Daily adaptive radiotherapy in head and neck squamous cell carcinoma can negatively impact organ at risk dosimetry.

Purpose: Adaptive radiotherapy (ART) accounts for anatomical changes during treatment and may improve target coverage and spare organs-at-risk (OARs) in head and neck squamous cell carcinoma (HNSCC). This study aimed to develop a protocol defining dose evaluation criteria for target volumes and OARs to assess the need for ART. The protocol was applied to compare compliance between daily-ART and non-ART plans with varying CTV-PTV-margins. Materials and methods: A protocol defining dose criteria for target and OAR volumes was developed and used to evaluate ART in a retrospective cohort of 80 patients (2,170 fractions) treated with radiotherapy to 60.0-68.0 Gy (2 Gy/fraction). Synthetic CTs were generated from daily kVCTs and contours propagated using deformable image registration. Delivered doses (no-ART) and daily-ART treatments using 5 mm and 2 mm PTV plans were simulated. All daily distributions were evaluated using the protocol. Results: With 2 mm margins, daily-ART significantly reduced protocol violations compared to no-ART(7% vs 20.3%, p < 0.001), while no significant difference was observed with 5 mm margins(15.1% vs 17.3%, p > 0.8). For 2 mm margins, daily-ART resolved 610 violations but introduced 152 new violations. With 5 mm margins, 391 violations were resolved, while 472 violations were introduced. When adaptation was limited to fractions that initially presented violations, fewer new violations were introduced (2 mm: 60; 5 mm: 49). Conclusion: Daily-ART with 2 mm margins improved target coverage and reduced protocol violations, while larger margins increased OAR doses by enforcing full PTV coverage. Restricting daily ART to fractions with identified violations minimized the occurrence of newly introduced violations.