Evaluating a Novel Hybrid Planning Technique for Complex Cases in LINAC-Based Stereotactic Radiosurgery

Objectives: : SRS and fSRS are increasingly prevalent treatment options for patients with multiple brain metastases. Dynamic conformal arcs (DCA) and VMAT are two widely-used techniques for LINAC-based SRS planning with single-isocenter. Recently, a novel hybrid planning technique (combining the strengths of both VMAT and DCA in a single plan has become available. This innovative technique aims to optimize the SRS dosimetric outcomes, potentially benefitting the challenging cases posed by large irregularly shaped cavities or a target near/inside the brainstem in the midst of multiple other metastases, thereby improving the overall plan quality. In this study, we conducted a comprehensive dosimetric comparison between DCA, VMAT, and Hybrid for SRS/fSRS with multiple brain metastases including a large cavity or a target near/inside the brainstem.

Methods: : Ten patients (a total of 73 PTVs, five patients with a large cavity, and five with a target near/inside the brainstem) treated with SRS/fSRS were retrospectively planned with DCA, VMAT, and Hybrid technique by an experienced planner following institutional planning standards. All plans used a 6MV flattening-filter-free beam on Varian Edge® equipped with HD-MLC. DCA used double-pass 5 non-coplanar arcs and VMAT used double-pass 4 non-coplanar arcs. Hybrid used double-pass 5 non-coplanar arcs with 3 additional VMAT arcs at existing couch angles for treating the cavity. 3 to 5-fraction prescriptions with 21 to 35Gy were used and plans were normalized to 95% of PTV receiving the prescription dose. Plans using the three techniques were compared in total MU and plan quality evaluated using the Paddick Conformity Index, gradient index, normal brain volume receiving >X Gy (Vx), and organ-at-risk (OAR) maximum point (0.035cc) dose.

Results: : All plans met clinical dose constraints for OARs. Overall, VMAT plans demonstrated superior conformity compared to DCA and Hybrid plans. DCA plans generally exhibited superior gradient index compared to Hybrid and VMAT plans. Specifically, Hybrid plans exhibited significantly improved conformity for the target-applied VMAT technique compared to DCA plans (0.86 vs. 0.76 on average). Furthermore, Hybrid plans showed more favorable normal brain parameters compared to DCA plans (for all dose ranges) and VMAT plans (for low dose ranges below 10Gy).

Conclusion(s):: This study suggests that the Hybrid planning technique is a viable option for multiple brain metastasis cases involving a large cavity or a target near/inside the brainstem, offering enhanced conformity for challenging targets compared to DCA alone and reducing normal brain exposure compared to DCA and VMAT.