S2146
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
Purpose/Objective: 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 for assessing dose distributions and apply it to compare compliance between adaptive and non-adaptive plans with varying CTV-PTV-margins. Material/Methods: A protocol was developed by a multidisciplinary team based on previous literature and international guidelines that categorizes regions-of-interest (ROI) into four groups with specific failure criteria:CTV: Delivered dose to D98%,CVT<95% of the prescribed dose.Serial OARs with MUST constraints: Delivered max or near-max dose exceeds clinical threshold.Parallel OARs with SHOULD constraints: Delivered mean dose exceeds planned mean dose by more than 3 Gy.Other OARs with SHOULD or CAN constraints: Delivered near-max dose exceeds clinical threshold and planned dose by more than 3 Gy.The retrospective cohort included 80 patients, comprising 2,170 fractions, all treated with radiotherapy to a total dose of 60.0–68.0 Gy in 2 Gy fractions with 5 mm PTVs and daily kVCT imaging. An additional set of plans with 2 mm PTVs were constructed. Further, synthetic CTs were generated from daily kVCT images using deformable image registration (DIR) and contours were propagated onto the daily images. On each synthetic CT, delivered treatment doses were calculated (referred to as no-ART) and daily-ART treatments were simulated, both based on plans with 5 mm and 2 mm PTV margins. Daily dose distributions were accumulated. All daily and accumulated dose distributions were evaluated using the protocol. Results: In the 2 mm margin scenario, daily-ART significantly reduced number of protocol failures compared to no- ART (7% vs. 20.3%, p<0.001) (Fig.1). No significant change in protocol failures was observed with daily- ART using 5 mm margin (15.1% vs. 17.3%). Daily-ART at 2 mm resolved 610 failures while introducing 152, as compared to 391 resolved and 472 introduced at 5 mm (Fig.2). Adapting only on failing fractions resulted in substantially fewer introduced failures (60 for 2 mm; 49 for 5 mm). The number of daily failures were associated with failure in the accumulated distribution, across all scenarios (p<0.03).
Conclusion: A protocol for assessing dose distributions in HNSCC was successfully developed. Daily-ART with reduced 2 mm PTV margins improved target coverage and reduced the number of failures. Larger margins increased the dose to OARs by enforcing full PTV coverage. Limiting daily adaptation to failing fractions minimized introduced failures and unnecessary replanning. Keywords: ART Protocol, H&N, dose tracking,
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