ESTRO 2021 Abstract Book

S603

ESTRO 2021

edition) between May 2018 to April 2019. All patients received 2 cycles of taxane based induction chemotherapy (IC) followed by definitive chemoradiotherapy (66-70Gy in 33-35 fractions using 3D-CRT technique) with weekly concurrent cisplatin (40mg/m 2 ). Serum cfDNA levels were quantified by qPCR in blood samples collected prior to treatment (S1), 2 weeks after IC (S2), post 20 fractions of RT (S3) and 3 months post treatment completion (S4). Response assessment was done 3 months post treatment completion based on WHO response criteria. Correlation between S3-S1 and response was evaluated by Spearman’s rank order correlation. P value of < 0.05 was considered statistically significant. Results Median age was 55 years (range 32-67) with M: F ratio of 31:3. Tumour site was oropharynx (55.9%), larynx (29.4%) and hypopharynx (14.7%). 23 patients were of T3-T4, 10 were T2 and 1 had T1 disease. 23 patients were of N2 stage. 26% patients were of stage III and 74% were of stage IVA. The median RT dose was 70Gy (range 66-70). 30 patients received ≥ 5 cycles of concurrent chemotherapy. The median cumulative cisplatin dose was 405 mg (range 260-560). The crude complete response (CR) rate, partial response rate and progression rates were 82.35%, 14.7% and 2.94% respectively at 3 months post completion of radiotherapy. CR rate was 100%, 84.2% and 40% for patients of larynx, oropharynx and hypopharynx respectively. The table given below shows cfDNA levels in ng/ml. S1 S2 S3 S4 MEDIAN 212.4 135.6 119 100.3 RANGE 31.4 - 660.8 17.8 - 723 15.1 - 583.5 19.7 - 1043.8 Declining trend in cfDNA levels was seen from S1 to S4 in all patients who had CR. 4 of 28 patients having CR had increased S3 levels vs S1. 5 of 6 patients having non-CR had increased S3 levels from S1. The median difference between S3 and S1 for the entire cohort was -73.6 ng/ml (range -480.6 – 186.2). Spearman correlation coefficient between S3-S1 and response (CR vs non-CR) was 0.48 ( p = 0.004 ).

Conclusion Moderate positive correlation was found between change in cfDNA levels (between initial and mid-RT) and tumour response in LAHNSCC treated with chemoradiotherapy. Decrease in cfDNA levels in the mid of treatment may predict complete response to chemoradiotherapy. As predicting response in the mid of treatment by any radiological or clinical means is difficult, cfDNA may be a good predictive biomarker for treatment personalization. PD-0772 DW-MRI changes in swallowing structures during RT as a biomarker for dysphagia in HNC patients S. Stieb 1 , A.S. Mohamed 1 , S. Ahmed 1 , K. Wahid 1 , R. He 1 , A.S. Garden 1 , S.J. Frank 1 , D.I. Rosenthal 1 , G.B. Gunn 1 , K.A. Hutcheson 2 , S.Y. Lai 2 , C.D. Fuller 1 1 The University of Texas MD Anderson Cancer Center, Department of Radiation Oncology, Houston, TX, USA; 2 The University of Texas MD Anderson Cancer Center, Department of Head and Neck Surgery, Houston, TX, USA