Validation of Reduced Planning Target Margins In a Prospective Clinical Trial for Stereotactic Accelerated Partial Breast Online Adaptive Therapy
Poster
2026 AAPM Poster.pdf Hosted by AAPM · 1.3 MBAbstract
Purpose
Online adaptive radiotherapy (oART) mitigates interfraction motion and enables potential reduction of planning target volume (PTV) margins relative to standard-of-care approaches. This study evaluates the robustness of 2 mm PTV margins in a prospective stereotactic accelerated partial breast irradiation (APBI) clinical trial utilizing HyperSight-based CBCT-guided oART with surface-guided RT (SGRT).
Methods
Thirteen patients treated with APBI in five fractions (65 total fractions) on the RAD 1802 prospective clinical trial were retrospectively analyzed. All patients received free-breathing oART with a minimum of three CBCT acquisitions per fraction: a planning CBCT, a pre-delivery verification CBCT with positional correction (immediately prior to beam-on), and a post-treatment CBCT acquired immediately after beam-off (no correction applied). Two fractions per patient were selected for recalculation (n = 26/65), including re-contouring of normal structures and gross tumor volume (GTV), derivation of clinical target volumes (CTV), and adaptive plan dose recalculation on verification and post-treatment CBCTs. Fractions in which SGRT triggered re-imaging and mid-treatment re-alignment were also analyzed (n = 4). Target coverage metrics were assessed for CTVs and PTVs on the planning CBCT and for CTVs on recalculated plans.
Results
For plans recalculated on the verification and post-treatment CBCTs (n = 60), CTV coverage remained robust, with V100% ≥ 96.5% and D95% ≥ 101%. Coverage was superior to the PTV on the initial planning CBCT (V100% ≥ 90.6%, n = 30) and slightly reduced relative to the planning CTV (V100% ≥ 99.3%, n = 30). In contrast, unacceptable coverage was observed in 3 of 4 CBCTs acquired following SGRT-triggered motion events (CTV V100% < 95%).
Conclusion
These findings support the clinical feasibility of a 2 mm PTV margin for APBI delivered using CBCT-guided oART. Additionally, the results highlight the value of SGRT for detecting intrafraction motion and prompting timely re-imaging and realignment when utilizing tight margins.