ESTRO 2025 - Abstract Book

S1132

Clinical – Head & neck

ESTRO 2025

Material/Methods: A cohort of patients treated with radical radiotherapy (RT) for HNC and who underwent replanning during treatment were identified. Comparative VMAT plans were generated for each patient, using both a 1 mm and a 5 mm PTV margin, for both the original plan and re-planned treatment. OAR dosimetric data for the 1mm and 5mm comparative plans was used to calculate the change in normal tissue complication probability (NTCP) for xerostomia and dysphagia from margin reduction [2,3]. Reproducibility and stability of the effect of PTV margin reduction on predicted toxicity was assessed by comparing outcomes using the baseline and replanned scans. Results: Nine patients were assessed. Mean time to initiate replanning during treatment was 31 days (14 – 35 days). Figure 1 illustrates the reduction in the probability of xerostomia and dysphagia 6 months post-radiotherapy using NTCP modeling of variable PTV margins on pre-treatment planning scans. The mean probability of moderate-to-severe xerostomia was reduced from 48.5% to 42.1%, and of grade 2-4 RTOG swallowing dysfunction from 37.2% to 28.3% from PTV margin reduction. Marked inter-patient variation in benefit from margin reduction was identified with a range of reduction from 0 – 14.5% and 4.9 – 12.8% for xerostomia and dysphagia, respectively. Comparison of the absolute risk reduction (ARR) in NTCP between the original planned treatment and the re-planned treatment demonstrates preservation of the benefit of margin reduction. There is some variation between patients, partially attributable to anatomical changes and response during treatment. The median difference in ARR between the original and re-planned treatments was 2.9% (0.5 – 10.4%) and 3.2% (0 – 5.9%) for dysphagia and xerostomia, respectively. This is illustrated in Figure 2.