S2863
RTT - RTT education, training, and advanced practice
ESTRO 2026
CBCT just before rCT were imported in RayStation v2023B. A cCBCT was generated using deformable registration to the pCT. Clinical Target Volume (CTV) of the primary tumor (CTVprim) and the elective neck (CTVelec) and Organs At Risk (OARs) were deformed to cCBCT. The original plan was recalculated on cCBCT without reoptimization. Evaluation included: (1) geometric comparison of deformed CTVs on cCBCT to manual delineated CTVs on rCT using Dice (DSC), surface Dice (sDSC) and Hausdorff 95% (HD95) metrics; (2) dosimetric assessment and offline plan adaptation decision-making was based on combining the following criteria for both CTVprim and CTVelec: CTV V95% < 98%, Δ D98% > 3%, D2% > 107%, and PTV V95% < 95%. OARs (spinal cord, brainstem, salivary glands, oral cavity, pharyngeal constrictors) were assessed for Dmax and Dmean against institutional constraints. Results: For CTVprim the mean DSC (+1SD) was 0.82 (0.09), sDSC 0.80 (0.17) and HD95 5.8 mm (2.9 mm), and for CTVelec 0.82 (0.04), 0.84 (0.10) and 5.8 mm (2.2 mm), respectively, indicating that deformed CTV structures on CBCT are geometrically acceptable. Nine of fifteen patients did not meet the clinical goal for CTV V95% (range: CTVprim 81.4–100%, CTVelec 80.3–100%; Fig. 1), and seven of these also failed the Δ D98% criterion ( Δ D98% [+1SD]: − 5.0% [6.5%] and − 3.2% [3.9%], respectively). Five patients exceeded PTV constraints, while D2%, OAR Dmax, and Dmean aims were never violated. Agreement between workflows was observed in twelve of fifteen patients; in three cases, manual review identified adaptation needs not flagged by the semi-automatic approach (false negatives). Figure 2 illustrates anatomical and dosimetric differences between pCT and cCBCT.
Conclusion: Integrating quantitative evaluation on cCBCTs proved feasible and aligned well with clinical decisions from the manual workflow. The semi-automatic approach provides objective decision support for offline plan adaptation, offering a promising approach to automate daily assessment for streamlining the
triggering of offline plan adaptation. Keywords: triggered offline adaptation
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