ESTRO 2023 - Abstract Book

S1728

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ESTRO 2023

Figure 2. For 20 patients, (nominal/worst-case scenarios): (a) and (b) Difference in D mean/V95% for iCTV. (c) Difference in integral dose to the full body volume (d) Difference in relevant metrics OARs. Target coverage was equivalent for both modalities (no significant difference, p > 0.05, for V95% and D mean), considering nominal and worst-case scenarios (Figure 1a,1b). However, arcPT managed to slightly decrease the integral body dose (median difference equal to 5Gy, p = 0.015) and the dose to some relevant OARs in the nominal case, like lungs (median Δ Dmean = 0.45 Gy, p= 0.002) and heart (median Δ Dmean = 0.8 Gy, p = 1.9e-06). The largest improvement was seen in the spinal canal, where arcPT plans decreased median D0.05cc by 4Gy. Similar trends are observed for the worst case scenario (Heart Δ Dmean = 1.2 Gy, p =1.9e-06, Lungs Δ Dmean = 0.23 Gy, p = 0.02), indicating that the plans are robust against setup/range errors. For arcPT, the worst-case median body D1 is about 1Gy larger, which might be a sign of lower robustness in terms of hot spots. Regarding Δ NTCP (IMPT-arcPT) for lung toxicity, arcPT showed an average decrease of (0.7 ± 1) % [range –1.8 to 2.6%]. Conclusion ArcPT can preserve and slightly improve IMPT benefits for esophageal cancer in terms of target coverage and OAR sparing, while potentially decreasing lung toxicity. Further investigation is needed to assess potential advantages in treatment time. (1) Engwall et al. 2022 (2) Thomas et. al 2019 M. Schafasand 1,2 , A.F. Resch 1 , E. Traneus 3 , L. Glimelius 3 , A. Nachankar 4 , M. Stock 1,5 , A. Carlino 1 , J. Gora 1 , D. Georg 2 , P. Fossati 4,5 1 MedAustron Ion Therapy Center, Medical Physics, Wiener Neustadt, Austria; 2 Medical University of Vienna, Radiation Oncology, Vienna, Austria; 3 RaySearch Laboratories, Physics, Stockholm, Sweden; 4 MedAustron Ion Therapy Center, Radiation Oncology, Wiener Neustadt, Austria; 5 Karl Landsteiner University of Health Sciences, Oncology, Krems an der Donau, Austria Purpose or Objective The treatment of radioresistant tumors is a major indication for carbon ion beam radiotherapy (CIRT) due to the high ionization density, often quantified by the linear energy transfer (LET). Recently it was observed that the clinical outcome declined with increasing tumor volume; presumably caused by reduced high LET components in the mixed radiation field with increasing volume. The purpose of this study was to identify a single value predictor for mixed radiation fields in CIRT by investigating small and large volumes. Materials and Methods Ten single-phase or sequential boost CIRT treatment plans for different tumor entities were selected. The total relative biological effectiveness (RBE)-weighted dose (computed with the local effect model I (LEMI)) was 73.6 Gy(RB|LEMI) or 76.8 Gy(RBE|LEMI) in 16 fractions. The plans were grouped based on the volume of the low dose target (VlD). Targets with a VlD<500 cc were considered as small (n=5) and those with VlD ≥ 500 cc as large. The evaluation was performed for the high dose volume (VhD; i.e. the volume that received the total dose). For the single-phase cases, no differentiation between low dose and high dose target was done. For each plan, the high-LET-dose (hLD, which is the physical dose filtered based on LET) was scored over 18 different LET thresholds in a research version of the treatment planning system RayStation (RaySearch Laboratories, Sweden). For each plan a voxel-based evaluation of the fraction of hLD to physical dose (hLDf) was performed as a function of LET threshold to find an appropriate LET threshold for the whole study. For the selected LET threshold, the hLDf volume histogram of each plan was extracted for its VhD and compared between the two cohorts by performing a t - or U -test (depending on the distribution of the data points). PO-1964 Towards a novel treatment plan evaluation concept in carbon ion radiotherapy based on high-LET-dose