Validation of Reduced Planning Target Margins In a Prospective Clinical Trial for Stereotactic Accelerated Partial Breast Online Adaptive Therapy
Abstract
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.