Dynamic Collimator Enhances SRS Plan Quality: Evaluation of Rapidarc Dynamic for Single-Isocenter Multi-Target (SIMT) Stereotactic Radiosurgery
Abstract
Purpose
RapidArc Dynamic (RAD) is a novel treatment delivery technique that combines rotational arc therapy with static angle modulated ports (STAMPs). This study evaluates the feasibility, dosimetric performance, and delivery efficiency of RAD for single-isocenter multi-target (SIMT) stereotactic radiosurgery (SRS), with comparison to the current standard linac-based VMAT.
Methods
Nineteen SIMT SRS cases (5–17 lesions per patient) were retrospectively studied using RAD in the Varian Eclipse™ treatment planning system (v18.1). Each plan consisted of 4-8 arcs with one non-contributing STAMP per arc (arc-dominant). Collimator rotation for both STAMP and arc components was automatically optimized. Dose calculation was performed using the Acuros XB algorithm. Plans were normalized to ensure that at least 99% of the target volume received the prescription dose. Dosimetric metrics, including conformity index (CI), homogeneity index (HI), and normal brain dose volumes (V60%, V30%, V25%, and V20%), were evaluated and compared between RAD and clinical VMAT plans. Delivery efficiency was assessed using total monitor units (MU) and simulated beam-on time. Statistical comparisons were performed using the paired Wilcoxon signed-rank test.
Results
RAD achieved significantly improved CI (1.43±0.12) compared to clinical VMAT 1.61±0.26 (p<0.05). For HI, RAD achieved 0.15± 0.02 while VMAT was 0.15 ± 0.04 (p=0.488). Mean brain V60% was 12.32cc for RAD and 11.92cc for VMAT. For RAD, V30% decreased from 73.55cc to 54.08cc, V25% decreased from 118.78cc to 86.53cc, V20% deceased from 207.23cc to 158.17cc. The plan mean MU demonstrated a 37.92% reduction for RAD (7223±2266) compared to VMAT (11634±5081). Simulated delivery showed that average delivery time was 418.84s for RAD and 568.85s for VMAT.
Conclusion
RapidArc Dynamic incorporating dynamic collimator motion, achieved plan quality comparable to standard VMAT for SIMT SRS while significantly reducing monitor units and delivery time. Dynamic collimation contributed to decreased low-dose normal brain exposure, potentially offering greater flexibility for clinical management, particularly in re-irradiation scenarios.