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Robustness Evaluation of Physical versus Virtual Bolus for Breast VMAT Using RegGAN-Based Synthetic CT
Yong Sang1, Lingke Kong2, Jianan Wu1
1Department of Radiation Oncology National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital & Shenzhen Hospital Chinese Academy of Medical Sciences and Peking Union Medical College Shenzhen China.
Precision Radiation Oncology
|August 13, 2026
Summary
Virtual bolus (VB) offers slightly better target dose coverage robustness than physical bolus (PB) in volumetric modulated arc therapy (VMAT) for breast cancer with skin involvement, despite minor differences.
Area of Science:
- Radiation Oncology
- Medical Physics
- Deep Learning in Medical Imaging
Background:
- Volumetric modulated arc therapy (VMAT) for breast cancer with skin involvement necessitates bolus use for adequate surface dose.
- Both physical bolus (PB) and virtual bolus (VB) are established, but their dosimetric robustness against interfraction variations is understudied.
- Deep learning-based synthetic CT (sCT) was employed to assess the robustness of PB versus VB strategies.
Purpose of the Study:
- To evaluate the dosimetric robustness of physical bolus (PB) and virtual bolus (VB) strategies in VMAT for breast cancer with skin involvement.
- To compare the impact of interfraction anatomical variations on target coverage and organ-at-risk sparing for both bolus methods.
- To utilize deep learning-generated synthetic CT (sCT) for robust dosimetric analysis.
Main Methods:
- Retrospective analysis of 10 breast cancer patients treated with VMAT and skin involvement.
- Generation of two treatment plans per patient: planpb (physical bolus) and planvb (virtual bolus).
- Conversion of pretreatment cone beam CT (CBCT) images from fractions 1, 6, and 11 to sCT using a registration-based generative adversarial network (RegGAN) for robustness evaluation.
Main Results:
- No significant initial differences in target coverage or OAR sparing between PB and VB plans.
- Virtual bolus (planvb) demonstrated statistically superior robustness for Vpd (P < 0.05) on sCT, with a small mean deviation improvement (-1.5% vs -0.8%).
- Physical bolus (planpb) exhibited better robustness concerning the conformity index (CI).
Conclusions:
- The RegGAN-based sCT framework confirms comparable dosimetric performance between PB and VB strategies.
- Virtual bolus offers marginally improved robustness in target dose coverage against daily anatomical variations.
- Physical bolus demonstrated superior robustness for CI, suggesting a trade-off between different dosimetric parameters.
