Evaluation of respiratory motion robustness of expanded-field VMAT in breast radiotherapy using dynamic phantom film dosimetry

Respiratory motion can displace the superficial breast target into the beam penumbra or beyond the planned multileaf collimator aperture, causing target-edge underdosing. Expanded-field volumetric modulated arc therapy (E-VMAT) creates a superficial and tangential aperture reserve, but its performance during continuous motion has not been directly evaluated using radiochromic film dosimetry. This study evaluated whether E-VMAT reduces motion-related underdosing at the superficial anterior edge of the planning target volume in breast radiotherapy. Hybrid intensity-modulated radiotherapy (H-IMRT), non-expanded VMAT (N-VMAT), and E-VMAT plans were generated using identical target contours, prescriptions, and primary optimization objectives. An anthropomorphic thorax phantom with a patient-specific 3D-printed breast attachment was mounted on an in-house three-dimensional motion platform and irradiated under three predefined anteroposterior surface-normal motion conditions. Each plan-motion combination was measured in triplicate at five film planes. Dose degradation was assessed using 3%/3 mm gamma loss, A95 loss, Cold95 area, 95% isodose retreat-equivalent distance, mean-dose retention, and a descriptive protection index. Under moderate and extreme motion, N-VMAT gamma loss was 24.6% ± 10.2% and 53.9% ± 14.6%, while A95 loss was 33.1% ± 24.4% and 73.7% ± 31.3%, respectively. E-VMAT maintained gamma loss at ≤9.3% and A95 loss at ≤11.9% across all conditions. Relative to N-VMAT, E-VMAT reduced A95 loss by 83.8%–86.0% and gamma loss by 82.7%–95.7% under moderate and extreme motion. E-VMAT reduced but did not eliminate respiratory motion-induced superficial target-edge dose degradation. Patient-specific volumetric four-dimensional validation remains necessary before clinical implementation.