ESTRO 2023 - Abstract Book

S355

Sunday 14 May 2023

ESTRO 2023

Conclusion We present evidence of causality between mean dose to the medulla and dysphagia 1 year after head and neck RT. To our knowledge, this is the first time that causal inference with an expert consensus causal diagram is used to find RT dose- effect relationships. Adoption of causal analysis in outcome modelling will help overcome current limitations in methodologies, as we can investigate causation rather than associations. Our analysis paves the way for clinical trials of medulla sparing to ultimately develop interventions to improve quality of life of cancer survivors. 1 Haaglanden Medical Center, Medical Physics, Leidschendam, The Netherlands; 2 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands; 3 Haaglanden Medical Center, Radiation Oncology, Leidschendam, The Netherlands Purpose or Objective Radiotherapy treatment planning of head-and-neck cancer is challenging for its anatomical complexity, numerous surrounding OARs and multiple dose levels involved. In robust planning, the CTV dose is directly optimized, omitting the use of a PTV. We investigated the potential of scenario-based robust VMAT planning in head-and-neck cancer to improve target coverage and reduce NTCPs for xerostomia and dysphagia. Materials and Methods Eighteen pharyngeal head-and-neck cancer patients were included. Planning was performed with RayStation version 9B and delivered in 2021. For each patient, the clinical PTV-based SIB VMAT plan was first robustly evaluated for CTV coverage, using 14 setup error scenarios with a 3 mm translation, corresponding to the applied PTV margin. This evaluation focussed on the voxel wise minimum (vw min) CTV dose [1]. Analogous to the clinical PTVs, primary CTVp and elective CTVe were cropped to 5 mm from the body contour, to avoid dose calculation in the dose build up region. CTV coverage was considered acceptable if V95% ≥ 98% in the vw-min distribution (robust CTV coverage). Next, two patient groups were identified based on the robust CTV coverage of the clinical plans. Group I (5 patients): for clinical plans with insufficient robust coverage of either of the CTVs, a robust min dose objective was added for the corresponding CTV and robust optimization with 6 scenarios was performed, aiming to achieve minimal robust CTV coverage and reduce OAR doses. Group II (13 patients): for clinical plans with adequate robust CTV coverage, first further optimization was performed to reduce OAR doses. Next, as for Group I, a robust min dose objective was added when robust CTV coverage was getting compromised. Plans were compared based on PTV coverage and robust CTV coverage. Moreover, using OAR mean doses, NTCP values were evaluated for the risk of dysphagia and xerostomia [2]. OC-0451 NTCP reduction in head-and-neck cancer VMAT using robust planning M. van Schie 1 , A. Petoukhova 1 , L. Rossi 2 , L. Kwakkel-Huizenga 3 , S. Hutschemaekers 3 , B. Heijmen 2

Results