Pseudo skin flash on VMAT in breast radiotherapy: Optimization of virtual bolus thickness and HU values

Purpose: Optimisation strategies for volumetric modulated arc therapy (VMAT) in most treatment planning systems for breast cancer do not account for patient positioning, breathing, or anatomical changes. To overcome this limitation, a pseudo-skin flash strategy using a virtual bolus has been proposed. Using this strategy, we determined optimal thickness and value of Hounsfield units (HU) assigned to the virtual bolus to ensure adequate CTV irradiation. Materials and methods: We modified the original computed tomography data (CT0) by adding combinations of thicknesses and densities of a virtual bolus on PTVs (CT') of seven bilateral breast cancer patients. Using a single optimization objective template, we obtained a VMAT plan on CT' and recalculated this on the CT0. Optimal CT' parameters were defined as those that minimized dose differences between CT' and CT0 plans regarding PTV and OAR dose-volume parameters. We studied bolus parameters regarding robustness by shifting the isocenter 5 and 10 mm in the breathing direction for each CT0 plan. Results: The minimal dosimetric impact was between - 400 and - 600 HU depending on bolus thickness. OARs doses were not significantly affected. Best robustness was found for - 500 HU and 15 mm bolus thickness against shifts of up to 10 mm in the breathing direction. Conclusion: Our results support a bolus thickness equal to the CTV-PTV margin plus 5 mm and a virtual bolus HU value around - 500 and - 400 depending on the bolus thickness chosen. These findings could play a useful role in maximising robustness and minimising the need for plan renormalization.