162. A study on the correlation between plan parameters and gamma index analysis of Volumetric Modulated Arc Therapy.

Abstract Purpose To evaluate the effect of plan complexity parameters in patient-specific quality assurance (QA). Methods Evaluation of 902 clinical VMAT plans, optimized using a 3° control point separation, was achieved by local gamma index analysis (3%, 3 mm) between delivered and calculated dose. Confidence limit (CL) for 93.5% interval (p = 0.065) was evaluated too [1]. All plans were delivered by a 6MV Linac to a biplanar diode array for patient-specific QA. Plan complexity was investigated throughout different parameters: average leaf travel (LT), average in-field leaf travel (LTi) [2] , modulation complexity score applied to VMAT (MCSv) and a multiplicative combination of them (LTMCS) [3]. Pearson's correlation analysis was performed between gamma passing rates, CL and each parameter. Results 98.45% of plans was characterized by a gamma passing rates above 90%, while only 14 plans yielded values under the acceptance threshold. CL resulted to be more restrictive: 95,34% of plans satisfied the passing criteria, while 42 plans presented CL > 1. Nevertheless, gamma passing rates and CL showed a strong correlation (Pearson absolute value above 0.93). Plan complexity parameters underlined a moderate correlation with gamma passing rates, with an absolute value between 0.39 and 0.47. More deeply, a negative correlation was observed with LT, while MCS and LTMCS presented a positive correlation. CL showed similar results characterized by an absolute Pearson factor between 0.47 and 0.51. Results of Pearson analyses are summarized in Table 1. Pearson Coefficient Gamma passing rate CL LT −0.47 (−0.42–−0.52) 0.51 (0.46–0.55) LTi −0.44 (−0.38–−0.48) 0.47 (0.42–0.52) MCSv 0.39 (0.34–0.45) −0.47 (−0.42–−0.52) LTMCS 0.44 (0.39–0.49) −0.51 (−0.46–−0.55) Conclusions Local gamma analysis results underlined a significant correlation with plan complexity parameters, with larger correlations obtained with CL. [ABSTRACT FROM AUTHOR]