ESTRO 2020 Abstract book

S348 ESTRO 2020

a 10mm margin into the CTV (volumetric protocol). The purpose of this study was to examine both toxicity and tumour control related to this protocol amendment. Material and Methods Overall survival and recurrence at 2 years were used to compare tumour control in the two contouring cohorts. For toxicity, the cohorts were compared by both the number of severe (grades 3-5) and all grades acute and late toxicities. In addition, quality of life and swallowing were compared using EORTC-C30 and MD Anderson Dysphagia Inventory (MDADI) respectively. Results Of 327 patients included in this study, 185 were contoured according to the anatomical protocol and 142 by the volumetric protocol. The two cohorts were well balanced for treatment arm (cisplatin versus cetuximab) and stage. With a minimum of two years follow up there was no significant difference in overall survival or recurrence between the two contouring protocols. Similarly there was no significant difference in the rate of reported severe or all grades acute or late toxicity and no sustained significant difference in quality of life. However, for dysphagia there was a statistical and clinically significant difference favouring the volumetric cohort at 1 year which persisted to 2 years in the MDADI domains (physical p=0.008; overall function p=0.015). Conclusion In the context of a randomised trial, measurable improvement in long term dysphagia has been demonstrated with a reduction in CTV without compromising tumour control. Further reduction in CTV should be subject to similar scrutiny within the confines of a prospective study. Contouring quality assurance both within randomised trials and in routine clinical practice is essential. OC-0574 Experience with Dutch National Indication Protocol_v1 model-based selection of proton therapy in HNC M. Tambas 1 , J.G.M. Van den Hoek 1 , R.G.J. Kierkels 1 , D. Scandurra 1 , A. Wolters 1 , D. Mulder 1 , E. Oldehinkel 1 , T.W.H. Meijer 1 , S. Both 1 , R.J.H.M. Steenbakkers 1 , J.A. Langendijk 1 1 University of Groningen- University Medical Center Groningen, Radiation Oncology, Groningen, The Netherlands Purpose or Objective In the Netherlands, head and neck cancer (HNC) patients qualify for proton therapy based on model-based selection as defined in the National Indication Protocol (NIP). Recently, the first version of the NIP (NIPv1) in HNC has been updated (NIPv2) and clinically implemented in September 2019. The aim of this study was to evaluate the first experience in model-based selection of HNC patients according to NIPv1. Material and Methods Three NTCP-models were used for model-based selection, including one for physician-rated grade≥2 dysphagia, patient-reported grade≥2 xerostomia and tube feeding dependence (TFD). The ΔNTCP-thresholds used for qualifying for protons were ≥10% and ≥5%, for grade ≥2 and TFD, respectively and/or ≥15% for the summed risk reduction (ΣΔNTCP) for the two grade 2 side effects. All HNC patients who were subjected to the model-based selection procedure according to the NIPv1 from January 2018 to September 2019 were included. A photon plan was created in all patients with optimal sparing of the five organ at risks (OARs)(oral cavity, contralateral parotid

glands and 3 swallowing muscles) relevant for the ΔNTCP- profile (model-based optimization). Patients were selected for a plan comparison if the NTCP-value of photon plan was higher than ΔNTCP threshold defined in NIPv1. Then, a robust proton plan was created and optimized using equivalent target dose with photon plan. Results Of the 227 patients with HNC treated with primary radiotherapy, 61 (27%) were not considered for a model- based selection procedure for various reasons including metal artefacts and very early-stage glottic cancer. The remaining 166 (73%) patients were entered in the model- based selection procedure and 141 (62%) qualified for a plan comparison. Eventually, 81 (36%) patients were selected for proton therapy (Fig. 1). Of these, 31 (22%), 16 (11%) and 27 (19%) and 7 (5%) were selected on basis of ΔNCTP values for dysphagia, xerostomia, ΣΔNTCP of xerostomia + dysphagia, and TFD respectively (Fig. 2). Thirty-one (43%) of the selected patients qualified two or more selection criteria. The selection rate was higher among patients with larger PTV volumes, pharyngeal tumours, and baseline complaints. A significant reduction in all OARs doses and NTCP values were obtained with protons compared with photons in both selected and non-selected patients, but these differences were more pronounced in selected patients.

Figure 1. A flowchart of preselection and selection phases of patients for proton therapy.