Plan complexity quantification of dual-layer multi-leaf collimator for volumetric modulated arc therapy with Halcyon linac.

This study characterized the plan complexity of the dual-layer multi-leaf collimator (MLC) performance for volumetric modulated arc therapy (VMAT) in Halcyon. Plan complexity metrics were computed independently for each MLC layer (proximal and distal): modulation complexity score (MCS), plan averaged beam area, irregularity, and modulation (PA, PI, and PM) were obtained. These were investigated when weighted by proportions of the effective proximal MLC and distal MLC (MCSw, PAw, PIw, and PMw). Conventional plan complexity metrics for the effective 5.0 mm resolution MLC (MCS5, PA5, PI5, PM5), small aperture score (SAS), effective distal MLC score (EDS), and MU were also evaluated. Forty-five consecutive VMAT plans were analyzed retrospectively for treatment sites of the prostate, head and neck (HN), and other parts. The mean values of the metrics for proximal MLC and distal MLC were 0.310 and 0.245 (MCS), 71.09 cm2 and 55.92 cm2 (PA), 6.24 and 10.19 (PI), and 0.593 and 0.645 (PM), respectively. The degree of plan complexity in the distal MLC was larger than in the proximal MLC. The percentage of the effective distal MLC was higher than that of the proximal MLC (mean EDS = 0.555). The MCSw, PAw, PIw, and PMw showed significant differences between the prostate and HN groups like the PA5, PI5, and PM5; however, the MCS5 did not. Additionally, the MCS5 correlated with MU and PM5, while the MCSw correlated with SAS and PA5. The plan complexity for each MLC layer and subsequently combining these complexities might be useful to better understand the dual-layer MLC performance.