S1784
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
Conclusion: Variable-RBE calculations (McNamara) increased overall heart dose estimates – especially D1cm3(8.5±1.5Gy RBE for α / β =2Gy; 6.5±1.3Gy RBE for α / β =3Gy). Yet cardiac metrics remained within PROTECT thresholds and were stable over time, consistent with minimal CTVtotal change. References: [1] Hahn C et al. Acta Oncol. 2022;61(2):206-214. doi: 10.1080/0284186X.2021.1992007.[2] Mortensen HR et al. Radiother Oncol. 2024;190:109980. doi: 10.1016/j.radonc.2023.109980.[3] Gergelis KR et al. J Thorac Dis. 2020;12(11):7002-7010. doi: 10.21037/jtd- 2019-cptn-06.[4] McNamara AL et al. Phys Med Biol. 2015;60(21):8399-416. doi: 10.1088/0031- 9155/60/21/8399.[5] Hoffmann L et al. Radiother Oncol. 2022;172:32-41. doi: 10.1016/j.radonc.2022.04.029. Keywords: Oesophageal cancer, Proton therapy, LET Optimal proton slot allocation for lung cancer patients who present tumor shrinkage during treatment Nathan Torelli, Ylva Bornhauser, Jan Unkelbach Department of Radiation Oncology, University Hospital Zurich and University of Zurich, Zurich, Switzerland Purpose/Objective: Access to proton therapy remains limited in many countries, and patients are often referred for protons only when the expected reduction in normal tissue complication probability (NTCP) compared to photon therapy is clinically significant. However, such decisions are typically based on the initial planning CT and do not account for anatomical changes during treatment, such as tumor shrinkage, that may alter the relative benefit of protons over photons. This study Digital Poster Highlight 4896 investigated whether patients initially selected for proton therapy could safely transition to photon therapy following tumor regression, and how the released proton treatment resources could be optimally reallocated to maximize the overall population benefit. Material/Methods: Twenty lung cancer patients who underwent adaptive radiotherapy due to tumor shrinkage were retrospectively analyzed. Each patient had at least two planning CT scans acquired at different time points. Intensity-modulated proton therapy (IMPT) and photon therapy (IMRT) plans were generated for each scan. The clinical benefit of protons over photons was quantified as the reduction in NTCP for radiation pneumonitis ( Δ NTCP, updated Appelt model [1]). Patients were assumed to be initially referred for
8.5±1.5Gy RBE and 6.5±1.3Gy RBE, respectively. If substantial tumour shrinkage had occurred, increasing McNamara doses would be expected as the heart migrated toward the distal edge. However, Figure 2 shows no such increase. This aligns with volumetrics: the total clinical target volume (CTVtotal) deviated on average 1.0±7.4% during treatment, indicating no shrinkage.
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