Main Session
Sep
27
PQA 01 - Gastrointestinal Cancer and Central Nervous System
2094 - Evaluation of Single- vs. Dual-Arc Dynamic Collimator VMAT for Stereotactic Spine Radiosurgery Using a Modified Modulation Complexity Metric
Presenter(s)
Shu-Fan Wang, MS - National Yang Ming Chiao Tung University, Tinan City, Tainan
S. F. Wang1, P. W. Lee1, C. J. Wu1, and F. M. Hsu1,2; 1Department of Radiation Oncology, National Taiwan University Cancer Center, Taipei, Taiwan, 2Division of Radiation Oncology, Department of Oncology, National Taiwan University Hospital, Taipei, Taiwan
Purpose/Objective(s):
Stereotactic spine radiosurgery (SSRS) requires rapid dose fall-off and high plan modulation to meet stringent spinal cord constraints. The conventional modulation complexity score for VMAT (MCSv) accounts for aperture-area variability (AAV) and leaf-sequence variability (LSV). A recently proposed modified MCSv (nMCSv) includes a collimator rotation factor to quantify the impact of dynamic collimator rotation on plan complexity. A novel VMAT planning algorithm incorporating dynamic collimator rotation (VMATP) increases planning degrees of freedom, potentially reducing the number of arcs required for SSRS delivery. This study aimed to evaluate the effect of arc reduction in VMATP on plan complexity using nMCSv and to assess the relationship between complexity, plan quality, and delivery efficiency.Materials/Methods:
Twelve patients with cervical-to-lumbar spine metastases were replanned to 20 Gy in a single fraction using the Eclipse treatment planning system (v18.1, Varian Medical Systems). For each patient, single-arc VMATP (S-VMATP) and dual-arc VMATP (D-VMATP) plans were generated using seven static modulated ports per arc, with identical objectives and dose constraints; arc number was the only variable. All plans met the RTOG 0631 criteria and were normalized to ensure that 90% of the PTV received the prescription dose. Arc-reduction effects (S-VMATP vs D-VMATP) on plan complexity were quantified using the nMCSv, which increased with the degree of dynamic collimator modulation. Plan quality was evaluated using target coverage, spinal cord sparing, the Paddick conformity index (PCI), and the gradient index (GI). Treatment delivery time, optimization time, and total monitor units (MUs) were recorded as efficiency metrics. A two-tailed paired t-test was used to determine whether differences between S-VMATP and D-VMATP were significant (p = 0.05).Results:
The S-VMATP plan showed a lower average nMCSv than the D-VMATP plan (S-VMATP 0.31±0.05 vs D-VMATP 0.32±0.05, p=0.01), consistent with reduced dynamic collimator modulation. Dosimetric metrics were similar between S-VMATP and D-VMATP, including PTV D95% (S-VMATP 18.77±0.52 Gy vs D-VMATP 18.79±0.54 Gy), spinal cord D0.03cc (S-VMATP 11.53±0.48 Gy vs D-VMATP 11.55±0.51 Gy), PCI (S-VMATP 1.47±0.77 vs D-VMATP 1.48±0.78), and GI (S-VMATP 3.30±0.34 vs D-VMATP 3.29±0.33). With arc reduction, S-VMATP demonstrated lower plan complexity, leading to improved planning and delivery efficiency. S-VMATP reduced total MUs (S-VMATP 6497±843 MUs vs D-VMATP 6577±804 MUs, p<0.05), shortened treatment delivery time by 5% (S-VMATP 4.6±0.6 mins vs D-VMATP 4.9±0.5 mins, p<0.001), and reduced optimization time by 14% (S-VMATP 1.4±0.3 mins vs D-VMATP 1.6±0.4 mins, p<0.001).Conclusion:
The nMCSv may support the clinical adoption of single-arc VMATP for SSRS by demonstrating reduced plan complexity, comparable plan quality, and improved planning and delivery efficiency compared with dual-arc VMATP.