ESTRO 2021 Abstract Book

S795

ESTRO 2021

such as India where tobacco consumption is very high.

PO-0954 REPORT; REirradiation and PD-1 blockade On Recurrent squamous cell head and neck Tumors Å. Bratland 1 , H.A. Eide 1 , J.A. Kyte 1 1 The Norwegian Radiumhospital, Cancer clinic, Oslo, Norway Purpose or Objective Recurrent and second primary tumors are frequent in squamous cell carcinomas of the head and neck (HNSCC). In the phase I trial, REPORT the aim was to evaluate the safety of a new treatment option. Radiotherapy (RT), together with surgery, remains the main treatment modality in HNSCC. Despite aggressive multimodal primary treatment of HNSCC patients, recurrent and second primary tumors in previously irradiated areas are a common clinical challenge. Re-irradiation is a possible choice of treatment, offering long-term survival for selected patients. However, re-irradiation is complicated with severe acute and late toxicity. Recurrent disease is further associated with a risk of developing systemic disease. There is an urgent need for new treatment approaches and combinations to improve tumor control and to reduce toxicity in this patient group. RT can induce immunogenic cell death, which serves as a trigger for the immune system (“in situ vaccine”). There is a high expression of PD-L1 in HNSCC and this expression is further enhanced by RT. Combining re-irradiation with PD-1/PD-L1 inhibitors thus represent a promising treatment option for the future. Materials and Methods REPORT is a phase I study exploring the combination of the monoclonal PD-1 inhibitor nivolumab with hyper fractionated re-irradiation in patients with recurrent squamous cell head and neck carcinoma. The primary objectives were safety and tolerability, and to determine a safe dose of nivolumab when administered concomitant with high dose re-irradiation. Enrollment in this phase I trial is currently completed. Results We have included 20 patients, 55 % male with a mean age of 70 years. A majority of the patients were diagnosed with recurrent oral cavity carcinomas. Only two patients had a HPV positive tumor at baseline. 13 of the patients (65 %) received a total dose of 60 Gy. The treatment has so far been well tolerated with few side effects. Conclusion Based on our preliminary experience, combined radiotherapy and immunotherapy is a feasible treatment option for recurrent HNSCC, with an acceptable safety profile. 1 University Hospital and Medical Faculty, Eberhard Karls University Tübingen, Department of Radiation Oncology, Tübingen, Germany; 2 German Cancer Consortium (DKTK), partner site Tübingen, and German Cancer Research Center (DKFZ), Heidelberg, Germany; 3 University Hospital and Medical Faculty, Eberhard Karls University Tübingen, Section for Biomedical Physics, Department of Radiation Oncology, Tübingen, Germany; 4 German Cancer Consortium (DKTK), partner site Tübingen, and German Cancer Research Center (DKFZ) , Heidelberg, Germany; 5 University Hospital and Medical Faculty, Eberhard Karls University Tübingen, Department of Radiation Oncology , Tübingen, Germany; 6 German Cancer Consortium (DKTK), partner site Tübingen, and German Cancer Research Center (DKFZ) , Heidelberg, Germany; 7 University Hospital and Medical Faculty, Eberhard Karls University Tübingen, Department of Radiology, Tübingen, Germany; 8 University Hospital and Medical Faculty, Eberhard Karls University Tübingen, Section for Experimental Radiology, Department of Diagnostic Radiology, Tübingen, Germany; 9 University Hospital and Medical Faculty, Eberhard Karls University Tübingen, Section for Biomedical Physics, Department of Radiation Oncology , Tübingen, Germany Purpose or Objective As diffusion weighted imaging (DWI) has been shown to be a prognostic marker in head and neck cancer (HNC), treatment on combined MR-Linacs (MRL) may be a promising approach to monitor treatment response with DWI and may offer the benefit of response adaptive radiotherapy. As a first step for future real-time interventions at the MRL we report on 6 consecutive patients treated at the 1.5T MRL with serial DWI during radiotherapy (RT). Materials and Methods Six serial patients with Stage II-IVA HNC treated with combined radiochemotherapy (n=5) or accelerated RT (n=1) on the 1.5T MRL were analyzed. N=3 tumors were p16 negative, n=3 p16 positive with n=1 being in a high-risk situation based on smoking status with >30 pack years. Tumor sites were OPC n=5 and n=1 hypopharyngeal carcinoma. Echo-planar DW-MRI with b-values of 0, 150, 500 s/mm 2 was performed on the MRL at least once per week. GTV of the primary tumor (GTV-P) and lymph nodes (GTV-LN) were delineated on b500 images for each time point. Individual lymph nodes were combined in one GTV-LN. ADC maps were calculated using mono-exponential fitting of signal amplitudes of the b150- and b500-images. Tumor volume and ADC mean values were analyzed for GTV-P and GTV-LN at the different time points. Results All tumors showed a steady decrease in tumor volume and increase in mean ADC. Median (min, max) increase of ADC from baseline to last week of RT was 0.63·10 -3 mm 2 /s (0.15 – 0.69·10 -3 mm 2 /s) and 0.14·10 -3 mm 2 /s (0.06 – 0.26·10 -3 mm 2 /s) for GTV-P and GTV-LN, respectively. From baseline to last week of therapy median decrease in tumor volume was 79.7% and 66.9%, while median increase in ADC mean was 52.4% and 15.0% for GTV-P and GTV-LN, respectively. Change of tumor volume and mean ADC from baseline to last week of therapy are shown in table 1 for all patients. Temporal changes of tumor volume and ADC mean for GTV-P and GTV-LN are shown in figures 1 and 2. In pat#4 problems in DWI acquisition at baseline occurred, while in pat#2 fx32 the tumor was PO-0955 Tumor volume and ADC changes during the time course of RT on a 1.5T MR-Linac in HNC S. Boeke 1,2 , J. Habrich 3 , M. Nachbar 3 , C. Gani 1,4 , S. Butzer 5 , J. Boldt 5 , C. Marks 5 , D. Zips 5,6 , K. Nikolaou 7,4 , F. Schick 8 , D. Thorwarth 9,4