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Impact of Monaco sequencing parameters on monitor units, plan quality, and optimization time for Elekta Unity liver
Bryan C Bates1, Guanghua Yan1, Amanda Schwarz1
1Department of Radiation Oncology, University of Florida, Gainesville, Florida, USA.
Background:
Magnetic resonance-guided radiation therapy (MRgRT) using the Elekta Unity MR-linac offers significant advantages for liver stereotactic body radiation therapy (SBRT). However, the Unity is limited by a low dose rate and the lack of volumetric-modulated arc therapy (VMAT), resulting in prolonged treatment times. Reducing monitor units per fraction (MU/Fx) by optimizing Monaco sequencing parameters may improve treatment efficiency.
Purpose:
To evaluate the impact of Monaco sequencing parameters on MU/Fx, plan quality, and optimization time for Unity liver SBRT.
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, minimum segment width (MSW), minimum segment area (MSA), and fluence smoothing (FS). The MU/Fx, optimization time, and estimated delivery time were recorded for each plan. Dosimetric and hotspot constraint compliance and 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 (3.4%-50.8% and 3.7%-43.2%, respectively) for all patients' clinically acceptable plans. Using a high FS yielded modest MU/Fx reductions (3.0%-17.6%) in eight patients. Minimum MU per segment and MSW showed 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 the largest planning target volumes (PTVs), optimization time decreased with fewer segments (1.1%-416%) and with increased MSA (22.5%-48.1%). Across all patients, optimization time decreased with increasing minimum MU per segment (3.0%-47.2%). Estimated delivery time strongly correlated with MU/Fx (R2 = 0.8278).
Conclusions:
Adjusting Monaco sequencing parameters-particularly lowering the maximum number of segments, increasing the MSA, or using a high FS-can reduce MU/Fx and treatment time while maintaining acceptable plan quality on a patient-specific basis.
