A Practical Distributed Virtual Bolus Strategy for Motion-Robust Superficial Dose Delivery in Breast Volumetric...

Maintaining adequate superficial dose coverage in breast volumetric modulated arc therapy (VMAT) remains challenging under interfractional motion and anatomic variation, largely due to the limited robustness of conventional skin flash strategies. This study evaluates the limitations of commonly used approaches and proposes a practical distributed virtual bolus strategy to improve motion robustness without dose escalation.

Six flash strategies-no flash, virtual target expansion with water- or air-equivalent density, and 3 virtual planning bolus configurations (single, dual, and triple)-were evaluated in 12 left-sided breast VMAT plans. The proposed triple virtual planning bolus strategy distributed discrete bolus segments along the anterior-lateral breast contour to maintain angular optimization support during arc delivery. Robustness was assessed using deformation-based motion simulations with displacements up to 15 mm. Superficial dose was evaluated using 3-mm and 5-mm skin shells, and target robustness was quantified using dose-volume metrics, relative coverage retention, and high-dose penalty reduction.

All strategies achieved comparable nominal plan quality. Virtual target expansion with water-equivalent density produced the largest superficial hotspot burden and pronounced instability after density removal (planning clinical target volume 2 [PCTV2] V107%: 42.99% ± 11.57%). Conventional planning bolus strategies showed direction-dependent robustness loss under lateral and composite motion. In contrast, the triple virtual planning bolus strategy maintained stable superficial coverage across all simulated scenarios, preserving PCTV2 V95% above 97% for displacements up to 15 mm while reducing high-dose penalty by up to 56.59% relative to virtual target expansion.

A distributed virtual bolus strategy enables motion-robust superficial dose delivery in breast VMAT while limiting high-dose escalation. This approach provides a practical and physics-informed solution for improving motion tolerance in arc-based breast radiation therapy without increasing planning complexity.