Main Session
Sep
29
PQA 05 - Physics
2960 - Dosimetric Impact of Dynamic Collimator Rotation In Hippocampal-Avoidance Whole-Brain Radiotherapy: Comparison with Static Collimator VMAT and Helical Tomotherapy
Presenter(s)
Chih-Kai Chang, MS - National Taiwan University Hospital, Taipei, Taipei
C. K. Chang1, S. H. Lu1, H. T. Lan1, S. H. Kuo1,2, and C. W. Wang1; 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):
To investigate the dosimetric advantages of dynamic collimator rotation during volumetric modulated arc therapy (VMAT) for hippocampal avoidance whole-brain radiotherapy (HA-WBRT). Performance of VMAT with dynamic collimator rotation and static angle modulated ports (VMATp) was compared with static collimator VMAT (VMATs) and helical tomortherapy (HT).Materials/Methods:
We retrospectively analyzed eight patients treated with HA-WBRT (30 Gy in 12 fractions) via HT. For each patient, two additional VMAT strategies were re-planned: VMATp (fully optimized collimator angles, one static beam with arc dominant weighting) and VMATs (fixed collimator angles), both using four 6MV coplanar arcs. Plans were normalized to match the PTV coverage of the original HT plan and optimized using identical DVH criteria to ensure fair comparison. We analyzed PTV coverage, homogeneity index (HI), and organs-at-risk (OARs) according to RTOG 0933 guidelines. Treatment efficiency was evaluated via total monitor units (MU) and estimated delivery time. Paired t-tests were used for statistical analysis.Results:
Regarding the hippocampus, VMATp demonstrated superior sparing, significantly reducing Dmax by 11.7% (14.32 ± 0.31 vs. 16.00 ± 0.77 Gy; P < 0.001) and D100% by 3.3% (7.78 ± 0.17 vs. 8.03 ± 0.39 Gy; P = 0.039) compared to VMATs. Notably, VMATp achieved hippocampal sparing comparable to HT (14.69 ± 1.17 and 7.73 ± 0.80 Gy; P > 0.05), meeting RTOG 0933 criteria. For the optic pathway within the PTV, VMATp showed a statistically significant but clinically minor reduction in Dmax compared to VMATs (0.8%; 31.56 ± 0.63 vs. 31.83 ± 0.44 Gy; P < 0.05). In terms of target quality, VMATp offered superior homogeneity, reducing HI by 9.7% compared to VMATs (0.11 ± 0.03 vs. 0.12 ± 0.03; P = 0.008) and outperforming HT (0.13 ± 0.06). Regarding PTV coverage, VMATp reduced D2% by 0.86% vs. VMATs (32.24 ± 0.15 vs. 32.51 ± 0.21 Gy; P < 0.001) and increased D98% by 3.67% vs. HT (28.72 ± 0.89 vs. 27.67 ± 1.51 Gy; P = 0.01). Efficiency analysis revealed that VMATp reduced MUs by 3.55% vs. VMATs (803 ± 21 vs. 832 ± 23 MU; P = 0.003). While delivery times were similar between VMATp and VMATs (5.05 ± 0.27 vs. 4.91 ± 0.04 min), both were approximately 77.3% faster than HT (22.27 ± 2.49 min), potentially minimizing intrafraction motion.Conclusion:
Dynamic collimator rotation introduces an additional degree of freedom to VMAT, allowing for superior dose shaping around the hippocampus. VMATp significantly improves hippocampal sparing and target homogeneity compared to VMATs, achieving a dosimetric profile comparable to, and in some aspects superior to, HT. Balancing quality and efficiency, VMATp represents a promising treatment option for HA-WBRT.