Impact of MLC shape smoothing on VMAT plan complexity and agreement between planned and delivered dose

Sammanfattning: Background/Purpose: The aperture shape controller (ASC) was introduced in the Eclipse treatment planning system to reduce plan complexity by counteracting irregular multileaf collimator (MLC) shapes. This should lead to decreased difference between the planned and measured dose distribution. The ASC was investigated to examine if it could limit plan complexity so that the planned and measured dose distributions are more consistent with each other, without compromising plan quality. The different ACS levels were studied with the aim to find the most optimal level regarding both agreement between measured and planned dose as well as plan quality. Material and methods: Fifteen patients from three treatment sites; prostate, prostate including adjuvant lymph nodes (prostate lgl) and head and neck (H&N), were used in this study. A volumetric modulated arch therapy (VMAT) treatment plan using ASC level "Very low" was optimized for each patient. Six VMAT plans were then re-optimized for each of the patients using the same optimization objectives as for the original "Very low" plan, the only parameter changed was the ASC level. The plan quality was evaluated using homogeneity index (HI), conformity index (CI) and dose-volume histograms (DVH) parameters where the statistical significance was examined through Friedman tests and post hoc Wilcoxon tests. Two different complexity metrics, the modulated complexity score (MCSv) and the edge area metric (EAM), were calculated for each treatment plan. The agreement between planned and delivered dose distribution was evaluated through measurements on a Varian TrueBeam using a Delta4 phantom+ (ScandiDos AB). The correlation between the gamma analysis pass rate (3 % / 2 mm and 2 % / 2 mm) and plan complexity was analyzed using scatter plots and statistically investigated using Spearmans correlation tests. Result: Generally, only small differences were observed in plan quality between the different ASC levels. In general, a higher ASC level decreased the complexity, and this trend was most prominent for the EAM. All plans passed the clinical pass rate at SUS (90 % for 3 % / 2 mm). A correlation between both investigated pass rates and EAM (p = 0:000 and p = 0:000) and MCSv (p = 0:003 and p = 0:045) was detected for the prostate patients. No correlation was observed for the prostate lgl and H&N. Conclusions: In general, the plan complexity decreased without compromising plan quality for higher ASC levels. However, a better agreement between the planned and delivered dose was not found. An ASC level between "Very low" and "High" is optimal as the plan quality is not affected and the plan complexity is reduced for these levels.