S1948
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Conclusion: Dose calculation directly on HS-CBCT was reliable and closely matched sCT-based dose calculation for both target and organ-at-risk doses. All differences were within clinically acceptable limits. Only D99% showed larger variation at the PTV edge, which should be interpreted with appropriate tolerance. These findings support the use of HS-CBCT for online adaptive radiotherapy in esophageal cancer, while confirmation in a larger patient cohort is warranted. Keywords: CBCT dose accuracy, adaptive RT, esophageal cancer Influence of monitor unit limits on Hyperarc SRS plan quality and dosimetry for multiple brain metastases. Helen Truong 1 , Helen Gustafsson 2 , Rachel Poldy 2 , Kimberley Legge 2 , Farhan Syed 1,3 1 Radiation Oncology, Canberra Health Services, Canberra, Australia. 2 Medical Physics and Radiation Engineering, Canberra Health Services, Canberra, Australia. 3 Irradiation Immunity Interaction Laboratory, John Curtin School of Medical Research, Canberra, Australia Purpose/Objective: Linear accelerator-based multiple target single- isocentre stereotactic radiosurgery (SRS) is Digital Poster 3773 increasingly being adopted in clinical practice due to its delivery efficiency, accessibility and comparable clinical outcomes to GammaKnife1. The associated volumetric modulated arc therapy (VMAT) plans are often highly modulated, which can affect dosimetry, deliverability and plan quality assurance (QA) pass rates2. The aim of this study was to investigate the effect of manipulating modulation on plan dosimetry in HyperArc SRS planning for multiple brain metastases. Material/Methods: Six previously treated HyperArc SRS cases were retrospectively replanned (Varian Eclipse, v16.0) using identical beam geometry and optimisation objectives, varying only the MU per Gy constraint (400–750 MU/Gy). Cases were grouped by planning target volume (PTV) count into cohorts with less than or equal to 10 PTVs (“Low PTV”) or more than 10 PTVs (“High PTV”). For each plan, we extracted target maximum dose (Dmax), RTOG conformity index (CI) and OAR doses.
clinical plan definition on both datasets (HS-CBCT and sCT). Evaluated metrics included target coverage (D0.1cm ³ , D99%, V95%) as well as heart and lung Dmean and volume-based metrics (heart V30Gy; lung V10Gy and V20Gy). Dose differences (HS-CBCT minus sCT) were summarized as mean ± standard deviation. Results:
Across all 22 fractions, HS-CBCT based dose calculation showed close agreement with the sCT (Table 1). For the planning target volume (PTV), the mean differences were Dmean of 1.0 ± 0.6%, D0.1 cm ³ of 0.7 ± 1.5%, and V95% of 2.7 ± 2.6%. D99% differed by 9.4 ± 2.4%, reflecting the higher sensitivity of near-minimum-dose regions to image density and scatter differences on CBCT. Figure 1 shows a representative fraction, illustrating that the dose differences were predominantly located in regions adjacent to the PTV boundary, particularly near the lung–diaphragm interface where motion and heterogeneous tissue composition influences CBCT HU accuracy. All HS-CBCT calculations achieved the PTV V95% ≥ 98% goal, whereas only 5 sCT-based calculations met this criterion. For organs of interest, heart Dmean and heart V30Gy differed by 1.0 ± 0.5% and 2.4 ± 1.8%, respectively. Lung Dmean differed by 0.6 ± 0.6%, with V10Gy and V20Gy differing by 0.9 ± 1.2% and 0.4 ± 5.8%. All OAR differences were within clinically acceptable limits and showed no systematic trend.
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