A patch-based bolus technique for total scalp irradiation: dosimetric impact of conformity and reproducibility

Total scalp irradiation (TSI) for scalp angiosarcoma remains technically challenging because of the large, highly curved target geometry and the need for reliable bolus application. This study aimed to develop a practical method to improve bolus conformity using commercially available bolus and to evaluate the dosimetric impact of bolus reproducibility in volumetric-modulated arc therapy (VMAT)-based TSI. An anthropomorphic head phantom was used to simulate TSI conditions. A three-segment 'patch bolus' method was developed to enhance conformity and setup stability. Treatment plans were generated using HyperArc with identical optimization parameters for three conditions: virtual bolus, single-sheet bolus and patch bolus. To assess reproducibility, the patch bolus was reapplied and rescanned 10 times, and dose distributions were recalculated on each dataset after rigid registration to isolate bolus-related variations. The patch bolus method substantially reduced air gaps compared with the single-sheet approach and achieved target coverage comparable to the virtual bolus condition. In contrast, the single-sheet bolus resulted in inferior coverage in regions with poor contact. Repeated application of the patch bolus demonstrated high reproducibility, with variations within 2% for most dose-volume metrics. Although relative variations exceeding 5% were observed for the lens, absolute doses remained low and clinically negligible. These findings indicate that VMAT-based TSI is relatively robust against minor bolus variations. The proposed patch bolus method provides improved conformity, reproducibility and workflow efficiency, representing a simple and clinically feasible strategy for TSI using external beam radiotherapy.