Measurement-Based Evaluation of the Robustness of Virtual Bolus–Generated Skin Flash In Breast Volumetric Modulated Arc Therapy
Abstract
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.