Abstract
Purpose: To evaluate the dosimetric performance and planning/delivery efficiency of a dual-layer MLC system for treating multiple brain metastases with a single isocenter.Materials and Methods: 10 patients each with 6–10 targets with volumes from 0.11 to 8.57 cc, and prescription doses from 15 to 24 Gy, were retrospectively studied. Halcyon has only coplanar delivery mode. Halcyon V1 MLC modulates only with the lower layer at 1 cm resolution, whereas V2 MLC modulates with both layers at an effective resolution of 0.5 cm. For each patient five plans were compared varying MLC and beam arrangements: the clinical plan using multi-aperture dynamic conformal arc (DCA) and non-coplanar arcs, Halcyon-V1 using coplanar-VMAT, Halcyon-V2 using coplanar-VMAT, HDMLC-0.25 cm using coplanar-VMAT, and HDMLC-0.25 cm using non-coplanar-VMAT. All same-case plans were generated following the same planning protocol and normalization. Conformity index (CI), gradient index (GI), V12Gy, V6Gy, V3Gy, and brain mean dose were compared.Results: All VMAT plans met clinical constraints for critical structures. For targets with diameter < 1 cm, Halcyon plans showed inferior CI among all techniques. For targets with diameter >1 cm, Halcyon VMAT plans had CI similar to non-coplanar VMAT plans, and better than non-coplanar clinical DCA plans. For GI, Halcyon MLC plans performed similarly to coplanar HDMLC plans and inferiorly compared to non-coplanar HDMLC plans. All coplanar VMAT plans (Halcyon MLC and HDMLC) and clinical DCA plans had similar V12Gy, but were inferior compared to non-coplanar VMAT plans. Halcyon plans had slightly reduced V3Gy and mean brain dose compared to HDMLC plans. The difference between Halcyon V1 and V2 is only significant in CI of tumors less than 1cm in diameter. Halcyon plans required longer optimization than Truebeam VMAT plans, but had similar delivery efficiency.Conclusion: For targets with diameter >1 cm, Halcyon's dual-layer stacked and staggered MLC is capable of producing similar dose conformity compared to HDMLC while reducing low dose spill to normal brain tissue. GI and V12Gy of Halcyon MLC plans were, in general, inferior to non-coplanar DCA or VMAT plans using HDMLC, likely due to coplanar geometry and wider MLC leaves. HDMLC maintained its advantage in CI for smaller targets with diameter <1 cm.
Highlights
Stereotactic radiosurgery (SRS) has gained substantial popularity in the radiation oncology community, especially in smaller clinics
Halcyon version 2 (V2) MLC showed improved conformity index (CI) for small targets compared to version 1 (V1) MLC
V2 MLC performed very compared to CIs achieved with HD MLC, which had twice finer modulation resolution perpendicular to leaf travel (0.25 cm for HD MLC vs. 0.5 cm for Halcyon V2 MLC)
Summary
Stereotactic radiosurgery (SRS) has gained substantial popularity in the radiation oncology community, especially in smaller clinics. It has become a standard and effective way to manage brain metastases for cancer patients [1,2,3,4]. There have been accumulated studies indicating that, for cancer patients with metastatic brain tumors, SRS provides effective and safe treatment compared to whole-brain radiation therapy (WBRT) alone, both as a monotherapy and combined with WBRT [5, 6]. Additional benefits of using SRS to control brain metastasis include substantial time and resource savings for the clinical care team, as well as preserving options of re-irradiation either to the remaining part of the brain or another part of the body. The use of volumetricmodulated arc therapy (VMAT) and high-definition multileaf collimators (HDMLC) enabled precise control of beam apertures to create conformal dose to the target while maintaining fast dose fall-off outside the target
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