S2193
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
Mini-Oral 4706 Evaluation of the effect of rotations on coverage of nodal boosts in gynae SIB patients Robert Smyth 1,2 , Cathy Fleming 1 , Ciaran Malone 1,3 1 Physics, St. Luke's Raidation Oncology Network, Dublin, Ireland. 2 School of Physics, University College Dublin, Dublin, Ireland. 3 Radiotherapy, University Medical Centre Rotterdam, Rotterdam, Netherlands Purpose/Objective: Limited CBCT field-of-view (FOV) can prevent verification of distal nodal boosts in cervix SIB plans. As the distance from isocentre (diso) increases, small residual rotations create larger geometric displacements that may decrease coverage. The aim of this study was to quantify how patient rotations affect dose to boosted nodal targets, and how effects scale with nodal location, diso, rotation axis, and volume, to enable prediction of coverage loss when CBCT FOV constraints are present. Material/Methods: Five patients with locally advanced cervical cancer treated with VMAT SIB plans (Monaco v6.1.3, Rx: pelvis [45 Gy/25 fx] with SIB [55 Gy/25 fx]) were retrospectively analysed. CTs and structures were rigidly rotated about the isocentre by ±3°, ±5°, ±7° for pitch, roll, and yaw, giving 18 rotations per patient for a total of 90 datasets. This was achieved using an in- house Python script and plans recalculated without re- optimisation. Outcomes were assessed as the change in D98% of rotated plans compared to the original plan ( ∆ D98%) for each SIB node PTV. diso values were measured between the treatment isocentre and centroid of each nodal PTV. Δ D98% was plotted versus diso using linear-quadratic regression with axis and node volume as covariates. Δ D98% values are reported as mean ± one standard deviation. Results: Across 34 boosted nodes, clinically realistic rotations ( ≤ 5°) were associated with reduced coverage of distal nodes. Pitch and yaw produced a quadratic loss of coverage with increasing diso (R2 = 0.36, 0.44), while roll produced a weak linear correlation (R2 = 0.02). Reductions of Δ D98% up to 6.79 ± 1.17 Gy (12 ± 2.9%) for 5° setup error were estimated for nodes at diso of 163 mm, the distance between the edge of FOV and treatment field for the machines in this study. The distance of a given node from the axis of rotation was a strong indicator for change in dose coverage (Figure 1). Node volume independently modified risk: larger nodes exhibited greater absolute Δ D98%, while smaller nodes showed higher variability in ∆ D98% (Figure 2).
Conclusion: When distal nodal boosts are partially or completely outside CBCT FOV, residual rotations ≤ 5° can materially reduce PTV SIB coverage, scaling with diso for pitch/yaw. Uniform margins and standard IGRT protocols may be insufficient for these nodes. Findings support (i) 6DoF correction to minimize rotational residuals, (ii) distance-stratified margins or tolerances for nodal boosts, and (iii) more advanced set-up protocols such as SGRT. References: Louise Vagner Laursen et al. “Residual rotational set- up errors after daily conebeam CT image guided radiotherapy of locally advanced cervical cancer”. en.
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