S1918
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
integration. This study evaluates whether DECT-based reconstructions can reproduce SECT-based dose distributions in photon radiotherapy of H&N cancer. Two approaches were assessed: (1) Virtual Monoenergetic Imaging (VMIopt) at 72 keV, optimized to match the SECT 120 kVp calibration curve, and (2) Rho reconstruction, directly mapping electron density. Material/Methods: Retrospective imaging data of 9 patients with histologically confirmed H&N carcinomas (soft palate, hypopharynx, floor of mouth, tongue, base of tongue, oropharynx, nasopharynx, oral cavity) were analyzed. Each underwent a CT session including contrast- enhanced SECT and DECT scans using a Somatom go.OpenPro CT scanner (Siemens Healthineers, Erlangen, Germany). All patients received primary radiochemotherapy with 33 × (1.7/2.0/2.1) Gy D95% prescribed to nested target volumes (PTVlow, PTVmed, PTVhigh). Treatment planning and dose calculation were performed in Eclipse TPS (v18.0, Varian Medical Systems, Palo Alto, CA, USA) using VMAT with 6 MV photons and multiple arcs. Dose–volume histogram (DVH) parameters (D95%, D98%, D2%, MinD1cc) were compared between SECT Qr40 as reference and DECT- based (VMIopt and Rho) dose distributions. To assess spatial dose agreement, voxel-based gamma analysis was performed using global 2%/2 mm and 1%/1 mm criteria. Results: The 72 keV VMIopt and Rho reconstruction showed excellent agreement with SECT-based dose distributions (Figure 1). Mean deviations for all investigated DVH parameter were below 0.27% or 0.29% across all PTVs, with maximum deviations below 0.60% or 0.63% for VMIopt and Rho, respectively. The narrow interquartile ranges and minimal outliers underline the high consistency and reproducibility of DECT-based dose calculations relative to the SECT reference. Gamma analysis further confirmed the high accuracy, with VMIopt achieving mean pass rates of (99.86 ± 0.05)% and (98.78 ± 0.58)% and Rho achieving (99.81 ± 0.13)% and (98.16 ± 0.85)% for the 2%/2mm and 1%/1 mm criteria, respectively, demonstrating near-perfect dose conformity to the SECT reference.
Conclusion: DECT-based dose calculation using VMI 72 keV or Rho reconstructions achieves high accuracy in photon radiotherapy of H&N cancer. VMIopt enables seamless TPS integration, while Rho allows direct electron density mapping. DECT thus combines superior tissue contrast with precise dose computation, supporting its clinical implementation without additional radiation exposure. References: [1] E.D. Roele et al., Dual-Energy CT in Head and Neck Imaging. Curr Radiol Rep 5, 19 (2017)[2] V.T. Taasti et al., Comparison of single and dual energy CT for stopping power determination in proton therapy of head and neck cancer, Phys Imaging Radiat Oncol 22, 6 (2018) Keywords: Dual-Energy CT, Dose calculation accuracy, H&N RT Poster Discussion 3157 Dosimetric Comparison of IMRT and VMAT on Circulating Immune Cell Depletion in Glioblastoma Radiotherapy Abdelkhalek Hammi 1 , Nadya Nadya Shusharina 2 , Eleni Gkika 3 , Youness Nour 3 1 Physics, TU Dortmund University, Dortmund, Germany. 2 Radiation Oncology, Massachusetts General Hospital, Boston, USA. 3 Radiation Oncology, University Hospital Bonn, Bonn, Germany Purpose/Objective: Radiation-induced lymphopenia (RiL) is associated with worse survival in glioblastoma (GBM), by impairing anti-tumor immunity [1-2]. Reducing dose exposure to circulating lymphocytes (CLs) is therefore a potential strategy to mitigate RiL. Modern treatment techniques, such IMRT and VMAT, differ in low-dose bath, delivery time, which may result in different blood exposure and CL’s depletion. We compared IMRT and VMAT for their dosimetric impact on CLs and the resulting risk of
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