Room: Karl Dean Ballroom A1
Purpose: To evaluate the performance of dual-layer MLC system for single-isocenter-multi-target SRS by comparison with other planning/delivery techniques.
Methods: Five patients treated with single isocenter multi-target SRS were retrospectively studied. Each patient had 6-9 treatment targets with volumes from 0.13 to 8.36 cc, and Rx from 15-24 Gy depending on target volume. The Halcyon delivery system has only coplanar delivery mode, and a dual-layer MLC with 1-cm leaf width. For each patient, three VMAT plans were generated with different MLC modulation resolution and beam arrangements: Halcyon-1cm-coplanar, Truebeam-0.25cm-coplanar, and Truebeam-0.25cm-noncoplanar. Coplanar plans had 2-3 arcs. The 5-arc Truebeam-0.25cm-noncoplanar plans were set as baseline in dosimetric comparison. All same-case plans were generated following the same planning protocol using 6X-FFF energy and normalized to 99% of any target receiving â‰¥100% Rx. Conformity-index (CI_RTOG), V12Gy, and brain mean dose (Brain_Dmean) were compared.
Results: All plans met clinical constraints for critical structures. CI_RTOG ranged from 1.03-2.37 for all plans. For target with diameter < 1cm, coplanar plans had inferior CI_RTOG compared to 5-arc Truebeam-0.25cm-noncoplanar baseline plans. Increases in CI_RTOG were: 0.27Â±0.29 for Halcyon-1cm-coplanar; and 0.13Â±0.21 for Truebeam-0.25cm-coplanar. For target with diameter > 1cm, coplanar VMAT plans showed similar CI_RTOG compared to baseline, with minimal difference between Halcyon-1cm-coplanar plans and other types of VMAT plans. All coplanar VMAT plans had increased V12Gy compared to baseline: 22.8%Â±14.5% for Halcyon plans, and 15.9%Â±10.5% for Truebeam-0.25cm-coplanar plans. However, Halcyon plans had slightly reduced Brain_Dmean by 2.8%Â±8.0% compared to baseline, whereas Truebeam-0.25cm-coplanar plans saw a 9.2%Â±11% increase.
Conclusion: For targets with diameter > 1 cm, Halcyon's 1cm-wide dual-layer MLC is capable of producing similar conformity compared to 0.25cm HDMLC while slightly reducing Brain_Dmean. Coplanar VMAT plans had inferior V12Gy compared non-coplanar. 0.25cm-wide HDMLC maintained advantage in CI for smaller targets.