ESTRO 2025 - Abstract Book

S991

Clinical – Head & neck

ESTRO 2025

Kingston, Canada. 7 Department of Otolaryngology, University of British Columbia, Vancouver, Canada. 8 Department of Otolaryngology Head and Neck Surgery, Princess Margaret Cancer Centre, University of Toronto, Toronto, Canada

Purpose/Objective: We constructed a nomogram to define the risk of locoregional failure (LRF) and quantify the benefit of post operative radiation therapy (PORT), for patients with pT1-T4 pN0 oral cavity squamous cell carcinoma (OSCC). Material/Methods: Patients with pT1-4 N0 OSCC treated with curative intent between August 1st 1994 and December 31st 2017 at four tertiary cancer centres were reviewed. Factors identified as statistically significant (p<0.05) in multivariable analysis (MVA) and three clinically relevant characteristics were utilized to construct a nomogram for defining a risk group classification for the LRF. Model validation was performed using a ten-fold cross-validation approach with 10 replicates. Subsequently, the association of PORT with outcomes was calculated for each individual risk group. Results: A total of 1,094 patients were included in the analysis, with a median follow up of 4.7 years among surviving patients (1 st and 3 rd quartile, 2.9-6.4 years). The nomogram was constructed based on five statistically significant factors identified through MVA (pT-category [pT2: 42 points and pT3-4: 100 points], histological grade 3 [80 points], perineural invasion [45 points], compromised resection margins (close [<5mm], involved, or undetermined) [37 points], inadequate neck dissection (less than 18 lymph nodes resected on each side) [56 points]), and three clinically relevant factors (primary oral tongue subsite [24 points], lymphovascular invasion [14 points], and current/ex-smoking history [11 points]) (figure 1). Four distinct risk groups were identified based on the three-year LRF probability : low-risk group (0 to 30 points, three-year LRF: 6% [95% CI: 2%-13%]), standard-risk group (31 to 110 points, three-year LRF: 12% [95% CI: 8%-16%]), intermediate-risk group (111 to 220 points, three-year LRF: 23% [95% CI: 19%-27%]), and high-risk group (221 to 340 points, three-year LRF: 27% [95% CI: 20%-34%]), p<0.001. The model demonstrated a C-index of 0.66. The use of PORT was associated with a significant reduction of the three-year LRF in intermediate- and high-risk groups (19% vs 28%, p=0.02 and 22% vs 38%, p=0.03, respectively), which translated into improved three-year overall survival (OS) rates (78% vs 70%, p=0.04 and 73% vs 45%, p<0.001, respectively) (figure 2).