Room: Exhibit Hall | Forum 3
Purpose: Spine stereotactic body radiotherapy (SBRT) require treatment plans with steep dose gradients and tight limit to cord maximal dose. A new dual-layer staggered 1-cm MLC in Halcyon treatment platform has improved leakage, speed, and DLG, compared to Millennium (0.5cm) and High Definition (0.25cm) MLC leaves in the TrueBeam platform. In this study, we investigate the effect of different MLC designs on plan quality of spine SBRT treatments.
Methods: 5 patients previously treated with spine SBRT were replanned according to the NRG-BR-001 guidelines for all three MLC systems with a prescription of 3000cGy in 3 fractions, 6xFFF beam, 800cGy/MU dose rate, and 3-arc VMAT technique. All plans were normalized to 95% of the target volume covered by prescription dose. For each case, planning objectives were adjusted manually by experienced planners to generate optimal plans for each platform and kept the same for different MLCs within the TrueBeam platform
Results: All treatment plans were able to achieve adequate target coverage while meeting dosimetric constraints to the OARs set by NRG-BR001 guidelines. Plans optimized for the 0.25cm TrueBeam MLC showed the sharpest dose fall-off, as demonstrated by the smallest conformity index (CI ) at 1.06Â±0.03 and lowest homogeneity index (HI) at 0.21Â±0.04, compared to CI 1.10Â±0.04 and HI 0.25Â±0.06 for 0.25cm TrueBeam MLC, and CI 1.13Â±0.04 and HI 0.26Â±0.06 for Halcyon. Halcyon plans had higher global Dmax (119.1% Â±3.4%): on average 3.4% higher than 0.25cm-MLC TrueBeam plans, but similar to 0.5cm-MLC TrueBeam plans.
Conclusion: Even with the 1-cm MLC leaf width, the Halcyon MLC is capable of generating clinically acceptable spine SBRT plans, but inferior in terms of dose fall-off and conformity. Our study indicates MLC width leaf does impact maximum dose to OARs and plan quality; however, it is not a limitation in achieving acceptable plan quality for spine SBRT treatments.