ESTRO 38 Abstract book

S509 ESTRO 38

integral dose is worse. In future all OARs that should be included in the optimization need to be specified in order to further harmonize treatment planning. PO-0944 Proton therapy for esophageal cancer; variable relative biological effect and heart dose C. Skinnerup Byskov 1 , A. Ivalu Sander Holm 1 , S.S. Korreman 1 , L. Hoffmann 1 , M. Nordsmark 2 , D. Sloth Møller 1 1 Aarhus University Hospital, Medical Physics, Aarhus C, Denmark ; 2 Aarhus University Hospital, Oncology, Aarhus C, Denmark Purpose or Objective For patients with esophageal cancer the standard treatment is radiotherapy (RT) and concomitant chemotherapy, possibly followed by surgery. Dose to the organs at risk (OAR) can be substantially reduced by proton therapy (PT), and a recent study has shown that posterior field directions constitute the most robust beam configuration for PT [Møller et al., RO 2018]. These PT studies were based on a constant proton relative biological effect (RBE) of 1.1 which disregards data suggesting that RBE varies with linear energy transfer (LET), physiological and biological factors. Heart dose has been found to increase the risk of heart toxicity [Darby et al., NEJM 2013]. The aim of this study was therefore to investigate the effect of variable RBE on dose to the heart in PT for patients with esophageal cancer using posterior field directions. Material and Methods Previously reported robustly optimized proton plans for 23 patients with esophageal cancer were used as baseline plans [Møller et al, RO 2018]. Two field configurations were applied for each patient, using one posterior field (I- plans) or two oblique posterior fields (V-plans) (Figure 1). The linear energy transfer (LET) and relative biological effect (according to the RBE model by McNamara et al. [PMB 2015]) were calculated in Eclipse using an Eclipse script developed by Sanchez-Parcerisa et al. [PMB 2016]. Mean heart dose (MHD), heart volumes receiving 25 Gy (V25Gy) and 40 Gy (V40Gy) and the near maximum dose (D2ccm) were measured and compared for the two different field configurations. Differences were tested using the Wilcoxon signed rank test, with p < 0.05 considered significant.

This is the first report of IPACS with focus on proton therapy treatment planning of head-and-neck patients. Material and Methods CT-data sets of five patients were included. During several face-to-face and online meetings a common treatment planning protocol was developed having objectives for target coverage and organ at risk (OAR) sparing together with their mutual priorization as well as a definition of robustness evaluation. Each centre used its own treatment planning system (TPS) and planning approach with some restrictions specified in the treatment planning protocol. In addition, volumetric modulated arc therapy (VMAT) photon plans were created. Results For CTV1 the average D 98% was 54.6±2.3 Gy(RBE) for protons and 54.9±0.5 Gy(RBE) for VMAT (aim was 56 Gy(RBE)). For CTV2 the average D 98% was 60.8±5.7 Gy(RBE) for protons and 61.6±3.4 Gy(RBE) for VMAT (aim was 70 Gy(RBE)). The average D 2% for the spinal cord was 25.1±8.5 Gy(RBE) for protons and 47.6±1.4 Gy(RBE) for VMAT (see also Figure 1). The average D 2% for chiasm was 46.5±4.4 Gy(RBE) for protons and 50.8±1.4 Gy(RBE) for VMAT respectively. Robust evaluation was performed and showed the least robust plans for plans with a low number of beams (see Figure 2).

Figure 1: For the nominal plans, the D 2% for the spinal cord for all 5 head and neck cases from each centre and the VMAT plan is shown. The planning aim was achieved by all plans. The dashed line shows the objective for spinal cord D 2% which was < 50 Gy (RBE).

Results For the I-plans and the V-plans respectively, the population median MHD was reduced with 7.4 Gy and 6.9 Gy compared to the IMRT plans (Figure 2). When applying the variable RBE model, doses to the heart increased compared to using RBE 1.1. For the I-plans and [V-plans], respectively, the MHD was increased by 1.0±0.5 Gy [1.3±0.7 Gy], V25Gy was increased by 1.9±1.0% [2.4±1.2%], V40Gy was increased by 2.3±1.3% [2.9±1.6%] and D2ccm was increased by 3.5±1.0 Gy [5.0±1.5 Gy] (population mean ±SD). Significant differences were found between the I-plans and V-plans for V25Gy and D2ccm.

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Figure 2: D 98% for CTV1 in the first treatment phase for the nominal plans and the results of the robustness analysis for all 5 head and neck cases from each centre and the VMAT plan is shown. The black bars show the band width (max and min) of D 98% for all the robustness scenarios. The dashed line shows the planning aim for CTV1 D 98% which was 56 Gy (RBE) respectively. Conclusion Despite the detailed treatment planning protocol, differences in dose distribution and reported parameter were still identified. Although VMAT is more robust than PT, OAR sparing at intermediate distance from CTVs and