARA principle to reduce auditory toxicity. Results: The prescribed dose covered a median 99.7% [97.2– 100] of the GTV volume. Median conformity index was 1.39 [1.10–2.30], and homogeneity index was 1.25 [1.12–1.30]. The obtained cochlear dose values are reported in Tab1. Treatment was well tolerated: 46% of patients reported only mild, transient symptoms (e.g., dizziness or fluctuating hearing), and no cranial nerve VII or V deficits were observed.Post-treatment MRI imaging showed a transient tumor volume increase in the single-fraction group (mean +45.1% ± 10.9%), followed by volume reduction - likely reflecting early inflammation and delayed regression. Among patients treated with fractionated SRS, early pseudoprogression occurred in 67% at 3 months, followed by tumor shrinkage in 53% at 9 months. Seventeen patients experienced temporary enlargement with symptoms, managed successfully with short corticosteroid courses. Conclusion: CyberKnife SRS, delivered in single or three fractions, demonstrated excellent dose conformity, minimal toxicity, and promising early tumor control in patients with small-to-medium ANs. Both regimens were well tolerated, with no severe adverse events. Particular attention was devoted to cochlear dose sparing while maintaining adequate target coverage and overall treatment effectiveness. References: Govindaraj R, Khong J, Byrne A, Zacest A, Roos D. The Effect of Cochlear Dose on Hearing Preservation After Low-Dose Stereotactic Radiosurgery for Vestibular Schwannomas: A Systematic Review. Adv Radiat Oncol. 2022 Aug 28;7(6):101059. doi: 10.1016/j.adro.2022.101059. PMID: 36420205; PMCID: PMC9677188.Timmerman R. A Story of Hypofractionation and the Table on the Wall. Int J Radiat Oncol Biol Phys. 2022 Jan 1;112(1):4-21. doi: 10.1016/j.ijrobp.2021.09.027. PMID: 34919882.Yamazaki, M., Takamatsu, S., Iwata, Y. et al. Notch appearance as a novel radiological predictor of transient expansion and good outcome of
Target-based dosimetric comparison of coplanar optimisation strategies for brain SRT on C-arm and ring-gantry systems Mark Ashburner, Arun Gandhi, Omer Ali Medical Physics, Te Whatu Ora Waikato, Hamilton, New Zealand Purpose/Objective: o evaluate and compare coplanar stereotactic radiotherapy (SRT) planning strategies between conventional C-arm and ring-gantry delivery systems. The study investigated different optimisation methods available on the Eclipse treatment planning system to assess viability of achieving target conformity and dose gradients comparable to standard non-coplanar HyperArc (HA) plans for multiple brain metastases to determine whether clinically acceptable alternatives to HyperArc can be achieved, to expand the system’s utility where dedicated c-arm delivery systems are unavailable. Material/Methods: Ten patients previously treated with multi-target brain SRT using HA on a TrueBeam linear accelerator were retrospectively selected. For each case (2–6 intracranial targets), plans were re-optimised for a Halcyon ring delivery system using identical dose prescriptions, structures, and constraints. Two coplanar optimisation methods were evaluated, manual Normal Tissue Optimisation (NTO) and Automatic NTOAll plans were generated with four full coplanar arcs and calculated using the AcurosXB algorithm (1.25mm optimisation grid, 1mm dose grid). Plan quality was assessed using the Paddick Conformity Index (PCI), Homogeneity Index (HI), Gradient Index (GI). Comparisons were performed per- target and per-plan. Statistical analysis included the Shapiro–Wilk test for normality, paired t-tests or Wilcoxon signed-rank tests, and Spearman/Pearson correlations to assess linear and non-linear trends with plan complexity. Results: Both optimisation strategies achieved clinically acceptable plans for all patients. Target conformity (PCI) was comparable between Auto- and Manual-NTO methods and the HyperArc reference (mean deviation <2\%). Variability was greater in small targets show in in Figures 1 and 2. Differences in dose homogeneity (HI) were minimal (mean HI <0.03), indicating
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