Measurement-Based Evaluation of the Robustness of Virtual Bolus–Generated Skin Flash In Breast Volumetric Modulated Arc Therapy
Poster
aapm2026_RY_20260701.pdf Hosted by AAPM · 1.2 MBAbstract
Purpose
This study aimed to quantitatively investigate the robustness of virtual bolus (VB)–generated skin flash under respiratory motion in breast volumetric modulated arc therapy (VMAT) using measurement-based dosimetry.
Methods
A custom-fabricated anthropomorphic phantom representing an Asian female was utilized. To generate the planning target volume (PTV), the clinical target volume, defined as the whole breast, was expanded by 5 mm. A VB structure was created by expanding the PTV by 5 mm beyond the body surface, leading to a skin-flash optimization region extending up to 10 mm outside the external contour. Three VB densities (0.4, 0.7, and 1.0 g/cm³) were assessed. VMAT plans were optimized employing two treatment planning systems: RayStation (RaySearch Laboratories) and Eclipse (Varian Medical Systems). Three VB-optimized plans were used for each TPS, yielding a total of six plans. All plans were optimized to meet identical clinical goals. Gafchromic EBT4 films, with the phantom placed on a motion platform, were used for dosimetric measurements. Under static conditions as well as respiratory motion modeled as a sinusoidal waveform with 5-mm amplitude in the anterior–posterior direction, measurements were repeated three times for each plan. Gamma analysis (3%/2 mm) was conducted to evaluate plan robustness.
Results
Gamma pass rates under static conditions were approximately 90% for all plans with minimal inter-plan variation. Gamma pass rates under respiratory motion for RayStation plans were 81.3%, 89.9%, and 91.4% for 0.4, 0.7, and 1.0 g/cm³ VB densities, respectively. Corresponding values for Eclipse plans were 80.9%, 82.5%, and 85.7%. Higher VB densities were associated with increased gamma pass rates under motion for both TPSs.
Conclusion
Measurement-based evaluation revealed that the robustness of VB-generated skin flash under respiratory motion is strongly dependent on VB density. These results indicate that the appropriate selection of VB density is critical for achieving motion-robust dose delivery.