ESTRO 2025 - Abstract Book

S1414

Clinical - Lung

ESTRO 2025

matching, shows that the tumor was within the ITV+3mm contour in all analysed breathing phases. The scoring of the overall treatment quality, based on the post-treatment 4D-CBCT analysis is indicated in Table 1.

Discussion 93% of the patients scored an overall treatment quality of sufficient or better, despite noticeable intra-fraction motion registered in 28% of the patients. The post-treatment imaging moment makes the resulting quality-scoring a “worst-case” indication. The definition of sufficient in the scoring matrix, in which the tumor is outside the ITV+3mm contour only for a brief moment, corresponds to the mid-ventilation treatment concept philosophy (2), with its foremost argument being the non-infinite-steep dose fall-off outside the PTV. Conclusion: This study demonstrates that in ≥93% of the patients, the overall treatment quality was scored as sufficient or better when applying the reduced 3mm ITV-PTV margin, based on a visual inspection of all breathing phases in post-treatment 4D-CBCT images of all fractions of 29 NSCLC patients and is therefore in line with Bellec et al. (1). References: (1) Bellec J, Arab-Ceschia F, Castelli J, Lafond C, Chajon E. ITV versus mid-ventilation for treatment planning in lung SBRT: A comparison of target coverage and PTV adequacy by using in-treatment 4D cone beam CT. Radiation Oncology [Internet], 2020 Mar 3; 15(1):1–10. (2) Wolthaus J, Sonke J, Van Herk M, Belderbos J, Rossi M, Lebesque J, Damen E, Comparison of different strategies to use four-dimensional computed tomography in treatment planning for lung cancer patients, Int. J. Radiation Oncology Biol. Phys., Vol. 70, No. 4, pp. 1229–1238, 2008 Keywords: ITV, margin, 4D-CBCT