S2255
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
changes, GTV volume, and maximum translational displacement. Results: A total of 65 GTVs were analysed. The median Δ D98% (Dsim − Dplan) was − 1.2% and Δ D95% was − 1.0%. Although differences were statistically significant (p < 0.001 in both metrics), the magnitude was small in most cases. A significant correlation was found between the maximum translational shift and Δ D98% ( ρ = − 0.41, p < 0.001), while GTV volume alone did not correlate with coverage loss (p = 0.97). Lesions with displacements > 3 mm (n = 3) showed median Δ D98% ≈ − 9%, and only one GTV (0.08 cm ³ ) presented Δ D98% > − 10% (Figure 1). Organs at risk showed negligible Dmax variation and no constraint violations, consistent with adapted clinical prescriptions near critical structures.
mL on average with compression (p<0.01). Statistically significant reductions in the left-right (p=0.04), anterior-posterior (p=0.04), and inferior-superior (p=0.01) iGTV dimensions were observed when using BPL1. No statistically significant differences between repeat scans with compression were measured for any metric. Conclusion: Unanimous reductions in iGTV dimensions demonstrate a significant motion management benefit when using pneumatic abdominal compression. Results were consistent between initial and repeat 4DCT scans with BPL1, indicating acceptable reproducibility. Forced shallow breathing with an indexed pneumatic abdominal compression system may allow for faster SABR delivery times while minimizing the treated area, achieving favorable dosimetric characteristics. Keywords: Abdominal compression, SABR, SBRT Dosimetric evaluation of patient position errors without intrafraction imaging in single-fraction SRS Sergi Serrano-Rueda, Jordi Saez, Carla Cases, Gabriela Antelo, Carme Ares, Cristian Candela-Juan Radiation Oncology, Hospital Clínic, Barcelona, Spain Purpose/Objective: To quantify the dosimetric impact of uncorrected 6D patient position errors that occur during couch rotation for non-coplanar, single-fraction cranial stereotactic radiosurgery (SRS) when intrafraction image guidance is not available. We aimed to determine the relationship between GTV coverage and (i) GTV volume, (ii) the magnitude of couch displacement. Material/Methods: Digital Poster 3731 Thirty-five single-fraction SRS cases (prescriptions: 12– 24 Gy, with a GTV-to-PTV margin of 1-1.5 mm) were retrospectively analysed. For each patient, translational (lat, long, vert) and rotational (pitch, roll, yaw) corrections recorded by the 6D IGRT system (ExacTrac Dynamic, Brainlab) were digitally applied to the planning CT using an in-house Python script. To simulate dose delivery without intrafraction correction, dose recalculation (Eclipse AAA, 1.25 mm) was performed per couch angle in the corresponding transformed CT and summed back on the original CT to obtain the simulated dose (Dsim). Dosimetric metrics (D95%, D98%, D2% for each GTV; Dmax for the closest OAR) were extracted and compared to the planned dose (Dplan) using the original contours. Statistical analysis included paired Wilcoxon tests (p < 0.05) and Spearman correlations between dosimetric
Conclusion: Simulated delivery without intrafraction imaging revealed measurable but generally small GTV underdosage. Potentially clinically relevant underdosage occurred only for small lesions combined with large (>2-3 mm) couch displacements. For such cases, either applying the corrections or increasing the GTV-to-PTV margin should be considered. Our current margin appears adequate for most cases despite the intrafraction deviations. The presented methodology provides a practical framework for institutions to evaluate whether their local PTV margins and setup tolerances sufficiently account for uncorrected rotational and translational patient displacements for multiple couch rotations. Keywords: intrafraction motion, SRS, couch rotations
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