Impact of Monaco Sequencing Parameters on Monitor Units, Plan Quality, and Optimization Time for Elekta Unity Liver SBRT Plans
Abstract
Purpose
To evaluate the impact of Monaco sequencing parameters on monitor units per fraction (MU/Fx), plan quality, and plan optimization time for liver SBRT treated on the Elekta Unity MR-linac.
Methods
Ten liver SBRT patients previously treated on the Unity were replanned. For each patient, 33 plans were generated by varying one of five sequencing parameters–maximum number of segments per plan, minimum MU per segment (MU/segment), minimum segment width (MSW), minimum segment area (MSA), and fluence smoothing (FS)–while holding the others constant. The MU/Fx, optimization time, and estimated delivery time were recorded for each plan. Dosimetric and hotspot constraint compliance, along with the RTOG 0915 conformality index (CI) and gradient index (RI), were used to assess plan quality.
Results
Reducing the maximum number of segments to no fewer than 30 and increasing the MSA produced the largest reductions in MU/Fx (10.2±4.8% and 19.8±9.9%, respectively) across all patients’ clinically acceptable plans. Using High FS yielded modest MU/Fx reductions (6.5±4.8%) in eight patients. Minimum MU/segment and MSW had negligible effects on MU/Fx among clinically acceptable plans. MSWs of 1.5 cm or greater resulted in degraded plan quality or clinically unacceptable plans. For the five patients with PTVs larger than 30 cm3, optimization times decreased with fewer maximum segments (47.3±29.0%) and with increased MSA (33.8±12.5%). Across all patients, optimization time decreased with increasing minimum MU/segment (26.2±15.8%). Estimated delivery time strongly correlated with MU/Fx (R2 = 0.8278).
Conclusion
Adjusting Monaco sequencing parameters–particularly lowering the maximum number of segments, increasing the MSA, using High FS, or increasing minimum MU/segment–offers a planning-level opportunity to improve treatment delivery times by reducing MU/Fx and/or plan optimization time while maintaining acceptable plan quality.