ESTRO 2025 - Abstract Book

S3339

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

3919

Mini-Oral Intrafraction Motion and Margin Assessment for CBCT-Guided Online Adaptive Radiotherapy in Head and Neck Cancer Mu-Han Lin 1 , Chien-Yi Liao 1 , Prashant Yarlagadda 2,1 , Sean Domal 1 , David Parsons 1 , Jing Wang 1 , Andrew Godley 1 , Steve Jiang 1 , Vladimir Avkshtol 3 , Dominic Moon 1 , David Sher 1 1 Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, USA. 2 Department of Radiation Oncology, University of Mississippi Medical Center, Mississippi, USA. 3 Department of Radiation Oncology, Kelsey-Seybold Clinic, Houston, USA Purpose/Objective: Online adaptive radiotherapy (oART) for head and neck squamous cell carcinoma (HNSCC) holds the potential to dramatically reduce planning margins. Fractionated daily oART for HNSCC with such margins has previously demonstrated favorable treatment outcomes [1]. However, careful margin assessment is critical due to the additional time required for real-time contouring and planning and consequential intrafractional motion. This study aimed to quantify the internal margin (IM) required for CBCT-guided oART in HNSCC. Material/Methods: This retrospective analysis included 24 HNSCC patients undergoing 35 fractions of simultaneous integrated boost IMRT (70/63/56 Gy) delivered via a CBCT-guided oART system. Immobilization was achieved with a 5-point mask. Weekly fractions (168 total) were analyzed. For each oART, a planning CBCT and a pre-treatment CBCT (just prior to beam delivery) were acquired. The oART plan was generated using actual contoured targets with a 1-mm axial and 2-mm superior-inferior (SI) planning target volume (PTV) expansion. Retrospectively, "gold standard" (GS) gross tumor volumes (GTV) and clinical target volumes (CTV) were contoured on both the planning and pre-treatment CBCTs without time constraints. Margins of 1- mm, 2-mm, 3-mm axial and 2-mm SI were applied to the planning GS contours to assess geometric overlap with pre-treatment GS targets, and the actual delivered plan was evaluated against pre-treatment GS contours. Geometric overlap (≥0.95 deemed feasible) and dosimetric coverage (V100% ≥95% and V95% ≥99%) were analyzed. Time from planning CBCT to beam-on and to the last beam delivery was recorded. Results: Geometric overlap and dosimetric coverage analysis showed that a 1-mm axial and 2-mm SI margin achieved > 99% geometric overlap for all GS targets and met dosimetric coverage goals: GTVp-70 (99.7±1.1%, 96.9±4.1%), GTVn-70 (99.0±2.8%, 96.6±5.8%), CTV-63 (99.4±1.4%, 98.7±1.9%), GTVn-63 (99.1±4.0%, 96.8±8.1%), and GTVn-56 (99.58±3.7%, 99.0±6.1%). The average time from planning CBCT to beam-on was 26 minutes (SD 6.5), reflecting the additional time required for oART compared to IGRT; while the total time to the last beam delivery was 32 minutes (SD 5.3).