Impact of plan parameters on the dosimetric accuracy of volumetric modulated arc therapy

Purpose: To evaluate the effect of plan parameters on volumetric modulated arc therapy (VMAT) dosimetric accuracy, together with the possibility of scoring plan complexity. Methods: 142 clinical VMAT plans initially optimized using a 4 degrees control point (CP) separation were evaluated. All plans were delivered by a 6 MV Linac to a biplanar diode array for patient-specific quality assurance (QA). Local Gamma index analysis (3%, 3 mm and 2%, 2 mm) enabled the comparison between delivered and calculated dose. The following parameters were considered for each plan: average leaf travel (LT), modulation complexity score applied to VMAT (MCSv), MU value, and a multiplicative combination of LT and MCSv (LTMCS). Pearson's correlation analysis was performed between Gamma passing rates and each parameter. The effects of CP angular separation on VMAT dosimetric accuracy were also analyzed by focusing on plans with high LT values. Forty out of 142 plans with LT above 350 mm were further optimized using a finer angle spacing (3 degrees or 2 degrees) and Gamma analysis was performed. The average Gamma passing rates obtained at 4 degrees and at 3 degrees/2 degrees sampling were compared. A further correlation analysis between all parameters and the Gamma pass-rates was performed on 142 plans, but including the newly optimized 40 plans (CP every 3 degrees or 2 degrees) in place of the old ones (CP every 4 degrees). Results: A moderate significant (p < 0.05) correlation between each examined parameter and Gamma passing rates was observed for the original 142 plans at 4 degrees CP discretization. A negative correlation was found for LT with Pearson's r absolute values above 0.6, suggesting that a lower dosimetric accuracy may be expected for higher LT values when a 4 degrees CP sampling is used. A positive correlation was observed for MCSv and LTMCS with r values above 0.5. In order to score plan complexity, threshold values of LTMCS were defined. The average Gamma passing rates were significantly higher for the plans created using the finer CP spacing (3 degrees/2 degrees) compared to the plans optimized using the standard 4 degrees spacing (Student t-test p < 0.05). The correlation between LT and passing rates was strongly diminished when plans with finer angular separations were considered, yielding Pearson's r absolute values below 0.45. Conclusions: At 4 degrees CP sampling, LT, MCSv, and LTMCS were found to be significantly correlated with VMAT dosimetric accuracy, expressed as Gamma pass-rates. These parameters were found to be possible candidates for scoring plan complexity using threshold values. A finer CP separation (3 degrees/2 degrees) led to a significant increase in dosimetric accuracy for plans with high leaf travel values, and to a decrease in correlation between LT and Gamma passing rates. These results indicated that the influence of LT on VMAT dosimetric accuracy can be controlled by reducing CP separation. CP spacing for all plans requiring large leaf motion should not exceed 3 degrees. The reported data were integrated to optimize our clinical workflow for plan creation, optimization, selection among rival plans, and patient-specific QA of VMAT treatments. (C) 2013 American Association of Physicists in Medicine.