Safety and benefit of using a virtual bolus during treatment planning for breast cancer treated with arc therapy

Abstract

Purpose: This study evaluates the benefit of a virtual bolus method for volumetric modulated arc therapy (VMAT) plan optimization to compensate breast modifications that may occur during breast treatment. Methods: Ten files were replanned with VMAT giving 50 Gy to the breast and 47 Gy to the nodes within 25 fractions. The planning process used a virtual bolus for the first optimization, then the monitors units were reoptimized without bolus, after fixing the segments shapes. Structures and treatment planning were exported on a second scanner (CT) performed during treatment as a consequence to modifications in patient's anatomy. The comparative end-point was clinical target volume's coverage. The first analysis compared the VMAT plans made using the virtual bolus method (VB-VMAT) to the plans without using it (NoVB-VMAT) on the first simulation CT. Then, the same analysis was performed on the second CT. Finally, the level of degradation of target volume coverage between the two CT using VB-VMAT was compared to results using a standard technique of forward-planned multisegment technique (Tan-IMRT). Results: Using a virtual bolus for VMAT does not degrade dosimetric results on the first CT. No significant result in favor of the NoVB-VMAT plans was noted. The VB-VMAT method led to significant better dose distribution on a second CT with modified anatomies compared to NoVB-VMAT. The clinical target volume's coverage by 95% (V95%) of the prescribed dose was 98.9% [96.1–99.6] on the second CT for VB-VMAT compared to 92.6% [85.2–97.7] for NoVB-VMAT (P = 0.0002). The degradation of the target volume coverage for VB-VMAT is not worse than for Tan-IMRT: the median differential of V95% between the two CT was 0.9% for VMAT and 0.7% for Tan-IMRT (P = 1). Conclusion: This study confirms the safety and benefit of using a virtual bolus during the VMAT planning process to compensate potential breast shape modifications.