ESTRO 2025 - Abstract Book

S1060

Clinical – Head & neck

ESTRO 2025

above/equal median) 2 , V 2 , chemotherapy (yes vs. no), pre-RT surgery (PORT vs. primary RT), RT technique (IMRT vs. VMAT) and fraction size (above vs. below/equal 2Gy/fraction), mandible volume (below vs. above/equal median). Results: The final dataset consisted of 408 ORN cases, of which 248 (60.8%) were OPC, 123 (30.1%) OCC and 37 (9.1%) other primary tumour sites. Median follow-up time was 4.6 (IQR 3.2-6.0) and median time to ORN was 1.2 (IQR 0.6-2.3, max 11.9) years. Patients with positive smoking status (p=0.012), dental extractions pre-RT (p=0.003), smaller mandible volumes (p=0.002), or treated with VMAT (p=0.011) were at risk of developing ORN at an earlier time point (Fig.1). At Cox regression analysis, only pre-RT dental extractions (p=0.002) and mandible volume (p=0.04) remained significantly associated with the timing of ORN onset. However, sensitivity analysis revealed that the effect of mandible volume was likely influenced by centre-specific differences rather than reflecting a true biological relationship (Fig.2). 70Gy (below vs. above/equal median)