S1542
Interdisciplinary - Quality assurance and risk management
ESTRO 2026
γ ‑ map deviations in patients with anatomical or positional changes. The γ ‑ analysis reliably detected patient motion, setup shifts, and anatomical variations between the first and subsequent kV ‑ CBCT sessions. Using the 3 %/3 mm/10 % criterion, approximately 10 % of head-and-neck patients and a smaller proportion of the remaining pathologies were identified as having compromised plans, prompting early replanning. Daily bolus verification detected
and γ-analysis (%, mm, % threshold) to identify dosimetric deviations relative to the first treatment session. The method aims to prevent underdosage, overdosing, and intervention delays. Material/Methods: DICOM Plan/Dose sets were generated from kV-CBCT images acquired in every treatment session over six months. Dose distributions were recalculated on each CBCT and compared to the first session to assess inter-fraction variations.The workflow involved selecting the same most change-prone slice (axial, sagittal, or coronal) in both planning CT and kV-CBCT. The dosimetric plan in kV-CBCT is recalculated without normalization, and repeated for session X. Both DICOM Plan/Dose sets were exported and analyzed in SNC Patient using γ-analysis (3 %, 3 mm, 10 %) to quantify inter-fraction deviations.Dosimetric plans, as well as bolus positioning, were evaluated daily in patients requiring surface compensation. Deviations beyond tolerance thresholds triggered clinical review and replanning when necessary. Figures 1 and 2 show representative breast case, highlighting workflow steps and γ-map visualization between fractions.
misplacements, which were corrected via communication with RTTs, improving dose
reproducibility. Overall, actionable feedback was provided in 10–12 % of cases, resulting in plan adjustments, patient repositioning, or bolus correction, enhancing both dose accuracy and patient safety [1,2].In adition, This CBCT-based dose verification workflow offers a rapid and reliable offline QA approach, capable of detecting anatomical and setup-related variations that impact dose delivery and patient safety. Prior studies confirm its feasibility, reporting γ pass-rates ≥ 90 % for CBCT-based adaptive verification [1]. Conclusion: Offline γ-analysis using kV-CBCT provides a practical and reproducible method for inter-session verification in radiotherapy. It enables early detection of dosimetric deviations, timely replanning, and improved daily bolus placement. Integrating this workflow enhances treatment precision, patient safety, and efficiency, particularly in high-precision treatments such as head-and-neck and thoracic
radiotherapy. References:
[1] Hussam Jassim, Hassan A Nedaei, Ghazale Geraily, Nooshin Banaee, Ali Kazemian. The geometric and dosimetric accuracy of kilovoltage cone beam computed tomography images for adaptive treatment: a systematic review. PMID: 37389008, PMCID: PMC10301728, DOI: 10.1259/bjro.20220062.[2] Marta Bogowicz, Didier Lustermans, Vicki Trier Taasti, Colien Hazelaar, Frank Verhaegen, Gabriel Paiva Fonseca, Wouter van Elmpt. Evaluation of a cone-beam computed tomography system calibrated for accurate radiotherapy dose calculation. PMID: 38487622, PMCID: PMC10937948, DOI: 10.1016/j.phro.2024.100566. Keywords: CBCT, Offline-Verification, Gamma-analysis
Results: Clinical implementation of the CBCT-based dose verification workflow over six months revealed distinct
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