ESTRO 2025 - Abstract Book

S2736

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

[3] Sobotta, B., Söhn, M. and Alber, M., 2012. Accelerated evaluation of the robustness of treatment plans against geometric uncertainties by Gaussian processes. Physics in Medicine & Biology , 57 (23). [4] Schobi, R., Sudret, B. and Wiart, J., 2015. Polynomial-chaos-based Kriging. International Journal for Uncertainty Quantification , 5(2).

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Digital Poster Evaluation of lymphocyte sparing by optimizing VMAT plans for locally advanced NSCLC Takahiro Kanehira 1 , Hiroshi Taguchi 2 , Koichi Miyazaki 3 , Taisuke Takayanagi 3 , Norio Katoh 4 , Yusuke Uchinami 4 , Keiji Kobashi 5 , Takayuki Hashimoto 5 , Hidefumi Aoyama 4 1 Department of Medical Physics, Hokkaido University Hospital, Sapporo, Japan. 2 Department of Radiation Oncology, Hokkaido University Hospital, Sapporo, Japan. 3 Research and Development Group, Hitachi, Ltd., Ibaraki, Japan. 4 Department of Radiation Oncology, Hokkaido University Faculty of Medicine and Graduate School of Medicine, Sapporo, Japan. 5 Global Center for Biomedical Science and Engineering, Faculty of Medicine, Hokkaido University, Sapporo, Japan Purpose/Objective: Radiation-induced lymphopenia has been reported to negatively affect survival in locally advanced non-small cell lung cancer (LA-NSCLC) patients. This study evaluates lymphocyte sparing by optimizing volumetric-modulated arc therapy (VMAT) plans. Material/Methods: Ten LA-NSCLC patients treated with concurrent chemoradiotherapy (60 Gy/30 fractions) using 3DCRT+IMRT/VMAT or VMAT alone were included retrospectively. Clinical IMRT/VMAT plans were normalized to 60 Gy to the planning target volume (PTV) D 50 , where D x defined as a minimum dose received by x% of the volume. Two lymphocyte sparing VMAT plans (P-LS1 and P-LS2) were created: P-LS1 reduced heart and thoracic vertebral body (TVB) doses without compromising lungs and body doses from the clinical plan; P-LS2 further reduced mean doses to the lungs and body relative to P-LS1. Dosimetric parameters assessed included mean doses to the lungs, heart, and TVB; integrated total dose volume (ITDV, mean body dose times body volume); conformity index (CI, the ratio of the 80%- isodose volume to the PTV volume); heterogeneity index (HI, the difference between D 5 and D 95 of the PTV divided by the prescribed dose). Lymphocyte counts during radiotherapy were estimated using a dynamic depletion model developed for the thoracic region [1], based on a compartment model by Jin et al. [2], incorporating dose-volume histograms (DVH) parameters of lungs, heart, TVB, and body. demonstrating organ sparing in P-LS1 and P-LS2. Across the cohort, P-LS1 and P-LS2 achieved statistically significant median dose reductions compared to the clinical plan: mean lung dose decreased by 0.2 Gy and 1.4 Gy, mean heart dose by 2.2 Gy and 1.7 Gy, mean TVB dose by 4.1 Gy and 2.9 Gy, and ITDV by 9084 and 23953 Gy ∙ cc, respectively (p<0.05) (Table 1). CI and HI worsened, with median increases in CI by 0.075 (P-LS1, p>0.05) and 0.253 (P-LS2, p<0.05) and in HI by 0.006 (P-LS1, p<0.05) and 0.018 (P-LS2, p<0.05). The lymphocyte prediction model estimated dynamic lymphocyte counts during radiotherapy (Figure 1). The median pre-treatment lymphocyte count of 1567/μl (range: 689–2244) was estimated to decrease to 239/μl at the last fraction in clinical plans. P-LS1 and P-LS2 improved the lymphocyte counts by 35/μl and 75/μl (p<0.05), respectively, compared to the clinical plans. Results: All plans met our clinical dose criteria. Figure 1 shows dose distributions and DVH curves for a patient,