ESTRO 2021 Abstract Book

S352

ESTRO 2021

treatment (Tambas et al, 2021). Most patients are selected based on the delta NTCP for dysphagia-related toxicities. The ongoing DAHANCA 35 trial is currently ongoing targeting patients with SCC of the larynx and pharynx planned for primary RT. After plan comparison (photons vs. protons), if protons are shown to reduce the anticipated absolute risk of dysphagia > Grade 2 or severe xerostomia Grade > 4 more than 5%, the patient is randomized to either proton or photon (2:1). The primary endpoint are dysphagia and xerostomia. Only solid data from clinical study could clarify the real advantage for using IMPT vs. VMAT in head and neck cancer and proton therapy centers should be encouraged to include patients in controlled clinical trials to reach a high level of evidence. SP-0465 Dose painting in head and neck radiotherapy D. Thorwarth 1 , F. Paulsen 2 , C. Pfannenberg 3 , G. Reischl 4 , C. La Fougère 5 , D. Zips 2 , S. Welz 2 1 Section for Biomedical Physics, Department of Radiation Oncology, Tübingen, Germany; 2 University Hospital for Radiation Oncology, Department of Radiation Oncology, Tübingen, Germany; 3 Diagnostic and Interventional Radiology, Department of Radiology, Tübingen, Germany; 4 Werner Siemens Imaging Center, Department of Preclinical Imaging and Radiopharmacy, Tübingen, Germany; 5 Nuclear Medicine and Clinical Molecular Imaging, Department of Radiology, Tübingen, Germany Tumor hypoxia has been shown to be a major cause of radiation resistance in head and neck cancer (HNC) and can be measured non-invasively using [ 18 F]fluoromisonidazole (FMISO) positron emission tomography (PET). The aim of this study (NCT02352792) was to validate the prognostic impact of hypoxia in locally advanced HNC assessed with dynamic FMISO PET with respect to outcome after radiochemotherapy (RCT). Furthermore, we aimed to test the feasibility and toxicity of hypoxia-guided dose painting and to evaluate the impact of hypoxia dose painting on local control (LC). Patients and methods Patients with HNC who were treated with curative RCT received a baseline dynamic FMISO PET/ computed tomography (CT) to derive hypoxic volumes (HV) in addition to routine planning CT and [ 18 F]fluorodeoxyglucose PET/CT for staging. Patients with hypoxic tumors were randomized between standard RCT (70 Gy/ 35 fx) [ST] or dose escalation (77 Gy /35 fx) [DE] to the HV. Patients without tumor hypoxia were treated with ST. Results The study was closed prematurely due to slow accrual. A total of 53 patients were enrolled, 39 had a detectable HV and were randomized into ST/DE. Median (range) HV in the ST and DE arm was 9.5 mL (0.2 – 58.5 mL) and 2.9 mL (0.2 – 100.4 mL), p=0.58. Dosimetric evaluation of the treatment plans showed that median (range) mean doses applied to the HV in the DE arm D mean,DE =71.3 Gy (69.5 – 73.7 Gy) were significantly higher compared to the ST arm with D mean,ST =69.8 Gy (68.5 – 70.9 Gy; p<0.001). Median follow up of for living patients was 62 months. Patients with non-hypoxic tumors presented with a 5- year LC of 100% compared to 74% in patients with tumor hypoxia (p=0.039). In the hypoxic group, patients treated with DE showed 19% higher LC rates with a 5-year LC of 84%, compared to 65% in the ST arm (p=0.150). Treatment adherence was 100% for radiotherapy. No significant difference was observed between the treatment groups with respect to acute and late toxicity with the exception of xerostomia as acute toxicity. Conclusion Tumor hypoxia assessed with dynamic FMISO PET has a clear prognostic impact on LC in HNC RCT. Hypoxia dose painting is tolerable without excess toxicity. No significant improvement of LC after dose painting in hypoxic tumors could be demonstrated. Abstract Text Purpose

SP-0466 Reirradiation in head and neck cancer P. Blanchard 1 1 Gustave Roussy Cancer Center, Radiation Oncology, Villejuif, France

Abstract Text Locoregional recurrences remain a frequent issue in head and neck cancer patients. This talk will focus on issues pertaining to diagnosis, patient selection, radiotherapy volume and doses, use of concomitant chemotherapy, with a focus on new techniques such as SBRT or proton therapy. The toxicity of reirradiation will be discussed as an important aspect of benefit/risk ratio in order to help patients make informed decisions.

Proffered papers: Proffered papers 28: Automated planning

OC-0467 Multi-criterial optimization of 200-400 MeV electron beams for strong improvement of photon VMAT A.W. Sharfo 1 , J. Dijkstra 2,1 , B. Heijmen 1 , D. Lathouwers 2 , S. Breedveld 1 1 Erasmus MC Cancer Institute, University Medical Center Rotterdam, Department of Radiotherapy, Rotterdam, The Netherlands; 2 Delft University of Technology, Department of Radiation Science and Technology, Delft, The Netherlands Purpose or Objective Very High Energy Electron (VHEE, 200-400 MeV) beam radiotherapy is being investigated as a potential replacement of photon therapy for selected tumor sites. Compared to photons, VHEE beams have a sharp penumbra at the level of the tumor, and a stronger dose fall-off at larger depths. This could potentially allow