ESTRO 2023 - Abstract Book

S133

Saturday 13 May

ESTRO 2023

Dresden, German Cancer Research Center (DKFZ), Heidelberg, Germany; 5 National Center for Tumor Diseases (NCT), Partner Site Dresden, Germany: German Cancer Research Center (DKFZ), Heidelberg, Germany, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany, and Helmholtz Association / Helmholtz-Zentrum Dresden - Rossendorf (HZDR), Dresden, Germany; 6 OncoRay – National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz- Zentrum Dresden–Rossendorf, Dresden, Germany; 7 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany; 8 TU Dortmund University, Department of Physics, Dortmund, Germany; 9 West German Proton Therapy Center Essen (WPE), University Hospital Essen, Essen, Germany; 10 Clinic for Particle Therapy, University Hospital Essen, Essen, Germany; 11 German Cancer Consortium (DKTK), partner site Essen, German Cancer Research Center (DKFZ), Heidelberg, Germany Purpose or Objective The variable relative biological effectiveness (RBE) of protons can lead to an increased risk of complications in normal tissue compared to the estimated risk based on the clinically applied RBE of 1.1. To demonstrate how accounting for variability in RBE affects the expected probability of side effects, we evaluated normal tissue complication probability (NTCP) models following proton beam therapy (PBT) of patients with primary brain tumors. Materials and Methods We retrospectively evaluated clinically applied PBT plans of 51 patients with primary brain tumors. The median prescribed dose was 60 Gy(RBE) [42.5–60 Gy(RBE)] administered in fractions of 2 Gy(RBE) [1.8–2.5 Gy(RBE)]. Dose and dose-averaged linear energy transfer were recalculated for these treatment plans using an in-house developed Monte-Carlo simulation framework [1] or the Monte-Carlo scoring extension of a research version of the treatment planning system RayStation (RaySearch Laboratories AB, Stockholm, Sweden). For each patient, the absorbed dose distribution was converted into two RBE-weighted absorbed dose distributions: one using a constant RBE of 1.1 (DconstRBE) and one using a variable RBE obtained from the Wedenberg et al. model [2] (DvarRBE) with an α / β ratio of 2 Gy and 10 Gy for organs at risk (OARs) and tumor, respectively. An example is presented in the Figure. Dose-volume parameters were extracted for different OARs and inserted into eleven published NTCP models for various radiation-induced side effects of the central nervous system [3]. For each patient and all adverse events, NTCPs were calculated based on DconstRBE and DvarRBE leading to NTCPconstRBE and NTCPvarRBE, respectively. Patient-specific differences were calculated using ∆ NTCP=NTCPvarRBE- NTCPconstRBE.

Results Consideration of RBE variability resulted in similar or slightly higher median NTCP values for all endpoints (Table). Pronounced relative changes were observed for blindness and necrosis. For individual patients, an increase in NTCP of more than 30% was observed, in particular for endocrine dysfunction, delayed recall, hearing loss and ocular toxicity when considering a variable RBE. The OARs related to these side effects are small in volume, underlining the local effect of an increased DvarRBE. For 34 patients (66.6%), the change in NTCP was greater than 5% for at least one of the considered endpoints.

Conclusion