ESTRO 2021 Abstract Book

S25

ESTRO 2021

PH-0049 Reduction of GTV to CTV radiation margin in head and neck squamous cell carcinoma is oncological safe and significantly reduced acute and late radiation-related toxicity A. Al-Mamgani 1 , R. Kessels 2 , A. Navran 3 , O. Hamming-Vrieze 4 , C.L. Zuur 5 , J.P. de Boer 6 , M.C. Jonker 4 , T. Janssen 4 , J. Sonke 7 , C.A. Marijnen 8 1 Netherlands Cancer Institute/Antoni van Leeuwenhoek Hospital, Department of Radiation Oncology, Amsterdam , The Netherlands; 2 Netherlands Cancer Institute/Antoni van Leeuwenhoek Hospital, Department of Biometrics, Amsterdam, The Netherlands; 3 Netherlands Cancer Institute/Antoni van Leeuwenhoek Hospital, Department of Radiation Oncology, Amsterdam, The Netherlands; 4 Netherlands Cancer Institute/Antoni van Leeuwenhoek Hospital, Department of Radiation Oncology , Amsterdam, The Netherlands; 5 Netherlands Cancer Institute/Antoni van Leeuwenhoek Hospital, Department of Head and Neck Surgery, Amsterdam, The Netherlands; 6 Netherlands Cancer Institute/Antoni van Leeuwenhoek Hospital, Department of Medical Oncology, Amsterdam, The Netherlands; 7 Netherlands Cancer Institute/Antoni van Leeuwenhoek Hospital,, Department of Radiation Oncology , Amsterdam , The Netherlands; 8 Netherlands Cancer Institute/Antoni van Leeuwenhoek Hospital,, Department of Radiation Oncology , Amsterdam, The Netherlands Purpose or Objective Despite the significant reduction of radiation-related toxicity in patients with HNSCC as result of the use of IMRT, the incidence and the severity of different acute and late toxicity such as painful mucositis, xerostomia and dysphagia are still high. We aim to investigate whether reducing the GTV to CTV margin will significantly reduce treatment-related toxicity without jeopardizing outcome in patients with HNSCC treated with definitive (chemo)radiation. Materials and Methods Between April 2015 and April 2019, 155 consecutive patients were treated with GTV-CTV margin of 10mm and subsequently another 155 patients with 6mm margin. The CTV-PTV margin was 3mm in both groups. All patients were treated with volumetric modulated arc therapy with daily-image guidance using cone-beam CT. End points of the study were acute and late toxicity and oncologic outcomes. Results Overall acute grade 3 toxicity was significantly lower in 6mm-group, compared to 10-group (48% vs. 67%, respectively, p=0.001). The same was true for grade 3 dermatitis (18% vs. 28%, p=0.043), grade 3 mucositis (18% vs. 34%, p=0.003), feeding tube-dependency at the end of treatment (25% vs. 37%, p=0.019), at 3 months (12% and 25%, p=0.005), at 6 months (6% and 15%, p=0.014), and 12 months (5% and 0.6%, p=0.036). Persistent mucosal ulceration beyond 6 months after radiotherapy was lower in patients treated by small margins (7% vs. 15%, p=0.044). The incidences of overall late grade ≥2 toxicity, xerostomia, and dysphagia were also significantly lower in the 6mm group (51% vs. 70% for overall grade ≥2 toxicity, 29% vs. 45% for grade ≥2 xerostomia, and 17% vs. 31% for grade ≥2 dysphagia, with p-values of 0.001, 0.007, and 0.008, respectively). The incidence of hypothyroidism for patients with a minimum follow up of 2 years is lower in patients treated with 6mm margins (26%vs. 40%, p=0.047). The 2-years rates of loco-regional control, disease-free and overall survival for the 6mm vs. 10mm group were 78.6% vs. 73.1% for LRC, 70.6% vs. 61.4% for DFS, and 83.2% vs. 74.4% for OS (p>0.05, all). Conclusion To the best of our knowledge, this is the first study to date reporting on the impact of reducing the GTV to CTV margin on toxicity and oncologic outcomes in patients with HNSCC. Margin reduction from 10 to 6 mm resulted in significant reduction of the incidence and severity of acute and late radiation-related toxicity without reducing local-regional control and survival. PH-0050 Detailed analysis of lymphatic progression patterns for oropharyngeal squamous cell carcinoma B. Pouymayou 1 , J. Hoffmann 2 , R. Ludwig 3 , M. Guckenberger 2 , P. Balermpas 3 , J. Unkelbach 3 1 University Hospital Zürich, Department of Radiation Oncology, Zürich, Switzerland; 2 University Hospital Zürich, Department of Radiation Oncology, Zürich, Switzerland; 3 University Hospital Zürich, Department of Radiation Oncology, Zürich, Switzerland Purpose or Objective Head & neck squamous cell carcinomas (HNSCC) spread lymphatically through the neck and form metastases in regional lymph nodes. While the general directions of spread are understood and the prevalence of lymph node level (LNL) involvement has been reported, the details of lymphatic progression patterns are insufficiently quantified. The purpose of this study is to investigate, how the risk of metastases in a given level depends on whether or not upstream levels harbour metastases, how much the risk increases for late versus early T-stage, and if progression patterns differ between HPV positive versus negative tumors. Quantifying these dependencies may eventually lead to more personalized definition of the elective nodal CTV and tailored radiotherapy treatments. Materials and Methods To address these questions, we retrospectively analysed all patients with newly diagnosed oropharyngeal HNSCC treated at our institution between 2013 and 2019, resulting in a dataset of 287 patients. For all patients, involvement of LNLs I-VII was recorded together with T-stage, midline extension, HPV status, smoking status, and primary tumor subsite. Using common criteria, LNL involvement was assessed individually based on available modalities (PET, MR, CT, FNA). To analyse the complex dataset, a graphical user interface linked to an SQL database was developed, which allows querying the number of patients with a certain combination of co-involved LNLs and tumor characteristics (Figure 1).