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Session: Therapy General ePoster Viewing [Return to Session]

The Plan Complexity of VMAT for the Pelvic Region Of  Halcyon Linac Using Dual-Layer Multi-Leaf Collimator

Q Zhu*, Z Wang, B Yang, Y Liang, H Zhu, W Zhang, J Qiu, Peking Union Medical College Hospital, Beijing, BeijingCN,


PO-GePV-T-225 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: This study is aimed to evaluate the plan complexity of the volumetric modulated arc therapy (VMAT)on Halcyon platform equipped with dual-layer multi-leaf collimator (MLC) for pelvic treatment plans, and evaluate the correlation between conventional complexity metrics and the γ pass rate of dose verification for VMAT plan based on Halcyon platform.

Methods: Plan complexity of 41 VMAT plans from the pelvic region including cervix and rectum treatment sites were evaluated using the metrics of modulation complexity score for VMAT (MCSarc), small aperture score (SAS), and plan normalized monitor unit (PMU). MCS and SAS were computed independently for each MLC layer (proximal and distal) and were evaluated when weighted by proportions of the proximal and distal MLC contributing to the field shape at each CP. The threshold distances of SAS were defined as 2mm, 5mm, 10mm, 15mm respectively. Portal dosimetry (PD) verification was performed to analyze the γ pass rate of each plan. Then, Pearson’s correlation analysis was performed between γ pass rates and the complexity metrics.

Results: The average values of MCSarc, SAS(2mm), SAS(5mm), SAS(10mm), SAS(15mm), PMU were 0.279, 0.038, 0.053, 0.090, 0.140, 906.344, respectively. The MCSarc and γ pass rates for the 1%/1mm, 2%/1 mm criteria were moderately correlated(1%/1mm: r=0.530,p=0.001; 2%/1 mm: r=0.408, p=0.008).The PMU and γ pass rates for the 1%/1mm criterion were strongly correlated(r=−0.796,p=0.001). The correlation coefficients (and p-values) were−0.682 (0.000), −0.372 (0.017) between PMU and γ pass rates for 2%/1 mm, 2%/2mm criterion respectively. There were no correlations between SAS and γ pass rates.

Conclusion: Both MCSarc and PMU show statistically significant correlations with γ pass rates, which may help us to understand the performance of dual-layer MLC better. Furthermore, these metrics may help us to identify the more complex plan that does not easily pass dose verification.


Not Applicable / None Entered.


Not Applicable / None Entered.

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