3086 - A Dosimetric Comparison of Standard vs. Novel VMAT Incorporating Dynamic Collimator Rotation and Static Angle Modulated Ports for Spine SBRT

Purpose/Objective(s): In spine stereotactic body radiation therapy (SBRT), the close proximity of the target to the spinal cord necessitates a steep dose gradient and the minimization of intrafractional error. A novel VMAT technique (VMATp) incorporating dynamic collimator rotation and static angle modulated ports (STAMPs) in a single continuous delivery has the potential to generate such steep dose distributions within a short delivery time. However, dosimetric data validating this approach currently remain lacking. This study aimed to investigate the dosimetric benefits of VMATp compared to standard VMAT for spine SBRT.

Materials/Methods: Twenty patients without spinal hardware who previously underwent spine SBRT were randomly selected for this retrospective analysis, with stratification across the cervical, thoracic, and lumbar spine. For each patient, two treatment plans were generated: VMATp and standard VMAT using 10 MV flattening filter-free (FFF) beams. The prescription dose was 24 Gy in 2 fractions, with the maximum dose within the planning target volume (PTV) constrained to < 170%. Standard VMAT plans utilized three full arcs, whereas the VMATp plans employed a single arc integrated with five STAMPs. To ensure an unbiased comparison, identical optimization parameters were applied to both techniques. We evaluated the achievement rates of various OAR constraints (e.g., D_0.03cc < 17 Gy for the spinal cord planning risk volume [PRV: spinal cord + 2 mm]). Plan quality was assessed using PTV D_95%, GTV D_98%, Paddick conformity index, D_0.03cc for the spinal cord PRV and thecal sac, and delivery time. Statistical differences between the two techniques were analyzed using a paired t-test, with statistical significance defined as p < 0.05.

Results: The OAR constraint achievement rate was 100% for VMATp, compared to 75% for standard VMAT. Furthermore, VMATp reduced delivery time by an average of 20% compared to standard VMAT, while significantly improving both target coverage and dose conformity. A detailed comparison of the dosimetric parameters is summarized in Table 1.

Conclusion: For spine SBRT, VMATp demonstrated superior dosimetric plan quality and delivery efficiency compared to standard VMAT. VMATp significantly improved target coverage and dose conformity while ensuring complete adherence to strict OAR constraints. Additionally, the 20% reduction in delivery time may effectively mitigate the risk of intrafractional error. These dosimetric and operational benefits suggest that VMATp is a highly effective and clinically viable treatment strategy.