(Saturday, 4/4/2020) [Mountain Time (GMT-6)]
Purpose: Maximum leaf speed is a configurable parameter of MLC in treatment planning system. This study is to investigate its influence on the quality of VMAT plans.
Methods: Seven MLCs with different maximum leaf speeds (1.0, 1.5, 2.25, 3.5, 5.0, 7.5, 10 cm/sec) were configured for an accelerator in Pinnacle3 treatment planning system. Correspondingly, seven treatment plans with the identical initial optimization parameter were designed with mdaccAutoPlan system. Six nasopharyngeal carcinoma (NPC) patients and nine rectal cancer patients were selected, which represent complex and simple clinical circumstances. VMAT plan quality was evaluated with PlanIQTM software.
Results: The relative change of plan scores achieved by the seven configured accelerator with specific maximum MLC speed (MMLS) for each patient are studied. Two apparent trend of MMLS influence on VMAT plan scores were observed: Plan scores increase with MMLS; Plan scores increase rapidly when MMLS increases from 1 to 3.5, thus relative change of plan score decrease in these MMLS range. The stationary point of maximum MLC speed (MMSSP) is defined as a specific MMLS when the relative change of plan scores is firstly less than 5% as MMLS increase from 1.0 to 10. For rectum plan, MMSSPs are 2.25 for 6 patients while 3.5 for other 3 patients. For NPC plan, MMSSP is 3.5 for 5 patients and 2.25 for only one patient.
Conclusion: This work indicates MMLS direct influence on VMAT plan quality. VMAT plan quality was improved significantly as MMLS increased from 1 to 3.5, afterwards marginal improvement was observed.
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