3047 - Dosimetric Advantages of VMAT Using Dynamic Multi-Leaf Collimation for Hippocampal-Avoidance Whole-Brain Radiotherapy: A Two-Arc Solution

12:30pm - 01:45pm ET

Poster Hall - Exhibit Hall A

Screen: 28

POSTER

Presenter(s)

Haoting Lan, MS - National Taiwan University Hospital, Taipei, Taipei

H. Lan¹, S. H. Lu¹, C. K. Chang¹, C. W. Wang¹, and S. H. Kuo²; ¹Division of Radiation Oncology, Department of Oncology, National Taiwan University Hospital, Taipei, Taiwan, ²Graduate Institute of Oncology, National Taiwan University College of Medicine, National Taiwan University, Taipei, Taiwan

Purpose/Objective(s): Hippocampal-avoidance whole-brain radiotherapy (HA-WBRT) requires complex arc configurations to achieve the steep dose gradients necessary for hippocampal sparing. VMAT_P allows for continuous, real-time collimator rotation during arc delivery, whereas static collimator angles per arc restrict traditional volumetric modulated arc (VMAT). This study investigates whether the improved beam-shaping flexibility provided by VMAT_P enables a streamlined two-arc strategy to provide superior organs-at-risk (OAR) protection and treatment efficiency during planning for HA-WBRT compared with a multi-arc VMAT.

Materials/Methods: Ten patients with brain metastases were analyzed to compare two strategies: (1) VMAT: 4 coplanar and 2 non-coplanar arcs; (2) VMAT_P: 2 coplanar arcs with dynamic collimator rotation optimization, supplemented by 3 static beams (arc-dominant setup). All plans (30 Gy at 10 fractions) were generated using a commercial treatment planning system and normalized to 95% planning target volume (PTV) coverage following RTOG 0933 criteria. Dosimetric parameters for the hippocampus, optic chiasm, ears, parotid glands, and anterior out-of-field regions were evaluated. Treatment efficiency was assessed via beam-on time and monitor units (MU). Statistical significance was determined using a paired t-test and Cohen’s d_z for effect size.

Results: VMAT_P plans met all RTOG 0933 constraints using only two arcs. Compared to VMAT, VMAT_P significantly reduced the hippocampal D_100% (VMAT_P 7.68Gy±0.45 vs. VMAT 8.35±0.92, p=0.031, Cohen’s d_z 0.81) and V10Gy (VMAT_P 63.67±19.10 vs. VMAT 72.9±17.29, p=0.024, Cohen’s d_z 0.86). Mean hippocampal dose showed a decreasing trend (3.7% reduction, d_z = 0.69). While achieving superior hippocampal sparing, VMAT_P plans caused a slight 1.0% increase in optic chiasm D_max compared to VMAT (p=0.001, d_z 1.55), though all values remained well within clinically acceptable limits (<37.5 Gy). Furthermore, VMAT_P's dynamic collimator rotatioin significantly suppressed out-of-field doses and diminished the mean dose to the anterior facial region (outside of PTV) (VMAT_P 5.69Gy±1.25 vs. VMAT 6.7±1.3, p <0.001, d_z =2.35). Crucially, VMAT_P improved efficiency by reducing total MUs by 12% (1010±66 vs.1142±156 MU, p <0.001) and the estimated treatment time by 60% (2.6±0.1 vs. 6.5±0.07 min, p <0.001).

Conclusion: Compared to more intricate multi-arc VMAT setups, HA-WBRT using the VMAT_P technique offers far lower out-of-field doses and better hippocampus sparing. The 60% delivery time reduction and reduced MU usage position VMAT_P as an efficient approach for managing patients with brain metastases receiving HA-WBRT while maximizing clinical productivity and patient comfort, even though the increase in optic chiasm dosage is clinically insignificant.