| Summary: | <strong>Purpose:</strong> Planning studies to compare x-ray and proton techniques and to select the most suitable technique for each patient are hampered by the non-equivalence of several aspects of treatment planning and delivery. A fair comparison should compare similarly advanced delivery techniques from current clinical practice, and also assess the robustness of each technique. This study therefore seeks to compare volumetric modulated arc therapy (VMAT) and single-field optimisation (SFO) spot scanning proton therapy plans created using with simultaneous integrated boost (SIB) for dose escalation in mid-oesophageal cancer, and to analyse the effect of set-up and range uncertainties on these plans. <strong>Methods and Materials:</strong> For 21 patients, SIB plans with a physical dose prescription of 2 Gy / 2.5 Gy per fraction (in 25 fractions) to PTV50Gy / PTV62.5Gy (primary tumour with 0.5cm margins) were created and evaluated for robustness to random set-up errors and proton range errors. Dose-volume metrics were compared for the optimal and uncertainty plans with p< 0.05 (Wilcoxon) taken as significant. <strong>Results:</strong> SFO reduced mean lung dose by 51.4% (range: 35.1 – 76.1%) and mean heart dose by 40.9% (15.0 – 57.4%) compared to VMAT. Proton plan robustness to 3.5% range error was acceptable: across all patients CTV D98 was 95.0-100.4% of prescribed dose (PD) and GTV D98 was 98.8-101%. Set-up error robustness was patient anatomy dependant and potential minimum dose per fraction was always lower with SFO than VMAT: CTV D98 was lower by 0.6-7.8% PD and GTV D98 lower by 0.3 - 2.2% of prescribed GTV dose. <strong>Conclusions:</strong> SFO plans achieve significant sparing of normal tissue compared to VMAT for mid-oesophageal cancer. Target dose coverage in SIB proton plans was less robust to random set-up errors, and may be unacceptable for certain patients. Robust optimisation to ensure adequate target coverage of SIB proton plans may be beneficial.
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